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1
Egan, John Mathew. "Solid state diffusion in Pd₂Si." Master's thesis, University of Cape Town, 1986. http://hdl.handle.net/11427/22141.
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The atomic transport processes ocurring in the Pd/Si system have been investigated. The Pd₂Si system has been studied to try and establish the mechanism(s) of diffusion by which the growth of Pd₂Si proceeds under thermal annealing. Using a deposited Ti marker, the dominant moving species during Pd₂Si formation in the temperature range 250-400°C has been determined to be silicon. Palladium transport appears to occur during the initial stages of formation of Pd₂Si. Once several hundred angstrom of Pd₂Si has been formed, palladium transport seems to be replaced by silicon transport. Silicon tracer experiments, in conjunction with Si selfdiffusion measurements. indicate that silicon mobility is exceptionally high during the formation of Pd₂Si on Si substrate. During growth. the mobility of silicon is orders of magnitude higher than under equilibrium conditions. This is thought to suggest a vacancy mechanism of diffusion, and is expected that large numbers of vacancies are generated at the growth interface during silicide formation. Silicon self-diffusion in Pd₂Si has been investigated. The results indicate that grain-boundary diffusion could be operative under equilibrium conditions. Under the assumption that grain-boundary diffusion does occur during thermal annealing in the range 350-550°C, it is deduced that at all times the grain-boundary diffusivity is so much greater than the lattice diffusivity, that the grain-boundaries are effectively able to act as sources for the grains. The activation energy for lattice self-diffusion of silicon in Pd₂Si which has grown on Si<100> substrate, has been determined to be 0.8±0.3eV. This value is thought to support recent kinetic results which indicate that the activation energy for growth of Pd₂Si is in the region of 1eV.
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Esser, M. J. Daniel. "Diode-end-pumped solid-state lasers." Thesis, Link to the online version, 2005. http://hdl.handle.net/10019/1020.
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Jackson, P. "Dipolar coupling in solid-state NMR." Thesis, Durham University, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.379058.
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Prakash, Arati Prakash. "Magneto Thermal Coupling in Solid State Transport." The Ohio State University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=osu1514502118670282.
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Brown, Marco J. "Selective inversion in solid-state deuteron NMR." W&M ScholarWorks, 1996. https://scholarworks.wm.edu/etd/1539623900.
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Deuteron NMR selective inversion (SI) is developed to study slow molecular motions in solids. Theoretical and practical aspects of selective inversion of spin-1 nuclei in solids are presented. Differences between powdered solids and liquids are considered. Double sideband modulated (DSBM) shaped pulses are shown to improve the performance of SI pulses. DSBM and DANTE selective pulses are compared for off-resonance SI applications. Common spin-{dollar}{lcub}1{rcub}\over{lcub}2{rcub}{dollar} shaped pulses are tested for suitability to solid state deuteron NMR. Simple, short pulses are shown to be most effective due to fast spin-spin relaxation and large underlying homogeneous linewidths in solids.;The power and utility of selective inversion is demonstrated by investigating the molecular dynamics of polycrystalline dimethylsulfone-{dollar}\rm d\sb6{dollar} (DMS). Quantitative information on the slow two-site jump motion in DMS is obtained. The complementary nature of selective inversion and quadrupolar echo lineshape (QELS) experiments is explored. Combination of QELS and SI extends the range of rates observed, increasing the accuracy of the information obtained. For DMS, motional rates were measured over a 80 K range (motional rates of {dollar}\rm 3\times 10\sp1{lcub}-{rcub}5\times 10\sp{lcub}4{rcub}\ s\sp{lcub}-1{rcub}),{dollar} permiting the barrier to slow rotation to be accurately characterized. SI was also used to observe the limiting homogeneous linewidth as a function of resonance offset and temperature. The temperature dependence of the homogeneous linewidth is shown to be sensitive to motion in the kilohertz range.;SI was used to quantify the molecular dynamics in some complex systems. SI and QELS experiments were performed on host urea-{dollar}d\sb4{dollar} in different urea inclusion compounds (UIC). Rotation rates about both the CO and CN bonds of urea-{dollar}d\sb4{dollar} in the UICs were determined as a function of temperature. Activation energies were obtained, and the results are discussed with respect to guest-host interactions in the UICs. Preliminary results are presented on quadrupolar echo lineshape simulations which include empirical, anisotropic homogeneous linewidth corrections. The modified simulations give significantly different best-fit motional rates than conventional QELS analysis. This allows some of the systematic errors of QELS analysis to be evaluated.;SI experiments were conducted, over a 125 K range, to study backbone motion in bisphenol-A polycarbonate. These experiments failed to detect the presence of any large angle slow motion of methyl groups in the isopropylidene moiety. This negative result suggests that cis-trans isomerization of the carbonate group is not the origin of the backbone motion.
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Vlassarev, Dimitar. "DNA Characterization with Solid-State Nanopores and Combined Carbon Nanotube across Solid-State Nanopore Sensors." Thesis, Harvard University, 2012. http://dissertations.umi.com/gsas.harvard:10310.
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A DNA molecule passing through a nanopore in a liner and sequential fashion allows for unprecedented interrogation of the polymer. Adding transverse electrodes that are comparable in size and sensitive to the DNA molecule, can further the attempts to rapidly sequence DNA. Carbon nanotubes are comparable in size and interact strongly with the DNA molecule. This makes them an excellent choice for integration with nanopores. Only the section of the carbon nanotube in immediate proximity to the nanopore should be sensitive to the DNA molecules. Atomic layer deposition of metal-oxides passivates the sections of the carbon nanotube that are not to interact with the DNA molecule. The coating also protects the thin film interconnects leading to the carbon nanotube. Hafnium oxide is superior to aluminum oxide in chemical resistance and electrical insulation but leads to a high failure rate of the carbon nanotube across nanopore devices. Aluminum oxide, combined with gold thin film interconnects to the carbon nanotube, produced the first functioning devices in electrolyte. These devices had concurrently functioning ionic (current across the nanopore) and transverse (current through the carbon nanotube) channels. No concurrent DNA translocation signal was recorded on the ionic and nanotube current traces. Analyzing the translocation events recorded on the ionic channel indicated that double-stranded DNA (dsDNA) passed through the carbon nanotube articulated nanopore an order of magnitude slower than it would have through a comparable unarticulated nanopore. The slower translocation observed is a necessary condition for sequencing. Investigating dsDNA translocation under various experimental conditions led to the discovery of a new interaction between the molecule and small nanopores. A dsDNA molecule is trapped when the electric field near the nanopore attracts and immobilizes a non-end segment of the molecule at the nanopore orifice without inducing folded translocation. In this work, the expression “trapped dsDNA” will exclusively refer to the immobilization of a dsDNA molecule at the orifice of the nanopore. The ionic current through the nanopore decreases when the dsDNA molecule is trapped by the nanopore. By contrast, a translocating dsDNA molecule under the same conditions causes an ionic current increase. Finite element modeling results predict this behavior for the conditions of the experiment.Physics
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Johnson, D. R. "The microstructure of all-solid-state batteries." Thesis, University of Oxford, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.375262.
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Elder, Ian F. "Diode-pumped two micron solid-state lasers." Thesis, University of Strathclyde, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.284812.
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Calmes, Lonnie Kirkland. "Solid-state Raman image amplification." Diss., The University of Arizona, 1998. http://hdl.handle.net/10150/288920.
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Amplification of low-light-level optical images is important for extending the range of lidar systems that image and detect objects in the atmosphere and underwater. The use of range-gating to produce images of particular range bins is also important in minimizing the image degradation due to light that is scattered backward from aerosols, smoke, or water along the imaging path. For practical lidar systems that must be operated within sight of unprotected observers, eye safety is of the utmost importance. This dissertation describes a new type of eye-safe, range-gated lidar sensing element based on Solid-state Raman Image Amplification (SSRIA) in a solid-state optical crystal. SSRIA can amplify low-level images in the eye-safe infrared at 1.556 μm with gains up to 106 with the addition of only quantum-limited noise. The high gains from SSRIA can compensate for low quantum efficiency detectors and can reduce the need for detector cooling. The range-gate of SSRIA is controlled by the pulsewidth of the pump laser and can be as short as 30-100 cm, using pump pulses of 2-6.7 nsec FWHM. A rate equation theoretical model is derived to help in the design of short pulsed Raman lasers. A theoretical model for the quantum noise properties of SSRIA is presented. SSRIA results in higher SNR images throughout a broad range of incident light levels, in contrast to the increasing noise factor with reduced gain in image intensified CCD's. A theoretical framework for the optical resolution of SSRIA is presented and it is shown that SSRIA can produce higher resolution than ICCD's. SSRIA is also superior in rejecting unwanted sunlight background, further increasing image SNR. Lastly, SSRIA can be combined with optical pre-filtering to perform optical image processing functions such as high-pass filtering and automatic target detection/recognition. The application of this technology to underwater imaging, called Marine Raman Image Amplification (MARIA) is also discussed. MARIA operates at a wavelength of 563 nm, which passes most efficiently through coastal ocean waters. The imaging resolution of MARIA in the marine environment can be superior to images produced by laser line scan or standard range-gated imaging systems.
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Van, der Westhuizen Gysbert Johannes. "Numerical design of an optical solid-state amplifier." Thesis, Stellenbosch : Stellenbosch University, 2007. http://hdl.handle.net/10019.1/21915.
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Canessa, E. "Statistical physics of colloidal dispersions." Thesis, University of East Anglia, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.314412.
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Lynch, Jonathan William. "Laser (Cooling) Refrigeration in Erbium Based Solid State Materials." Diss., Temple University Libraries, 2015. http://cdm16002.contentdm.oclc.org/cdm/ref/collection/p245801coll10/id/358684.
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PhysicsPh.D.
The objective of this study was to investigate the potential of erbium based solid state materials for laser refrigeration in bulk material. A great deal of work in the field has been focused on the use of ytterbium based ZBLAN glass. Some experiments have also reported cooling in thulium based solid state materials but with considerably less success. We proposed that erbium had many attractive features compared to ytterbium and therefore should be tried for cooling. The low lying energy level structure of erbium provides energy levels that could bring obtainable temperatures two orders of magnitude lower. Erbium transitions of interest for cooling fall in the near IR region (0.87 microns and 1.5 microns). Lasers for one of these transitions, in the 1.5 micron region, are well developed for communication and are in the eye-safe and water and atmosphere transparent region. Theoretical calculations are also presented so as to identify energy levels of the eleven 4f electrons in Er3+ in Cs2NaYCl6:Er3+ and the transitions between them. The strengths of the optical transitions between them have been calculated. Knowledge of such energy levels and the strength of the laser induced transitions between them is crucial for understanding the refrigeration mechanisms and different energy transfer pathways following the laser irradiation. The crystal host for erbium was a hexa-chloro-elpasolite crystal, Cs2NaYCl6:Er3+ with an 80% (stoichiometric) concentration of erbium. The best cooling results were obtained using the 0.87 micron transition. We have demonstrated bulk cooling in this crystal with a temperature difference of ~6.2 K below the surrounding temperature. The temperatures of the crystal and its immediate surrounding environment were measured using differential thermometry. Refrigeration experiments using the 1.5 micron transition were performed and the results are presented. The demonstrated temperature difference was orders of magnitude smaller. Only a temperature of ~0.015 K below the temperature of the surrounding environment was observed in this case. These results are in agreement with another group’s that has observed cooling, though a slightly poorer temperature difference, using this transition of erbium (Condon et. al., 2009). Cooling was also attempted in the 0.87 micron transition of another crystal host, KPb2Cl5:Er, which has a concentration of about one percent of erbium. We did not observe any cooling in this crystal. However, the first cooling reports in erbium based systems were with this crystal where another group observed cooling by 0.7 K using the same transition (Fernández, García-Adeva, & Balda, 2006).
Temple University--Theses
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Orr, Simon Timothy. "Multinuclear solid-state NMR of fuel cell materials." Thesis, University of Warwick, 2010. http://wrap.warwick.ac.uk/35532/.
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This thesis describes the application of multinuclear solid-state NMR to three materials systems: first, components of polymer-based proton-exchange fuel cells including the fluoropolymer membranes (Chapter 4) and the precious metal supported catalysts (Chapter 5); secondly, the formation of a complex bismuth niobium aluminoborosilicate glass-ceramic with novel dielectric properties (Chapter 6); finally, platinum (II) dialkyldithiophosphates which belong to a class of compounds (metal dialkyldithiophosphates) some of which are used in mineral separation processing (Chapter 7). A full investigation into the effects of different conditions during sample preparation and 19F NMR experiments on fluoropolymer membranes recommended unmilled preparation, dry storage and magic angle spinning below 24 kHz for the study of structural differences between membranes. The application of 19F NMR to a range of commercial and experimental fluoropolymer membranes revealed that the equivalent weight does not affect the mobility of the polymer molecules such that can be detected by this technique. Calculations of equivalent weight from 19F NMR differed with quoted values by up to 14%. Discrepancies were smallest in the short sidechain polymers, as low as 3%. The assignment of spectra was invariant with sidechain structure apart from a change in the number of ester links. The presence or absence of oxygen affected chemical shielding even around nuclei separated by several bonds. Differences in 1H linewidths between membranes could not be interpreted without the control and comparison of manufacturing techniques. It is desirable to remove the necessity for organic solvents in membrane casting. However, membranes cast from aqueous solution do not possess the same properties as those from propanol. It had been proposed that rapid drying of water cast membranes would result in a structure more similar to those from organic solvent. 1H NMR revealed that the opposite is the case, rapid drying makes the ordinarily more inhomogeneous aqueous membranes even more so. The application of both 19F and 1H NMR revealed that the monomolecular layers of fluoropolymer deposited on the surface of fuel cell catalysts to aid proton conductivity are categorically different in nature to the same materials in the bulk state. 19F NMR suggests a polymer structure either more disordered, greatly less mobile or both. 1H NMR displayed water environments that could not be reconciled to the standard model of rapid exchange between bulk water and water associated with acid groups. Spectral differences caused by solvent and polymer loading were discussed. The first complete and quantitative Fourier transformed 195Pt NMR spectra of platinum fuel cell catalysts, acquired using a field sweeping method, are analysed for deviation from the cubooctohedral particle model and surface oxidation. A combination of 11B, 27Al and 29Si studies of the BN1 ceramic system after different temperature heat treatments confirmed much of the previous work on phase evolution. However, it was shown that kyanite does not make up a significant proportion of the material until heat treatment reaches 1000 ºC and that aluminium impurities in bismuthbiobate crystals appear to increase with treatment temperature. The nature and abundance of glassy phases in the system are explored for the first time. Field sweep 195Pt NMR was employed to characterise the 195Pt chemical shift anisotropy of five platinum (II) dialkyldithiophosphates complexes. Additionally the 31P chemical shift anisotropies of two of the complexes, previously unpublished are presented.
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Bhattacharya, Prodipta. "Solid state NMR studies of ferroelectric relaxor materials." Thesis, University of Warwick, 2005. http://wrap.warwick.ac.uk/66666/.
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Multi-nuclear solid state nuclear magnetic resonance has been used to investigate the local atomic structure of the relaxor ferroelectric materials, lead magnesium niobate titanate (PMN-PT) and sodium potassium bismuth titanate (NKBT). In addition to these two series of materials, numerous precursor and model niobate compounds have also been analysed in order to gain a insight into the structures and phases present in these materials. The PMN-PT series was investigated using 93Nb, 207Pb and 170 NMR techniques. A total of 14 PMN-PT samples, from pure PMN to PMN-90PT, were investigated in order to fully understand the transitions taking place over the entire compositional range. 9~b proved to be the most informative nucleus, owing to its high sensitivity to the changes occurring at the B-site of the perovskite structure. We discovered three distinct niobium environments. We then proposed a new randomsite random-layer model explaining the distribution of the cations among two different layers ß' and ß". The high level of correlation between the theoretical predictions and the experimental results suggests that there are actually two different ways that PMN-PT behaves, one for titanium concentrations less than 25% and the other for concentrations over 25%. This was also clearly visible in our PMN-PT spectra, as a sharp line present in titanium concentrations below 25%, that disappears in the concentrations above 25%. We have also tied in our results with the existing literature on PMN-PT to identify possible links to the dielectric response and phase transitions in the material. NKBT was investigated using both 23Na and 39K MAS NMR techniques. The 23Na data proved most informative and results were obtained at different fields and different spinning speeds. We were then able to extract calculated isotropic chemical shift values and quadrupolar parameters to understand the subtle changes taking place. The preliminary results hint that there are some interesting changes taking place around the morphotropic phase boundary in the material.
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Vlachou, Maria C. "17O solid state NMR study of ceria systems." Thesis, University of Warwick, 2017. http://wrap.warwick.ac.uk/108288/.
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Ceria is a highly commercial, valuable material. Its diverse set of real-world applications demonstrates its far-reaching influence on modern society. Indeed, one only has to consider its extensive employment as an essential component of automotive, three-way catalysis (TWC) for reducing pollutants in vehicular exhaust gases; a technology that is used on a daily basis by millions of people, to appreciate its importance. Its popularity is partially due to its extraordinary oxygen storage capacity, i.e. its ability to release and store oxygen through redox reactions mediated by the two oxidation states of cerium (Ce3+/Ce4+). This unique ability can be heavily influenced by ceria’s particle size and shape, dopant concentration and surface chemistry, and is still a very active area of research today. Whilst there have been over 26,000 scientific publications on ceria since the 1950s, and although the bulk-ceria oxygen chemical shift (877 ppm) was first observed with 17O solid-state NMR in 1989, it is only within the past decade that solid-state NMR has been employed as a probe for ceria’s structure-function relationships. This study presents an extensive 17O solid-state NMR investigation into various ceria systems, in conjunction with complementary Raman spectroscopy (for which the literature is abundant). An alternative spectral assignment for the 17O NMR spectrum of nanoceria based on experimental deductions and DFT quantum mechanical calculations is proposed. The current working assignment for nanoceria is based on the work of Wang et al., where DFT calculations of a core-shell model are used to deconvolute the NMR data. In contrast to this model, it is suggested that the most upfield peak at δCG = 822 ppm (that is actually part of a multicomponent region centred at ~830 ppm), corresponds to oxygen species displaced from their standard sublattice positions, to occupy a Frenkel type defect site. The resonance regions observed at ~830 and 920 ppm are proposed to be bulk oxygen environments feeling the effects of the Frenkel defect and concomitant oxygen vacancy. For reduced ceria systems, a broad component is detected that spans > 1000 ppm, and is assigned to oxygen directly bonded to Ce3+, in agreement with the current working model. Finally, it is observed that the most deshielded peak at ~1030 ppm is multi-component and (one of these components) possesses an extensive spinning sideband manifold in comparison to the other 17O resonances. Variable temperature investigations show a small inverse temperature dependence of the peak positions, suggesting weak pseudocontact paramagnetic shifts are influencing the spectrum. In light of this, the peak at ~1030 ppm is assigned to oxygen in closer proximity to (but not directly bonded) to Ce3+, as is reported by current 27Al solid-state NMR studies of Ce3+ doped systems and as is reflected by the extensive sideband manifold. The novel 17O preparation treatments implemented in this work were engineered to probe oxygen environments near defects. Ceria’s defects are manipulated by exposure to certain temperature/atmospheric conditions, and when these replicate those of possible catalyst operating temperatures with 17O enriched gas, 17O is able to probe the distribution of oxygen in distinct sites that are important to the OSC process. A prereduction of ceria was therefore implemented, following 17O2 reoxidation to target these sites. This process was also able to identify a surface reorganization mechanism in which a low reoxidation temperature/ pressure of 17O2 is insufficient to reverse the onset of bulk oxygen diffusion induced by the reduction, an effect which is seen to be enhanced with the loading of Pd. Furthermore, the storage of these treated systems dictates the evolution of the 17O species, with an almost closed-system (to air) detecting the pathway of reoxidised oxygen species to more stable sites over time. Ceria zirconia systems are also investigated in this work. The 17O solid-state NMR spectra show an even greater sensitivity to the 17O2 enrichment conditions. The pre-reduction of the systems induces a greater oxygen removal compared to pure ceria, and thus an increase in Ce3+ paramagnetic centres, i.e. the broad component (> 1000 ppm) characterising the oxygen-Ce3+ bond reveals a greater relative intensity. The complication of increasing Ce3+ paramagnetic centres near the 17O adsorption sites is evidenced by the significant loss in 17O spectral resolution. These effects are exacerbated when ceria zirconia is supported with Pd, known to (stably) reduce the state of the system even further. A straight exchange of the 17O isotope in ceria zirconia helps to inhibit these effects, allowing observation of (1) the chemical shift of the bulk-ceria oxygen move to a more shielded position compared to the pure ceria material at 877 ppm (caused by the inherent contraction of the ceria zirconia lattice), and (2) a broad resonance at ~730 ppm, attributed to oxygen species bridging cerium and zirconium.
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Kuan, Aaron. "Ultra-Thin Solid-State Nanopores: Fabrication and Applications." Thesis, Harvard University, 2016. http://nrs.harvard.edu/urn-3:HUL.InstRepos:33493498.
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Solid-state nanopores are a nanofluidic platform with unique advantages for single-molecule analysis and filtration applications. However, significant improvements in device performance and scalable fabrication methods are needed to make nanopore devices competitive with existing technologies. This dissertation investigates the potential advantages of ultra-thin nanopores in which the thickness of the membrane is significantly smaller than the nanopore diameter. Novel, scalable fabrication methods were first developed and then utilized to examine device performance for water filtration and single molecule sensing applications.
Fabrication of nanometer-thin pores in silicon nitride membranes was achieved using a feedback-controlled ion beam method in which ion sputtering is arrested upon detection of the first few ions that drill through the membrane. Performing fabrication at liquid nitrogen temperatures prevents surface atom rearrangements that have previously complicated similar processes. A novel cross-sectional imaging method was also developed to allow careful examination of the full nanopore geometry.
Atomically-thin graphene nanopores were fabricated via an electrical pulse method in which sub-microsecond electrical pulses applied across a graphene membrane in electrolyte solution are used to create a defect in the membrane and controllably enlarge it into a nanopore. This method dramatically increases the accuracy and reliability of graphene nanopore production, allowing consistent production of single nanopores down to subnanometer sizes.
In filtration applications in which nanopores are used to selectively restrict the passage of dissolved contaminants, ultra-thin nanopores minimize the flow resistance, increasing throughput and energy-efficiency. The ability of graphene nanopores to separate different ions was characterized via ionic conductance and reversal potential measurements. Graphene nanopores were observed to conduct cations preferentially over anions with selectivity ratios of 100 or higher for pores as large as 20 nm in diameter, suggesting that porous graphene membranes can be used to create highly effective cation exchange membranes for electrodialysis filtration. These surprisingly high selectivities cannot be explained by current models of ionic conduction in graphene nanopores, motivating the development of a new model in which elevated concentrations of mobile cations near the graphene surface generate additional ion selectivity.Engineering and Applied Sciences - Applied Physics
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Maker, Gareth Thomas. "Diode laser pumped solid state lasers." Thesis, University of Southampton, 1990. https://eprints.soton.ac.uk/397281/.
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This thesis deals with the development of diode laser pumped solid state lasers. The earliest work presented enabled 125W peak power, single frequency Q-switched pulses to be obtained from a 100mW diode laser pumped Nd:YAG laser. Using a 500mW diode laser as a pump source for C.W. Nd:YAG and Nd:YLF oscillators an Yb:Er fibre laser was pumped, producing 0.75mW C.W. power at a wavelength of 1.56µm. Acousto-optic mode-locking techniques were used to provide C.W. mode-locked pulse durations in Nd:YAG and Nd:YLF of 55ps and 18ps respectively, at repetition rates of 240MHz. Frequency modulation mode-locking was shown to be a superior technique, giving pulse durations of 11.5ps and 10ps in diode laser pumped Nd:YAG and Nd:YLF oscillators respectively. FM operation of diode laser pumped Nd:YAG lasers was investigated, yielding a maximum FM bandwidth of 70GHz. Spatial hole burning was considered to be an important factor in this result. Using a 1W diode laser to pump a mode-locked and Q-switched Nd:YLF oscillator peak power levels of 70kW were obtained at a wavelength of 1.047µm. Frequency doubling this output in MgO:LiNbO3 with an energy conversion efficiency of 47% enabled other tunable lasers to be pumped using the second harmonic. Firstly, a synchronously pumped rhodamine 6G dye laser is described which is capable of producing 3.2ps mode-locked pulses in a Q-switched envelope with peak powers of around 10kW. Secondly, a synchronously pumped doubly resonant optical parametric oscillator tunable between 983nm and 1119nm is described. Lastly, a Ti:Sapphire laser producing 400ns pulses with peak powers of 3W at a wavelength of 755nm is demonstrated. This oscillator could be wavelength tuned between 746nm and 838nm. A highly efficient method of frequency doubling C.W. mode-locked lasers was developed. Using an external resonant cavity a frequency doubling energy conversion efficiency of 61% to 532nm was achieved, giving 87mW average power in 8.5ps pulses.
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Haydock, Sarah Amanda. "In situ vibrational spectroscopy of thin organic films confined at the solid-solid interface." Thesis, University of Oxford, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.270369.
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Westwood, Stephen Mark. "The physics of magnetic particle inspection." Thesis, Durham University, 1993. http://etheses.dur.ac.uk/5525/.
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This thesis describes the results of an investigation into the Physics of Magnetic Particle Inspection. This is a non destructive technique used to detect the presence of surface breaking cracks in ferrous metals. On applying a magnetic ink to the surface of the part being magnetised, particles will be attracted towards any regions of flux discontinuity. This allows the defect to be visible to the eye. The main areas of study were the magnetic properties of the inks used, a computer simulation of the indication process and work of a more practical nature concerning MPI. From the magnetic measurements it was found that field induced agglomeration of the particles imparts the ink with a coercivity and remanence which varies with concentration. Low field time decay measurements show the existence of two reversal mechanisms within the magnetic ink, i.e. Brownian and Neel. The effect interparticle interactions have on a single aggregate has been studied by the use of remanence curves and this was found to lower the aggregates' magnetisation. This therefore lowers the magnetic force on a particle during the inspection process. The spin reversal mechanism has also been determined. The computer simulation of the indication mechanism originally written by McCoy has been improved to accommodate time varying waveforms. Results from this investigation indicate that the rms of the waveform is the important parameter when specifying field values for magnetisation levels for low remanence steels. This is contrary to the British Standard BS 6072 but in agreement with the draft European Standard. It was also found that increasing the paint layer thickness decreased the number of particles at a crack. This variation was found to be similar to the variation of the magnetic leakage field from the crack with increasing paint thickness. From a study of the effects of viscosity and radii on the number of particles arriving at a crack, we find that for particles whose radius is greater than 10 microns no change is observed as the viscosity is altered. Thyristor controlled power supplies are increasingly being used to magnetise inspection pieces and for this reason the relationship between the mean and rms of the waveform has been determined. It was found that the relationship between the two is non linear. This has implications if the user wishes to remain within BS 6072. Experimental verification of the simulation results is also presented. An offshoot of this is a possible method of determining the crack width using MPI, this is also presented. Finally it was noted that upon increasing the applied magnetic field, particles present at the crack rotated through 180 degrees. This also occurred upon decreasing the magnetic field. Investigation of this unusual effect are reported as well as a possible mechanism for it.
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Drummond-Brydson, Richard. "Electron energy loss spectroscopy in solid-state chemistry." Thesis, University of Cambridge, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.237906.
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Cheng, Chi-Feng. "Solid-state NMR studies of mesoporous molecular sieves." Thesis, University of Cambridge, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.243004.
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Brierley, Richard Thomas. "Far-from-equilibrium many-body physics in solid state systems." Thesis, University of Cambridge, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.610799.
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Bates, Matthew. "Theoretical investigation of solid state cooling using spin models." Thesis, University of Warwick, 2015. http://wrap.warwick.ac.uk/79564/.
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A mean field spin model coupled to a thermal model of a solid is used as a description of the electrocaloric effect in relaxor ferroelectrics at both first and second order transitions. This theoretical model is also used in efforts to find the tricritical point of transitions to maximise the adiabatic temperature change due to the electrocaloric effect while minimising the hysteresis losses in a cooling cycle. The electrocaloric effect is the adiabatic temperature change exhibited by a material under the sequential application then removal of electric fields. Relaxor ferroelectrics are materials with two interaction scales, local interactions in polarised nano regions (PNRs) and longer range interactions between PNRs. Electric dipoles are modelled as spins and their alignment due to an external field is used to examine polarisationelectric field loops. The results are compared to experimental data to determine values of parameters that may be used in simulations to model the electrocaloric effect. I reproduced simulations of the electrocaloric effect in the ferroelectric material lead zinc niobate-lead titanate [Pb(Zn1=3Nb2=3)O3-PbTiO3] by coupling the dipolar entropy of the spins to the entropy of a thermal lattice one can examine the temperature increase that must occur as the dipolar entropy decreases in an external field. The results of this simulation agree with experiment. Novel work is carried out in a prediction of the strength of the electrocaloric effect in polyvinylidene fluoride trifloroethylene, P(VDF-TrFE), using a simulation where the material parameters are determined from the comparison of simulated polarisation-electric field hysteresis loops with experimental results. The simulations suggest that P(VDF-TrFE) is a material worth investigating experimentally because even though it has a weak electrocaloric strength, under large fields the potential adiabatic temperature change at room temperature has promise for replacing conventional refrigerants. Due to the simplicity of this coupled model and the physical similarities between the electrocaloric effect and the magnetocaloric effect (an analogous effect observed in certain magnetic materials where an adiabatic temperature change is seen under the application and removal of a magnetic field) an investigation is also carried out on the magnetocaloric material iron rhodium (FeRh). In simulations I varied global and local stoichiometry to affect the ordering of spins and thus the entropy and strength of the magnetocaloric effect. I compared the results of our simulations with experiment and examine the variation in peak isothermal entropy change and adiabatic temperature change under variation in stoichiometry as well as the effects on the full width at half maximum. The results show that a simple model can give a qualitative representation of the effect. Work of a different nature is presented at the end of the thesis due to it being a short and complete topic which interested me during my PhD. Details are given on how Catalan numbers may be used to determine the depth of leaves in binary tree graphs and path length between them using both diagrammatic notation and the methodology of generating functions. An analytic solution is drawn from a combinatoric problem that initially appeared to only be soluble by brute force numerics.
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Blackband, S. J. "NMR imaging of liquid-solid systems." Thesis, University of Nottingham, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.356019.
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Fleming, F. A. "Proton conduction in solid germanium phosphates." Thesis, University of Leeds, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.356446.
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Bollig, Christoph. "Single-frequency diode-pumped solid-state lasers." Thesis, University of Southampton, 1997. https://eprints.soton.ac.uk/378587/.
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The work discussed in this thesis covers two broad areas: Novel techniques for the single-frequency operation of miniature, diode-pumped solid-state lasers and the high-power (i.e. multi-watt) operation of diode-bar end-pumped lasers in the eyesafe 2 μm wavelength region. A monolithic Nd-doped phosphate glass laser is described, in which uni-directional, hence single-frequency operation is enforced by the acousto-optic effect in the laser medium. The loss difference for the two counter-propagating waves relies on an acousto-optic self-feedback mechanism which can yield high loss differences even for very small diffraction efficiencies. Reliable single-frequency output is maintained indefinitely with an applied radio-frequency power of 0.2 W. Single-frequency output powers up to 30 mW for 400 mW of pump power are demonstrated. A technique is developed which facilitates reliable single-frequency operation of actively Q-switched lasers at repetition rates beyond the inverse lifetime of the upper laser level. Stable single-frequency operation of a Q-switched laser requires the initial establishment of a stable prelase which is free from spiking. Relying on the natural decay of spiking limits repetition rates and hence average power. Using feedback suppression of spiking, a Q-switched Nd:YAG laser is demonstrated which operates on a single frequency at repetition rates up to 25 kHz, with 88% of available cw power extracted. In the second part of this thesis, the high-power operation of diode-bar end-pumped solid-state lasers operating in the eyesafe 2 μm wavelength region is discussed. Efficient operation of a Tm:YAG laser end-pumped by a beam-shaped 20W diode bar is demonstrated. At a mount temperature of 20°C an output beam of 4.1 W with M2 values of 1.2 and 1.4 in the orthogonal planes is obtained for 13.5 W of diode power incident on the rod. This laser is then used to intracavity-pump a Ho:YAG laser, which avoids the upconversion problems usually associated with Tm3+-Ho3+-codoped lasers. At a mount temperature of 10°C for both the Tm:YAG and the Ho:YAG rods, an output power of the Ho:YAG of up to 2.1 W at 2097 nm is obtained for 9.2 W of diode power incident on the Tm:YAG rod.
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Goddard, Yanina Anatolievna. "Solid state NMR characterization of conductive polyanilines." W&M ScholarWorks, 2004. https://scholarworks.wm.edu/etd/1539623435.
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Different forms of ring deuterated polyaniline with different conductivity have been characterized by solid state deuteron nuclear magnetic resonance.;Quadrupole echo (QE) spectra of all forms of polyaniline consist of a superposition of lineshapes for nearly rigid aromatic rings and a small fraction of rings which undergo fast 180?? flips. The intensity of the fast flipping component is temperature dependent and different for conductive emeraldine salt (ES) and non-conductive emeraldine base (EB). This is a manifestation of the different structure and morphology of these polymers.;Simultaneous measurements of QE lineshapes and the relaxation time anisotropies allowed an accurate description of motion in polyanilines. Slow, small-angle libration in an asymmetric cone provided the best description for the "rigid" fractions of EB and ES. The broadening of deuteron QE lineshapes is consistent with the presence of a distribution of cone angles. Relaxation time measurements also reveal a relatively narrow distribution of librational rates for the EB sample. For ES, the magic angle spinning (MAS) spectra show the existence of two resolved signals with different relaxation rates, which are ascribed to microscopic domains with very different electrical properties. The unexpectedly short relaxation time found for nonconductive domains in ES can be explained by the presence of localized, unpaired electrons.;Spin count experiments proved that in highly conductive ES samples, loss of NMR signal intensity occurs not only because of high RF reflectance but also because of irreversible dephasing before signal acquisition due to interactions of nuclear spins with localized unpaired electrons.;Deuteron MAS spectra provided unique information about small frequency shifts. Compared to non-conductive EB, conductive emeraldine salts have an additional manifold of spinning sidebands, which is shifted ∼5.8 ppm towards higher frequencies. These shifted sidebands arise from quasi-metallic regions of the sample, where deuteron spins interact with delocalized electrons (Knight shift). The experimental temperature dependence of the intensity of the shifted peak can be explained using models developed for amorphous semiconductors. The observation of a Knight shift has an important consequence for the theory of electrical conduction in polyaniline: it implies that polarons are the charge carriers.
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Dauphinais, Guillaume. "Fabrication, structural relaxation, and flow in solid-state nanopores." Thesis, McGill University, 2012. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=107651.
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Single solid-stat nanopores have been fabricated in free-standing layers of amorphous silicon nitride. Using a focused electron beam and a focused ion beam, diameters ranging from about 0.7 nm to a few hundred nanometerswere achieved. Structural relaxation of nanopores fabricated by focused electron beam was observed. Depending on the initial diameter and on the length of the nanopores, two distinct phases for the relaxation process were found. An experimental setup for themeasurement of pressure-driven mass flow of helium through a single nanopore was built. The conductance of nanopores with diameters ranging from 25 nm to 315 nm was measured. A semi-phenomenological model was developed and it was shown to quantitatively describe the conductance of fluid through a short cylindrical nanopore.Des nanopores ont été fabriqués dans de minces couches suspendues de nitrure de silicium amorphe. En utilisant un faisceau d'électrons focalisé et un faisceau d'ions focalisé, des diamètres entre 0.7 nm et 315 nm ont été obtenus. La relaxation struturelle de nanopores fabriqués par faisceau focalisé d'électrons a été observée. Dépendamment du diamètre inital du nanopore et de sa longueur, deux phases distinctes ont été identifiées. Un montage expérimental permettant la mesure de l'écoulement de masse d'hélium causé par l'application d'une différence de pression a été réalisé. La mesure de conductance de nanopores ayant un diamètre compris entre 25 nm et 315 nm a été effectuée. Un simple modèle phénoménologique permet de décrire quantitativement l'écoulement de gaz dans un court nanopore cylindrique.
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Reams, Patrick William. "Solid-state N.M.R. studies of platinum and tin compounds." Thesis, Durham University, 1986. http://etheses.dur.ac.uk/6793/.
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High-resolution solid-state N.M.R. studies of dilute spins are now possible using cross-polarisation and MAS techniques. A systematic evaluation has been undertaken to determine their applicability to spin-½ metal nuclei, in particular (^195)Pt and (^119)Sn. In addition, an extensive (^13)C and (^31)p solid-state N.M.R. study has been carried out on a selection of Pt(II) complexes, supplying information on isotropic (scalar) coupling constants and shielding anisotropy. The majority of (^119)Sn and (^195)Pt spectra exhibit a multitude of spinning sidebands due to the large shielding anisotropy present. The tin systems under study have been of type R(_3)SnX (where R = Alkyl, phenyl and X = F, OH, CI); some are shown to be polymeric in the solid-state with penta-coordinate tin present. Where possible, correlations with X-ray crystallographic data and solution-state N.M.R. studies are giving. The interactions present in Pt(IV) compounds containing directly bonded quadrupolar nuclei have been studied and imply motional activity present in the solid- state. A more comprehensive study of these effects is given for two tin systems (Ph(_3)SnCl and (NH(_4))(_2)SnCl(_6)), whereby observed splittings can be accounted for by a combination of (^119)Sn-Cl dipolar and scalar coupling. The interplay of tensor properties between spin-½ nuclei, namely (1) dipolar coupling, (11) indirect (scalar) coupling and (111) shielding anisotropy is explored in solid-state (^195)pt-(^31)p and (^119)sn-(^19)f systems. The theory for such tensorial interplay is given for an AX(_2) system.
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Gullans, Michael John. "Controlling Atomic, Solid-State and Hybrid Systems for Quantum Information Processing." Thesis, Harvard University, 2013. http://dissertations.umi.com/gsas.harvard:11146.
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Quantum information science involves the use of precise control over quantum systems to explore new technologies. However, as quantum systems are scaled up they require an ever deeper understanding of many-body physics to achieve the required degree of control. Current experiments are entering a regime which requires active control of a mesoscopic number of coupled quantum systems or quantum bits (qubits). This thesis describes several approaches to this goal and shows how mesoscopic quantum systems can be controlled and utilized for quantum information tasks.Physics
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Marasli, Necmettin. "The measurement of solid-liquid surface energy." Thesis, University of Oxford, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.260154.
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Codd, Sarah. "3DFT NMR imaging of solid-like materials." Thesis, University of Kent, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.318105.
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Young, Kevin Edward. "Ionic conductivity in silicate - containing solid electrolytes." Thesis, University of Exeter, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.335654.
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Akeroyd, Frederick Anthony. "The sticking of molecules to solid surfaces." Thesis, University of Cambridge, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.309088.
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Smith, Thomas Benjamin. "Entanglement and measurement of solid-state qubits." Thesis, The University of Sydney, 2020. https://hdl.handle.net/2123/24894.
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A full-scale quantum computer requires physical qubits that can be controlled with high precision and accuracy. Unfortunately, few state-of-the-art qubits can perform all their elementary operations (preparation, measurement, single-qubit gates and two-qubit gates) with sufficient fidelity. In this thesis, we investigate alternative schemes for such operations in solid-state qubits. Specifically, two-qubit gates and measurements, which are often the noisiest of the four.
We first provide a preliminary introduction to quantum computing, and describe how quantum information can be encoded and manipulated in quantum systems. We include background information for the three different solid-state qubit architectures that feature in this thesis: spin qubits, superconducting qubits and Majorana qubits.
Following this, we investigate a scheme for mediating a two-qubit interaction between spin qubits via a multielectron quantum dot. We study a multielectron dot in detail, and characterise its exchange interaction with a single spin. With the aid of a theoretical model, we show that the multielectron dot possesses an irregular triplet-preferring ground state, analogous to Hund's rule from atomic physics. Using these findings, we then demonstrate that the multielectron dot can be used to mediate a fast, long-range exchange interaction between two spin qubits.
Subsequently, we examine two resonator-based measurement schemes for Majorana qubits. We first propose a readout technique based on a longitudinal qubit-resonator interaction. This leads to a measurement that is fast, high-fidelity and quantum non-demolition (QND). We then investigate a more conventional dispersive readout scheme. Not only does this yield a high quality measurement, but it can also offers a more protected readout mechanism in comparison to the dispersive readout of conventional superconducting qubits.
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Zailer, Isaac. "The physics and fabrication of low-dimensional hole systems." Thesis, University of Cambridge, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.339624.
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Glasson, Philip Henry. "Surface state electrons in a helium micro-channel." Thesis, Royal Holloway, University of London, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.272323.
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Jin, Gongjiu. "DLTS studies of surface state effects in GaAsFETs." Thesis, Lancaster University, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.316292.
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Noble, Christopher John 1967. "Applications of magnetic resonance in materials science and solid state physics." Monash University, Dept. of Physics, 2001. http://arrow.monash.edu.au/hdl/1959.1/9064.
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Kaiser, John Michael. "New probes of spin physics and correlation in solid-state NMR." Diss., [Riverside, Calif.] : University of California, Riverside, 2009. http://proquest.umi.com/pqdweb?index=0&did=1899476631&SrchMode=2&sid=3&Fmt=2&VInst=PROD&VType=PQD&RQT=309&VName=PQD&TS=1269285773&clientId=48051.
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Thesis (Ph. D.)--University of California, Riverside, 2009.Includes abstract. Available via ProQuest Digital Dissertations. Title from first page of PDF file (viewed March 10, 2010). Includes bibliographical references. Also issued in print.
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Sean, David. "On the mobility of partially denatured DNA in gel electrophoresis: a theoretical investigation." Thesis, University of Ottawa (Canada), 2010. http://hdl.handle.net/10393/28742.
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There are technologies which exploit a rapid reduction of the gel electrophoretic mobility of DNA arising from partial denaturation. The underlying phenomenon behind these experiments---the mechanisms which reduce the mobility---are not very well understood. Such is the purpose of my thesis.
The first chapter provides a brief introduction to the field of polymer physics. The subjects covered are carefully chosen to directly relate to the forthcoming research. There is a published semi-empirical formula used to model the rapid decrease of mobility which is largely considered to be consistent with experimental data. The second chapter of this thesis demonstrates that there is a fundamental confusion in the literature regarding the fitting parameter Lr, in the said formula. By going back to the original derivation, a physical interpretation can be given to L r. This interpretation yields theoretical values which are consistent with what has been published. However, we find that an underlying assumption---that the effect of the denaturation does not depend on its position along the DNA fragment---may systematically overestimate experimental observations of Lr.
To measure the impact of this assumption, a simulation model of DNA is presented. The article presented in the third chapter reveals that indeed, the position of the denatured region affects the migration of the DNA fragment. A refined version of the formula which takes these factors into account is proposed. The simulations also reveal that, for certain fields, an unexpected conformation completely dominates during migration of the fragment. This surprising result: a squid-like conformation, is explored in chapter four.
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McGowan, Shona. "«In situ» study of amorphous semiconductor crystallization by dynamic transmission electron microscopy." Thesis, McGill University, 2010. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=86999.
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Amorphous silicon is an important material of particular interest in the context of thin film transistors and display technologies. It also provides an ideal model system for the study of crystallization dynamics, and to this end we have investigated the crystallization of thin amorphous semiconducting films, including silicon and germanium, in the Dynamic Transmission Electron Microscope (DTEM) at Lawrence Livermore National Laboratory.Crystallization is initiated in the DTEM by a laser pulse that deposits sufficient heat into the system to activate the transition. A variable time delay after this initiation pulse, the sample is probed by a short photoelectron pulse in the TEM, generating a diffraction pattern or image with nanosecond time resolution. Thus, using the DTEM we can gain access to the kinetics of crystallization and details of the nucleation mechanism in situ through time-resolved diffraction patterns and images of the specimen.
In the crystallized silicon film, three distinct phenomena were observed as the incident fluence on the sample was increased: for low fluences, the film underwent solid state crystallization, for surface melting, large, radially oriented crystals were observed and when the entire film was melted, the silicon dewetted from the substrate and coalesced into crystalline droplets on the surface. Modelling of heat conduction in the laser-heated film supports this interpretation. Preliminary experiments were also carried out in amorphous germanium.
Le silicium amorphe est un matériau important en particulier, pour la fabrication de matériau polycristallin pour transistor couches minces. Il fournit également un système modèle idéal pour l'étude de la dynamique de cristallisation, et à cet effet, nous avons étudié la cristallisation des films couche mince en semiconducteurs amorphes, y compris le silicium et le germanium, dans le microscope Électronique en Transmission Dynamique (METD) à Lawrence Livermore National Laboratory.
La cristallisation est amorcée dans le METD par une Impulsion de laser qui dépose dans le système la chaleur suffisante pour activer la transition. Un délai variable après cette impulsion de déclenchement, l'échantillon est sondée par une impulsion de photoélectron dans le MET, produisant soit diffraction ou une image avec la résolution temporelle de nanoseconde. Ainsi, utilisant le METD nous pouvons accéder à la cinétique de la cristallisation et les détails du mécanisme de nucléation emphin situ par les diagrammes diffraction et les images de l'échantillion avec résolution temporelle sur une échelle de nanoseconde.
Dans la pélicule cristallisée de silicium, on a observé trois phénomènes distincts à mesure que le fluence incident sur l'échantillon était augmenté : pour les fluences réduits, la pellicule a subi la cristallisation l'état solide, pour la fonte à la surface, des grands cristaux radialement orientés ont été produits et quand la pellicule entier a été fondu, le silicium fondu a été fusionnés dans les gouttelettes cristallines sur la surface. La modélisation de la conduction de chaleur dans la pellicule chauffée par le laser soutient cette interprtation de l'évolution de la structure observée. Des exériences préliminaires ont été aussi effectuées en germanium amorphe.
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Yu, Victor. "Optics and chemical vapour deposition of graphene monolayers on various substrates." Thesis, McGill University, 2011. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=97052.
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Graphene-based devices are typically fabricated by exfoliation on substrates such as SiO2/Si. This is a disordered surface which can degrade the intrinsic properties of graphene such as mobility. Hence, in order to optimize device performance, we investigated the possibility to deposit graphene on more ordered surfaces such as GaAs, despite the difficulty in locating exfoliated graphene on such substrates. We show that the optical contrast of graphene can be greatly enhanced by increasing theincident angle of the light on the substrates (SiO2 and GaAs). In the second part, we move beyond the standard exfoliated graphene production to present a method for large-scale graphene synthesis by chemical vapour deposition (CVD). We varied various growth conditions to understand and optimize the mechanisms of graphene synthesis and the subsequent transfer techniques.Les dispositifs à base de graphène sont généralement fabriqués par exfoliation sur des substrats de SiO2/Si, mais ces surfaces désordonnées peuvent dégrader les propriétés intrinsèques du graphène, comme la mobilité. Par conséquent, en vue d'optimiser la performance des dispositifs, nous avons étudié la possibilité de déposer le graphène sur des surfaces plus ordonnées, comme le GaAs, malgré la difficulté augmentée de l'identification du graphène exfolié sur ces surfaces. Nous démontrons que le contraste optique du graphène peut être grandement renforcé en augmentant l'angle d'incidence de la lumière sur les substrats (SiO2 et GaAs). Dans la seconde partie, nous allons au-delà de la production standard du graphène exfolié pour présenter une méthode de synthèse de graphène à grande échelle par dépôt chimique en phase vapeur. Nous modifions les paramètres de croissance pour mieux comprendre et optimiser les mécanismes de croissance et les techniques de transfert subséquent.
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Feng, Zimin. "SymGF: a symbolic tool for quantum transport theory." Thesis, McGill University, 2012. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=107697.
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In this thesis, I report the development and application of a symbolic derivation tool named "SymGF'' - standing for Symbolic Green's Function, that can automatically and analytically derive quantum transport expressions and the associated Keldysh nonequilibrium Green's functions (NEGF). Quantum transport happens in open systems consisting of a scattering region coupled to external electrodes. When there are strong electron-electron interactions in the scattering region, analytical derivations of the Green's functions can be very tedious and error prone. Running on a personal computer, SymGF derives the necessary analytical formulas at a level of correlation specified by the user, using the equation of motion (EOM) method. The input to SymGF are the second quantized form the device Hamiltonian, the (anti)commutators of the operators that appear in the Hamiltonian, and a truncation rule for the correlators which determines the accuracy of the final outcome. The output of SymGF are the analytical expressions of transport properties such as electric current and conductance in terms of various Green's functions; as well as the Green's functions themselves in terms of the unperturbed non-interacting Green's functions that can be obtained straightforwardly. For systems where electron-electron interaction can be neglected, the transport problems can be easily solved and SymGF is not necessary - even though SymGF gives the same answer; but for interacting systems SymGF drastically reduces the mathematical burden of analytical derivations. We have tested SymGF for several transport problems involving Kondo resonances where analytical derivations were done by humans: exactly the same results were obtained by SymGF but in a tiny fraction of time. We have applied SymGF to new and very hard problems that resist analytical derivations by hand, including quantum transport in a double quantum dot system; transport through a single quantum dot in parallel to a direct lead-to-lead tunneling. Finally, at the post-analysis level, we have combined SymGF with an ab initio numerical modeling method to calculate quantum transport features involving atomistic degrees of freedom.Dans cette thèse, je présente le développement et lesapplications d'un outil de calcul symbolique connu sous le nom de "SymGF" (Symbolic Green's Functions) qui permet d'obtenir des expressionsanalytiques pour le transport quantique et les fonctions de Green de Keldyshpour les systèmes hors équilibre (FGHE). Le transport quantiquesurvient dans les systèmes ouverts constitués d'une régiondiffusante\ couplée à des électrodes externes. Lorsque lesinteractions électrons-électrons sont importantes dans cette région, la dérivation analytique des fonctions de Green peut devenirfastidieuse et propice aux erreurs. Par contre, à l'aide d'un ordinateurpersonnel, la méthode SymGF permet d'obtenir rapidement les formulesanalytiques nécessaires, en utilisant les équations du mouvement(EOM) du système, à un niveau de corrélation spécifiépar l'utilisateur. Les entrées nécessaires à la méthodeSymGF sont l'Hamiltonien en seconde quantification du système, lesrelations d'anti-commutation des opérateurs impliqués et les règles de coupure pour les fonctions de corrélation, ce qui déterminela précision du résultat final. Les sorties sont les expressionsanalytiques des propriétés de transport tels que le courant électrique et la conductance en fonction des différentes fonctions deGreen en plus de l'expression de ces dernières en fonction des fonctionsde Green du système non perturbé et sans interactions, pouvant être calculées directement.Pour les systèmes où les interactions électrons-électronspeuvent être négligés, le transport peut être étudiéplus facilement et la méthode SymGF n'est plus nécessaire, mêmesi elle reste toujours aussi juste. Par contre, pour les systèmes avecinteractions, elle permet de réduire drastiquement les difficultésmathématiques reliées à la dérivation analytique. Nousl'avons testée pour plusieurs situations impliquant des résonancesde Kondo, où les dérivations analytiques avaient déjà été effectuées, et les résultats furent reproduis parfaitement enune fraction du temps. Nous avons aussi appliqué cette méthode àde nouveaux problèmes très compliqués qui résistaienttoujours au traitement analytique; tels le transport quantique dans undouble point quantique et le transport dans un point quantique en parallèle avec une simple jonction tunnel. Finalement, en vue d'analyses futures,nous avons combiné la méthode SymGF avec un méthode desimulation numérique ab initio afin de calculer les caractéristiques du transport quantique impliquant des degrés de libertéatomiques.
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Michaud-Rioux, Vincent. "Real space DFT by locally optimal block preconditioned conjugate gradient method." Thesis, McGill University, 2012. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=110628.
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In condensed matter physics, atomistic first principle calculations are often necessary to achieve a microscopic understanding of the observed experimental phenomena and to make quantitative predictions of physical properties. In practice, atomic scale systems have irregularities (e.g. surface roughness) or defects (e.g. substitutional atoms or vacancies) that are too strong to be ignored or treated as small perturbations. In this thesis, we report the development of a real space DFT code for studying atomic scale systems from first principles. Our code, named MatRcal, which stands for "Matlab-based real space calculator", is developed in the technical computing language Matlab. The physics is described by density functional theory. The method itself is based on projecting the Kohn-Sham Hamiltonian on a uniform Cartesian grid. High-order finite-differencing is used to discretize the Laplacian operator. The potential due to the atomic nuclei is approximated with ab initio pseudopotentials. The pseudopotentials are generated following the procedure proposed by Troullier and Martins. We use the fully separable form introduced by Kleinman and Bylander. We argue that the method is simpler and yet has many advantages compared with conventional spectral methods. We provide relevant mathematical techniques and implementation details. In particular, we present and compare different eigensolvers used to diagonalize the Kohn-Sham Hamiltonian. We validate our software by comparing the HOMO-LUMO gaps of many organic and inorganic molecules obtained using our method with those obtained with the commercial code Gaussian. Our results are in excellent agreement. Our method gains in computational speed and algorithm parallelism, and its power in handling real space boundary conditions will be a major advantage for future applications in nanoelectronic device modelling.En physique de la matière condensée, les calculs numériques sont souvent nécessaires pour parvenir à comprendre les phénomènes microscopiques observés lors d'expériences ou à prédire quantitativement des propriétés physiques. En pratique, les systèmes d'échelle atomique sont irréguliers (rugosité de surface) ou comportent des défauts (atomes de substitution ou lacunes), ce qui induit des effets trop sévères pour être ignorés ou traités comme des perturbations. Dans cette thèse, nous présentons une méthode qui permet d'étudier des systèmes d'échelle atomique à partir des lois fondamentales de la physique. Notre logiciel, nommé MatRcal, qui signifie "Matlab-based real space calculator", est développé dans le langage Matlab. La physique est décrite par la théorie de la fonctionnelle de la densité. La méthode projette l'Hamiltonien de Kohn-Sham sur un maillage Cartésien uniforme. Le calcul des différences finies est utilisé pour discrétiser l'opérateur Laplacien. Le potentiel dû aux noyaux atomiques est approximé par des pseudopotentiels non-empiriques. Les pseudopotentiels sont générés en suivant la procédure proposée par Troullier et Martins. Nous utilisons la forme séparable introduite par Kleinman et Bylander. Nous soutenons que la méthode est plus simple et pourtant présente de nombreux avantages par rapport aux conventionnelles méthodes spectrales. Nous introduisons plusieurs techniques mathématiques pertinentes à notre étude et certains détails d'implémentation. Entre autres, nous présentons et comparons plusieurs algorithmes de calcul de vecteurs propres utilisés pour diagonaliser l'Hamiltonien de Kohn-Sham. Nous validons notre méthode en comparant la largeur de bande interdite "HOMO-LUMO" de nombreuses molécules organiques et inorganiques prédites par notre méthode avec celles prédites par le logiciel commercial Gaussian. Notre méthode permet des gains en rapidité et en parallélisme, mais la possibilité de traiter des conditions limites non-périodiques sera le principal atout pour de futures simulations de dispositifs nanoélectroniques.
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Wang, Quanyong. "Crossover in directional solidification and C60 island morphology." Thesis, McGill University, 2009. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=32260.
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We simulate directional solidification using a phase field model solved with an adaptive finite element method. For small surface tension anisotropy directed at 45 degree relative to the pulling direction, we have observed three types of morphology: Dendritic, seaweed, and cellular. The seaweed pattern is characterized by alternating tip splitting and the branches have no orientation preference. A crossover from the seaweed to dendritic morphology was noticed when lowering the thermal gradient, consistent with previous experimental findings. We also investigate the influence of anisotropy on the morphology. We find that the thermal gradient, pulling velocity, and anisotropy can determine the crystal morphology. We show that the morphology of crystal structures can be unambiguously characterized through the local interface velocity distribution. As the thermal gradient is lowered, large-velocity shoulders begin to appear in both transverse and growth directions, becoming progressively broader as clearly defined dendrites emerge. We derive semi-empirically an estimate for the crossover from seaweed to dendrite as a function of thermal gradient and pulling velocity, which agrees with our simulation results. The ratio of thermal length to diffusion length has a sharp "transition" at the crossover region. In the second part of the thesis, a single variable dynamic model(Model B) is proposed to study C60 ultra-thin film growth on the alkali halide substrate. A triple-well free energy is proposed to explain the coexistence of three layers. The middle well is metastable representing a single layer. We successfully recover "branched" and "compact" patterns for both deposition aNous simulons la solidification directionnelle en utilisant un modèle de champ de phase résolu avec la méthode d'élément fini adaptative. Pour la petite anisotropie de tension superficielle dirigée à 45 degree relativement à la direction de traction, nous avons observé trois sortes de morphologies de solidification: dendritique, algue et cellulaire. Le schéma sous forme d'algues est caractérisé par le bout alternatif se dédoublant et dont les branches n'ont aucune préférence d'orientation. Une transition du schéma d'algues vers un morphologie dendritique a été noté lorsque le gradient thermique est abaissé. Ceci est compatible avec les résultats expérimentaux précédents. Nous étudions également l'influence de l'anisotropie sur la morphologie de cristal. Nous prouvons que le gradient thermique, la vitesse de traction, et l'anisotropie peuvent déterminer la morphologie de cristal. Nous prouvons que la morphologie des structures cristallines peut être clairement caractérisée par la distribution locale de la vitesse d'interface. Pendant que le gradient thermique est abaissé, les épaules de grand-vitesse commencent à apparaître dans des directions transversales et de croissance, devenant progressivement plus large en tant que dendrites bien définies émergent. Nous dérivons semi empiriquement une approximation pour la transition du schéma d'algues vers le schéma des dendrites en fonction du gradient thermique et la vitesse de traction, ce qui est conforme à nos résultats de simulation. Le rapport de la longueur thermique à la longueur de diffusion est caractérisée par une forte "transition" dans la région de la transition. Dans la
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Dean, Cory. "A study of the fractional quantum hall energy gap at half filling." Thesis, McGill University, 2009. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=40784.
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A complete understanding of the the ν = 5/2 fractional quantum hall effect (FQHE) continues to be among the most exciting problems in semiconductor physics. It is widely believed that this unique electron state is described by the Moore-Read Pfaffian wavefunction, resulting from a BCS-like pairing of composite fermions. In recent years this wavefunction has received special interest owing to its non-abelian quantum statistics which underlies a new paradigm for fault tolerant quantum computation. However, in spite of several theoretical advancements, an unequivocal experimental verification of the Moore-Read description is still missing. We studied the 5/2 state in a very high quality 2DEG sample with the lowest electron density reported to date, by nearly a factor of two. We demonstrate that large discrepancies between experimentally measured values of the 5/2 energy gap, and theoretical calculations based on the Moore-Read theory, can not be trivially attributed to disorder as has conventionally been assumed. Using a tilted field geometry, we investigated the effect of applying an in-plane magnetic field on the 5/2 state. We observe the 5/2 energy gap to collapse linearly with the in-plane field, whereas the neighbouring 7/3 shows a strong enhancement. The opposite behaviour between the two states is in startling contrast to theory which predicts both gaps should be similarly suppressed. Since the early theoretical foundation in support of the Moore-Read interpretation presumed the two states should behave the same, our experimental finding of opposite behaviour may necessitate a fundamental rethinking of the nature of the 5/2 FQHE. A crucial step towards verifying the Moore-Read description of the ν = 5/2 FQHE will be an unambiguous measurement of its spin state. In an effort to measure the 5/2 spin directly, we implemented a resistively detected nuclear magnetic resonance (RDNMR) technique. I report on our detailed study of the anomalous RDNMR lineshape arUne compr\'ehension compl\`ete de l'effet Hallquantique fractionnaire (EHQF) \`a $\nu = \frac$ continue \`afaire partie des probl\`emes les plus passionnants de la physiquedes semi-conducteurs. On estime g\'en\'eralement que cet \'etat\'electronique unique est d\'ecrit par la fonction d'onde deMoore-Read connue sous le nom de ``Pfaffien'', r\'esultant d'unappariement de type BCS de fermions composites. Cette fonctiond'onde a r\'ecemment suscit\'e un int\'er\^et tout particulier enraison de ses statistiques quantiques non-ab\'eliennes quisous tendent un nouveau paradigme pour le calcul quantique insensibleaux d\'efaillances. En d\'epit de plusieurs avanc\'ees th\'eoriques,il manque toutefois une v\'erification exp\'erimentaled\'efinitive de la description Moore-Read.Nous avons \'etudi\'e l'\'etat $\frac$ dans un gaz\'electronique bidimensionnel de tr\`es haute mobilit\'e ayant laplus faible densit\'e d'\'electrons report\'ee \`a ce jour, cecide par un facteur proche de deux. Nous d\'emontrons que lesimportants \'ecarts entre les valeurs du gap d'\'energie \`a$\frac$ mesur\'ees exp\'erimentalement et les calculsth\'eoriques bas\'es sur la th\'eorie Moore-Read ne peuvent pas\^etre uniquement attribu\'es au d\'esordre, comme cela atraditionnellement \'et\'e propos\'e. En utilisant un champ \`ag\'eom\'etrie inclin\'ee, nous avons \'etudi\'e l'effet del'application d'un champ magn\'etique dans le plan sur l'\'etat$\frac{5}{2}$. Nous observons une d\'ecroissance lin\'eaire du gapd'\'energie \`a $\frac$ en fonction du champ magn\'etiquedans le plan, tandis que le gap d'\'energie d'\'etat fractionnairevoisin $\frac{7}{3}$ exhibe une forte augmentation. Lecomportement oppos\'e des deux \'etats est en contraste avec lath\'eorie qui pr\'evoit que les deux gaps doivent \^etresupprim\'es par l'application d'un champ magn\'etique dans le plan, ce quin\'ecessite un nouvel examen de la nature de l'EHQF$\frac$.Une \'etape cruciale vers la v\'erificati
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Pouliot, Dominique. "Models of binary fluid phase separation in the inertial regime." Thesis, McGill University, 2010. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=94930.
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This thesis presents a theoretical analysis of the late stage kinetics of phase separation in a binary fluid. The infrared (long-wavelength) and long-time properties of the fluid system are studied using the techniques and ideas of the momentum shell renormalization group (RG). The non-linear effective one-fluid description of Sain and Grant is generalized to include memory effects through the construction of two non-Markovian noise correlators, and the full incompressible Navier-Stokes equation (NSE) is considered. The structure of the non-Markovian force-force correlators is justified heuristically, and the exponent of their decaying power law obtained using an energy equation and Novikov's theorem. The main motivations for this study are three-fold. The first goal is to gain a better understanding of how to model such binary fluid systems with a minimal number of key features, through a renormalization group (RG) approach. The second goal is to obtain dynamical information about the droplet power law growth, by finding an interval of allowed values for the exponent. This is achieved through a Reynolds number criterion, Re, which grows as L raised to a scaling exponent calculated with RG, and where L denotes a length scale of observation, in analogy with the Kadanoff blocks of critical dynamics. This in turn leads to an examination of which (first-order phase transition) universality class is associated with this kinetic process. The third goal is to estimate a velocity correlation length scale by means of another output of the RG procedure, namely the scaling exponent z. Results from both models, MI and MII, are in good agreement with data from numerical simulations, and also agree with the exponent 1/3 for the Lifshitz-Slyozov universality class, Model B for first-order phase transitions with a conserved order parameter. A discussion of the scaling of the velocity-velocity correlation is included. Our analysis includes partial hydrodyCette thèse présente une analyse théorique de la cinétique tardive de séparation de phase dans un fluide binaire. On utilise les idées et les techniques du groupe de renormalisation (RG) à enveloppe d'impulsion pour étudier les propriétés du système de fluides, pour des distances et des temps très longs. La description non-linéaire effective à un fluide de Sain et Grant est généralisée, dans le but d'inclure des effets de mémoire, grâce à la construction de deux corrélateurs de bruit non-Markoviens, et l'équation complète de Navier-Stokes (NSE) pour un fluide incompressible est considérée. La structure des corrélateurs non-Markoviens est justifiée de façon heuristique, et l'exposant du facteur de decroissance en puissance de t est obtenu en utilisant une équation d'énergie et le théorème de Novikov. Il y a trois motivations principales à cette étude. Le but premier est d'obtenir une meilleure compréhension de la façon de modéliser de tels systèmes fluides binaires, en utilisant un nombre minimal d'éléments clés, grâce à l'approche du groupe de renormalisation (RG). Le deuxième objectif est d'obtenir de l'information dynamique sur le taux de croissance en puissance de t du rayon R des gouttelettes, à travers le calcul d'un intervalle de valeurs permises pour l'exposant, et le critère de decroissance du nombre de Reynolds , Re, en puissance de L. L'exposant dynamique du nombre de Reynolds résulte du calcul de renormalisation, et L est l'échelle de longueur à laquelle les phénomènes sont observés. Ceci mène à la considération de la classe d'universalité (pour les transitions de phase du premier ordre) associée à ce processus cinétique. Le troisième objectif est d'estimer une échelle de grandeur des vitesses dans le fluide, au moyen d'un autre résultat du processus de renormalisation, l'exposant dynamique z. Les résultats des deux modèles, MI et MII, sont en bon accord avec les donné
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Hagedorn, Till. "Atomic contacts characterized by force and current." Thesis, McGill University, 2011. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=96938.
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Contacts between atoms present the reduction of an electronic system to the smallest scale accessible at the present. Field ion microscopy (FIM) and simultaneous scanning tunneling microscopy (STM) and atomic force microscopy (AFM) were used to characterize atomic contacts between a tungsten (W(111)) STM-tip and a gold (Au(111)) sample. The STM-tip was imaged with atomic resolution using FIM and reconstructed in a three dimensional ball model. Force and current were measured simultaneously while the tip approached an atomically flat Au(111) sample to form a contact.A detailed description of the tip production and characterization using an etching setup that was developed for this thesis is introduced. These tips are characterized by field ion microscopy (FIM). The adaptation of a well known three dimensional ball model reconstruction to the tungsten bcc(111) crystal is described and utilized to reconstruct FIM micrographs of W(111) STM-tips. Using several of such tips, the long-range forces of the tip-sample system were analyzed using recent theoretical models, concluding that van der Waals and electrostatic force contributions were negligible for tips with a radius smaller than r_tip = 15 nm within the precision of the FIM-STM/AFM system that was employed for the experiments.Force and current data for different structural modifications of the W(111)-Au(111) SPM tunneling junction are presented and discussed. Findings include that the W-Au junction in geometries which were examined in this thesis do not exhibit the conductive properties of a one-atom contact.A comparison of different theoretical approaches that treat the correlation of force and current in a STM type tunneling junction is presented and the results of this thesis support a F^2 ~ I_t proportionality for a three-atom tungsten tip contacting an atomically flat Au(111) sample.Un contact entre atomes est la réduction d'un système électronique à la plus petite échelle actuellement accessible. La microscopie à champ ionique (field ion microscopy, FIM) et la microscopie à effet tunnel (scanning tunneling microscopy, STM) simultanément à la microscopie à forces atomiques (atomic force microscopy, AFM) ont été utilisées pour caractériser le contact atomique entre une pointe de tungstène (W(111)), un palpeur pour le STM, et un échantillon d'or (Au(111)). La pointe a été imagée par FIM avec une résolution atomique et reconstruite par un modèle de balle tridimensionnel. La force et le courant ont été mesurés simultanément pendant que la pointe s'approchait de l'échantillon d'or (111) atomiquement plat pour y former un contact.Une description détaillée de la production et de la caractérisation des pointes par l'utilisation d'un système d'attaque électrolytique, développé pour cette thèse, est présentée. L'adaptation de la reconstruction par modèle de balle tridimensionnel au cristal de tungstène ccc(111) est décrite, puis utilisée pour reconstruire les pointes W(111). En utilisant plusieurs de ces pointes, les forces à longue portée du système pointe-échantillon ont été analysées en utilisant de récents modèles théoriques, ce qui a permit de conclure que les contributions des forces de van der Waals et des forces électrostatiques sont négligeables pour des pointes de rayon inférieur à r_pointe = 15 nm, compte tenu de la précision du système FIM-STM/AFM employé pour les expériences.En lien avec les simulations qui prennent en compte les forces d'interaction pointe-échantillon et les propriétés conductrices de la séparation des contacts, des données de force et de courant pour différentes modifications structurelles de la jonction tunnel W(111)-Au(111) sont présentées et analysées. Entre autres choses, il est montré que la jonction W-Au, pour les géométries étudiées dans cette thèse, ne présente pas les propriétés conductrices d'un contact monoatomique.En comparant différentes approches théoriques qui traitent de la corrélation entre la force et le courant dans une jonction tunnel de type STM, les données pour la jonction W(111)-Au(111) sont mises en relation avec ces lois de puissance. Les résultats de cette thèse indique une proportionnalité F^2 ~ I_t pour une pointe de tungstène à 3 atomes faisant contact avec un échantillon d'or (111) atomiquement plat.
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Payette, Christopher. "Transport in weakly coupled vertical double quantum dots: single-particle energy level spectroscopy and hyperfine interaction effects." Thesis, McGill University, 2011. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=96713.
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Performing transport measurements on weakly coupled vertical double quantum dots, we study by magneto-resonant-tunneling spectroscopy, single-particle energy spectra of the constituent dots over a wide energy window. The measured energy spectra are well modeled overall by ideal spectra calculated for elliptical and parabolic in-dot-plane confinement potentials. However, in regions where single-particle energy levels are naively expected to cross, we observe pronounced level anti-crossing behaviour and strong resonant current variations (both enhancement and suppression). Within a coherent tunneling picture, these effects can be attributed to coherent level mixing induced by weak perturbations in the nearly ideal dot confinement potentials. We analyze the energy spectra in detail, and focus on examples of two-, three- and four-level crossings where we observe the suppression of an otherwise strong current resonance, a signature of dark state formation due to destructive interference. The mixing we measure and model at two three-level crossings represents an all-electrical analogue of coherent population trapping. We also explore the limitations of the applicability of the coherent level mixing model and demonstrate in-situ alteration of the coupling between levels.We further examine the electron spin-nuclear spin (hyperfine) interaction. In the familiar two-electron spin blockade regime, on application of an out-of-dot-plane magnetic field, we observe current switching and hysteresis, and a funnel-like structure in the leakage current, all hallmarks of the hyperfine interaction. The measurements bring to light a strong gate voltage dependence, significant device-to-device variations, and an intricate bias voltage history dependence not accounted for in any existing model. Unexpectedly, we also observe signatures of the hyperfine interaction at high bias, well outside the spin blockade regime. We characterize these features and suggest how the hyperfine interaction may play a role at high bias, although the electronic states involved generally can not easily be identified. As a first step toward understanding this new regime, we describe another hysteretic funnel-like structure observed at high bias where the electronic states involved can be identified as two-electron states, so allowing us to postulate a specific mechanism for this funnel.En effectuant des mesures de transport sur des boîtes quantiques doubles faiblement couplées, nous étudions, par la spectroscopie par effet tunnel magnetorésonant, les spectres d'énergie de particule simple dans une grande fenêtre d'énergie. Les spectres d'énergie mesurés sont modélisés par les spectres calculés pour des potentiels de confinement elliptique et parabolique. Cependant, dans les régions où les niveaux d'énergie de particule simple doivent se croiser, nous observons des comportements de croisements évités et des variations du courant résonnant. Dans le cadre de l'effet tunnel cohérent, ces effets peuvent être attribués au mélange cohérent des niveaux induit par les faibles perturbations du confinement. Nous analysons les spectres d'énergie et focalisons sur des exemples de croisements de deux à quatre niveaux où nous observons la suppression d'une résonance de courant qui est une signature de la formation d'un état sombre dû à de l'interférence destructive. Le mélange que nous mesurons et modélisons à deux croisements de trois niveaux représente un piégeage cohérent de population.Nous examinons plus en détail l'interaction hyperfine entre les spins des électrons et des noyaux. Dans le régime du blocage de spin avec deux électrons, lors de l'application d'un champ magnétique hors plan, nous observons un courant intermittent avec de l'hystérèse et une structure en entonnoir dans le courant de fuite, qui sont des aspects marquants de l'interaction hyperfine. Les mesures dévoilent une dépendance sur la tension de grille, des variations d'un dispositif à l'autre et une dépendance sur l'histoire de la tension de biais qui apparaît dans aucun modèle existant. Nous observons également des signatures de l'interaction hyperfine à biais élevé, au-delà du régime du blocage de spin. Nous caractérisons ces aspects et suggérons comment l'interaction hyperfine peut jouer un rôle à biais élevé, quoique les états électroniques impliqués ne puissent pas être identifiés facilement. Comme première étape vers la compréhension de ce nouveau régime, nous décrivons une autre structure hystérétique en entonnoir observée à biais élevé où les états électroniques impliqués peuvent être identifiés comme des états à deux électrons, ce qui nous permet de postuler un mécanisme spécifique pour cet entonnoir.