Dissertations / Theses: 'Two Dimensional Electron Systems (2DES)'

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Rödel, Tobias. "Two-dimensional electron systems in functional oxides studied by photoemission spectroscopy." Thesis, Université Paris-Saclay (ComUE), 2016. http://www.theses.fr/2016SACLS197/document.

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De nombreux oxydes de métaux de transition (TMOs) possèdent des propriétés physiques complexes (ferroélectricité, magnétisme, supraconductivité à haute Tc ou magnétorésistance colossale). Les différents degrés de liberté (le réseau, la charge, le spin ou l'ordre orbitalaire) interagissent pour donner des phases différentes, très proches en énergie, qui vont former une grande variété d'états fondamentaux accessibles. La possibilité de fabriquer des hétérostructures de TMOs a encore accru la complexité de ces systèmes, de nouveaux phénomènes apparaissant aux interfaces. Un exemple typique est le gaz d'électrons bidimensionnel (2DEG) créé à l'interface entre deux oxydes isolants, LaAlO3 et SrTiO3, qui montre une transition métal-isolant, du magnétisme ou de la supraconductivité (contrôlée par une tension de grille). Le point de départ de cette thèse a été la découverte d'un 2DEG similaire à la surface nue de SrTiO3 fracturée sous vide, rendant possible l'étude de sa structure électronique par photoémission angulaire.Dans cette thèse, l'étude de surfaces préparées, plutôt que de petites facettes fracturées, a permis l'obtention de données spectroscopiques possédant des largeurs de raie proches des valeurs intrinsèques. Il est alors possible d'étudier les effets à N corps comme la renormalisation de la self-énergie due à l'interaction électron-phonon.Ces recherches sur la structure électronique du 2DEG à la surface de SrTiO3 ont pris un tour nouveau lorsqu'une texture de spin complexe y a été mesurée par photoémission résolue en spin. Nous présentons des résultats qui contredisent ces conclusions et nous discutons des raisons pouvant expliquer ce désaccord.Une des motivations de cette thèse était de savoir si la structure électronique et les propriétés du 2DEG pouvaient être contrôlées. L'étude du 2DEG sur des surfaces (110) et (111) de SrTiO3 révèle que sa structure de bandes (ordre orbitalaire, symétrie de la surface de Fermi, masses effectives) peut être ajustée en confinant les électrons sur des surfaces de différentes orientations du même matériau.Un succès majeure est la mise en évidence de 2DEGs à la surface de nombreux autres TMOs (TiO2-anatase, CaTiO3, BaTiO3) ou d'oxydes plus simples utilisés dans les applications (ZnO). Dans tous ces oxydes, nous avons identifié les lacunes en oxygène comme étant à l'origine de la création des 2DEGs.Dans l'anatase, ou d'autres TMOs en configuration électronique initiale d0, les lacunes en oxygène produisent à la fois des électrons localisés ou itinérants (le 2DEG). Il peut être subtile de prévoir quel est le cas est le plus favorable énergétiquement comme le démontre l'étude de deux polymorphes de TiO2, anatase et rutile. Dans CaTiO3, l’octaèdre formé par les atomes d'oxygène autour du Ti est incliné. Cette rupture de symétrie provoque un mélange des orbitales d et modifie le 2DEG. Dans BaTiO3, la création d'un 2DEG entraîne la coexistence de deux phénomènes normalement incompatibles, la ferroélectricité et la métallicité, dans deux zones spatialement distinctes du même matériau. Ce travail démontre qu'un 2DEG existe aussi à la surface de ZnO qui est, contrairement aux oxydes à base de Ti, plutôt un semiconducteur conventionnel, le caractère des orbitales pour les électrons itinérants étant alors de type s et non de type d.Le principal résultat est la mise au point d'une méthode simple et versatile pour la création de 2DEGs en évaporant de l'aluminium sur des surfaces d'oxydes. Une réaction d'oxydo-réduction entre le métal et l'oxyde permet de créer un 2DEG à l'interface entre le métal oxydé et l'oxyde réduit. Dans cette thèse, les 2DEGs ont été étudiés uniquement par photoémission sous ultra-vide. Cette méthode ouvre la possibilité d'étudier ces 2DEGs dans des conditions de pression ambiante en utilisant, par exemple, des techniques de transport, un pas important vers la production de masse et à bas coûts de 2DEGs dans les oxydes pour de futures applications
Many transition metal oxides (TMOs) show complex physics, ranging from ferroelectricity to magnetism, high-Tc superconductivity and colossal magnetoresistance. The existence of a variety of ground states often occurs as different degrees of freedom (e.g. lattice, charge, spin, orbital) interact to form different competing phases which are quite similar in energy. The capability to epitaxially grow heterostructures of TMOs increased the complexity even more as new phenomena can emerge at the interface. One typical example is the two-dimensional electron system (2DES) at the interface of two insulating oxides, namely LaAlO3/SrTiO3, which shows metal-to-insulator transitions, magnetism or gate-tunable superconductivity. The origin of this thesis was the discovery of a similar 2DES at the bare surface of SrTiO3 fractured in vacuum, making it possible to study its electronic structure by angle-resolved photoemission spectroscopy (ARPES).In this thesis, the study of well-prepared surfaces, instead of small fractured facets, results in spectroscopic data showing line widths approaching the intrinsic value. This approach allows a detailed analysis of many-body phenomena like the renormalization of the self-energy due to electron-phonon interaction.Additionally, the understanding of the electronic structure of the 2DES at the surface of SrTiO3(001) was given an additional turn by the surprising discovery of a complex spin texture measured by spin-ARPES. In this thesis data is presented which contradicts these conclusions and discusses possible reasons for the discrepancy.One major motivation of this thesis was the question if and how the electronic structure and the properties of the 2DES can be changed or controlled. In this context, the study of 2DESs at (110) and (111) surface revealed that the electronic band structure of the 2DES (orbital ordering, symmetry of the Fermi surface, effective masses) can be tuned by confining the electrons at different surface orientations of the same material, namely SrTiO3.A major achievement of this thesis is the generalization of the existence of a 2DES in SrTiO3 to many other surfaces and interfaces of TMOs (TiO2 anatase, CaTiO3, BaTiO3) and even simpler oxides already used in modern applications (ZnO). In all these oxides, we identify oxygen vacancies as the origin for the creation of the 2DESs.In anatase and other doped d0 TMOs, both localized and itinerant electrons (2DES) can exist due to oxygen vacancies. Which of the two cases is energetically favorable depends on subtle differences as demonstrated by studying two polymorphs of the same material (anatase and rutile).In CaTiO3, the oxygen octahedron around the Ti ion is slightly tilted. This symmetry breaking results in the mixing of different d-orbitals demonstrating again why and how the electronic structure of the 2DES can be altered.In BaTiO3, the creation of a 2DES results in the coexistence of the two, usually mutual exclusive, phenomena of ferroelectricity and metallicity in the same material by spatially separating the two.Moreover, this work demonstrates that the 2DES also exists in ZnO which is - compared to the Ti-based oxides - rather a conventional semiconductor as the orbital character of the itinerant electrons is of s and not d-type.The main result of this thesis is the demonstration of a simple and versatile technique for the creation of 2DESs by evaporating Al on oxide surfaces. A redox reaction between metal and oxide results in a 2DES at the interface of the oxidized metal and the reduced oxide. In this thesis the study of such interfacial 2DESs was limited to photoemission studies in ultra high vacuum. However, this technique opens up the possibility to study 2DESs in functional oxides in ambient conditions by e.g. transport techniques, and might be an important step towards cost-efficient mass production of 2DESs in oxides for future applications

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Yoon, Hosang. "Two-Dimensional Plasmonics in Massive and Massless Electron Gases." Thesis, Harvard University, 2014. http://nrs.harvard.edu/urn-3:HUL.InstRepos:13070026.

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Plasmonic waves in solid-state are caused by collective oscillation of mobile charges inside or at the surface of conductors. In particular, surface plasmonic waves propagating at the skin of metals have recently attracted interest, as they reduce the wavelength of electromagnetic waves coupled to them by up to ~10 times, allowing one to create miniaturized wave devices at optical frequencies. In contrast, plasmonic waves on two-dimensional (2D) conductors appear at much lower infrared and THz-GHz frequencies, near or in the electronics regime, and can achieve far stronger wavelength reduction factor reaching well above 100. In this thesis, we study the unique machinery of 2D plasmonic waves behind this ultra-subwavelength confinement and explore how it can be used to create various interesting devices. To this end, we first develop a physically intuitive theoretical formulation of 2D plasmonic waves, whose two main components---the Coulomb restoration force and inertia of the collectively oscillating charges---are combined into a transmission-line-like model. We then use this formulation to create various ultra-subwavelength 2D plasmonic devices. For the 2D conductor, we first choose GaAs/AlGaAs heterostructure---a 2D electron gas consisting of massive (m*>0) electrons---demonstrating plasmonic bandgap crystals, interferometers, and negatively refracting metamaterials. We then examine a 2D plasmonic device based on graphene, a 2D electron gas consisting of effectively massless (m*=0) electrons. We theoretically show and experimentally demonstrate that the massless electrons in graphene can surprisingly exhibit a collective mass when subjected to a collective excitation, providing the inertia that is essential for the propagation of 2D plasmonic waves. Lastly, we theoretically investigate the thermal current fluctuation behaviors in massive and massless electron gases. While seemingly unrelated on first sight, we show that the thermal current fluctuation is actually intimately linked to the collective mass of the massive or massless electron gas. Thus, we show that the thermal current fluctuation behaviors can also be described by the same theoretical framework introduced earlier, suggesting a possibility to design new concept devices and experiments based on this linkage.
Engineering and Applied Sciences

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Nahm, In Hyun. "Two dimensional disordered electron systems." Thesis, University of Southampton, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.330179.

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Madhavi, S. "Carrier Mobility And High Field Transport in Modulation Doped p-Type Ge/Si1-xGex And n-Type Si/Si1-xGex Heterostructures." Thesis, Indian Institute of Science, 2000. https://etd.iisc.ac.in/handle/2005/294.

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Modulation doped heterostructures have revolutionized the operation of field effect devices by increasing the speed of operation. One of the factors that affects the speed of operation of these devices is the mobility of the carriers, which is intrinsic to the material used. Mobility of electrons in silicon based devices has improved drastically over the years, reaching as high as 50.000cm2/Vs at 4.2K and 2600cm2/Vs at room temperature. However, the mobility of holes in p-type silicon devices still remains comparatively lesser than the electron mobility because of large effective masses and complicated valence band structure involved. Germanium is known to have the largest hole mobility of all the known semiconductors and is considered most suitable to fabricate high speed p-type devices. Moreover, it is also possible to integrate germanium and its alloy (Si1_zGex ) into the existing silicon technology. With the use of sophisticated growth techniques it has been possible to grow epitaxial layers of silicon and germanium on Si1_zGex alloy layers grown on silicon substrates. In tills thesis we investigate in detail the electrical properties of p-type germanium and n-type silicon thin films grown by these techniques. It is important to do a comparative study of transport in these two systems not only to understand the physics involved but also to study their compatibility in complementary field effect devices (cMODFET). The studies reported in this thesis lay emphasis both on the low and high field transport properties of these systems. We report experimental data for the maximum room temperature mobility of holes achieved m germanium thin films grown on Si1_zGex layers that is comparable to the mobility of electrons in silicon films. We also report experiments performed to study the high field degradation of carrier mobility due to "carrier heating" in these systems. We also report studies on the effect of lattice heating on mobility of carriers as a function of applied electric field. To understand the physics behind the observed phenomenon, we model our data based on the existing theories for low and high field transport. We report complete numerical calculations based on these theories to explain the observed qualitative difference in the transport properties of p-type germanium and ii-type silicon systems. The consistency between the experimental data and theoretical modeling reported in this work is very satisfactory.

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Madhavi, S. "Carrier Mobility And High Field Transport in Modulation Doped p-Type Ge/Si1-xGex And n-Type Si/Si1-xGex Heterostructures." Thesis, Indian Institute of Science, 2000. http://hdl.handle.net/2005/294.

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Abstract:

Modulation doped heterostructures have revolutionized the operation of field effect devices by increasing the speed of operation. One of the factors that affects the speed of operation of these devices is the mobility of the carriers, which is intrinsic to the material used. Mobility of electrons in silicon based devices has improved drastically over the years, reaching as high as 50.000cm2/Vs at 4.2K and 2600cm2/Vs at room temperature. However, the mobility of holes in p-type silicon devices still remains comparatively lesser than the electron mobility because of large effective masses and complicated valence band structure involved. Germanium is known to have the largest hole mobility of all the known semiconductors and is considered most suitable to fabricate high speed p-type devices. Moreover, it is also possible to integrate germanium and its alloy (Si1_zGex ) into the existing silicon technology. With the use of sophisticated growth techniques it has been possible to grow epitaxial layers of silicon and germanium on Si1_zGex alloy layers grown on silicon substrates. In tills thesis we investigate in detail the electrical properties of p-type germanium and n-type silicon thin films grown by these techniques. It is important to do a comparative study of transport in these two systems not only to understand the physics involved but also to study their compatibility in complementary field effect devices (cMODFET). The studies reported in this thesis lay emphasis both on the low and high field transport properties of these systems. We report experimental data for the maximum room temperature mobility of holes achieved m germanium thin films grown on Si1_zGex layers that is comparable to the mobility of electrons in silicon films. We also report experiments performed to study the high field degradation of carrier mobility due to "carrier heating" in these systems. We also report studies on the effect of lattice heating on mobility of carriers as a function of applied electric field. To understand the physics behind the observed phenomenon, we model our data based on the existing theories for low and high field transport. We report complete numerical calculations based on these theories to explain the observed qualitative difference in the transport properties of p-type germanium and ii-type silicon systems. The consistency between the experimental data and theoretical modeling reported in this work is very satisfactory.

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Oxley, John Paul. "Thermopower in two dimensional electron systems." Thesis, University of Nottingham, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.293657.

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Siegert, Christoph. "Disorder effects in two dimensional electron systems." Thesis, University of Cambridge, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.612312.

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Peck, Andrew John. "Lateral tunnelling in two-dimensional electron systems." Thesis, University of Bath, 1994. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.385345.

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Yeung, Yan Mui Kitty. "Engineering Plasmonic Waves in Two-Dimensional Electron Systems." Thesis, Harvard University, 2015. http://nrs.harvard.edu/urn-3:HUL.InstRepos:17467363.

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Plasmonic waves are waves of mobile charge carriers caused by their collective oscillations. They can be excited in solid-state conducting materials and behave distinctively in different numbers of dimensions. With fabrication technologies available for solid-state materials, one can functionalize the dimensional properties by engineering the boundaries and interfaces of the plasmonic wave medium. For instance, plasmonic waves in two-dimensional (2D) conductors, such as semiconductor heterojunction and graphene, exhibit strong subwavelength confinement – with a wavelength about a factor of 100 below the electromagnetic wavelength at the same frequency. Hence, 2D plasmonic devices can be constructed below the diffraction limit of light. To utilize this ultra-subwavelength confinement is the main motivation of this thesis. This thesis establishes the machinery behind the unique behaviors of 2D plasmons, and compares them to plasmons in higher dimensions, namely plasma oscillations in bulk materials and surface plasmons on conducting-insulating interfaces. The Coulomb restoring force and mobile charge carrier inertia causing the collective oscillations are formulated into a transmission-line model. This formulation is used to engineer ultra-subwavelength plasmonic circuits in gigahertz integrated electronics and terahertz metamaterials. As one of the demonstration platforms, we use GaAs/AlGaAs 2D electron gas. Amongst a variety of devices, the thesis focuses on an on-chip solid-state 2D plasmonic Mach-Zehnder interferometer operating at microwave frequencies. The gated 2D plasmonic waves achieve a velocity of ~c/300 (c: free-space speed of light). Due to this ultra-subwavelength confinement, the resolution of the 2D plasmonic interferometer is two orders of magnitude higher than that of its electromagnetic counterpart at a given frequency. Another material we use, which hosts mobile charge carriers in 2D, is graphene. We fabricate metamaterials in the form of graphene plasmonic crystals in a continuous graphene sheet with periodic structural perturbations. Plasmonic bands in the far-infrared are formed and excited via symmetry-based selection rules, in a manner akin to photonic crystals. The plasmonic bands can be engineered by manipulating the charge carrier concentration, the dimensions of the periodic lattice, the shape of the perturbation and the lattice symmetry. These demonstrations may generate new avenues for a wealth of subwavelength graphene plasmonic devices, such as band gap filters, modulators and switches.
Engineering and Applied Sciences - Applied Physics

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Li, Hongtao. "Electron-electron correlations and lattice frustration in quasi-two-dimensional systems." Diss., The University of Arizona, 2011. http://hdl.handle.net/10150/202769.

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Strong electron-electron correlations and lattice frustration are two physical interactions that pose serious challenges to condensed matter physics. A variety of exotic physical phenomena, for example, charge ordering, spin liquid, and unconventional superconductivity, are believed to arise from the interplay of the two interactions. In this dissertation, I examine two families of systems which exhibit both electron-electron correlations and lattice frustration – charge transfer solids and layered cobaltates. The half-filled band Hubbard model on the triangular lattice has been proposed by mean-field theories as the minimal model for the superconductivity in the charge transfer solids. In the first part of this dissertation, by using exact calculations, I prove the absence of superconductivity in this model. This result calls for a new theoretical approach to describe the rich physics in charge transfer solids. In the second part of this dissertation, I study charge transfer solids by focusing on its real bandfilling ¼. I show that a new kind of insulating phase, paired electron crystal, emerges from antiferromagnetism as the frustration is increased. The paired electron crystal state can explain the various insulating states adjacent to the superconducting state, thus provides a new avenue towards the understanding of the unconventional superconductivity in charge transfer solids and other ¼ filled systems. In the third part of this dissertation, I investigate the carrier concentration-dependent electronic behavior in layered cobaltates. I provide a natural yet simple explanation for this behavior. I show that it can be described within correlated-electron Hamiltonians with finite on-site and significant nearest neighbor hole-hole Coulomb repulsions. I also point out the similarities between organic charge transfer solids and layered cobaltates, which may involve superconductivity.

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Bao, Yunjuan. "Resonant spin Hall effect in two-dimensional electron systems." Click to view the E-thesis via HKUTO, 2005. http://sunzi.lib.hku.hk/hkuto/record/B34627819.

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Castleton, I. M. "Electron transport in coupled one- and two-dimensional systems." Thesis, University of Cambridge, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.597365.

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In this dissertation, transport measurements are presented on the coupling between two closely separated two-dimensional electron gases (2DEGs) formed within GaAs/AlGaAs heterostructures. The experiments fall into three main areas. Firstly, the electrical properties of a new type of switching transistor are investigated. A 2DEG is formed at the inverted and non-inverted heterojunction of a wide GaAs/AlGaAs quantum well. With the use of front and back gates, charge is moved from one 2DEG to the other in a manner similar to that proposed theoretically in the Velocity Modulated Transistor (VMT) device concept. In the VMT, switching is obtained through a mobility modulation rather than through a more conventional carrier density modulation, thereby minimising capacitive time delays. By adjusting the grown-in carrier density and mobility of each 2DEG, successful VMT operation is demonstrated, and a mobility modulation of a factor of 125 is achieved. In the second part, the resistance resonance in strongly coupled double 2DEGs is investigated. The size of the resistance resonance in three samples is measured as a function of the 2DEG mobility ratio, and the results are compared with predictions made by classical and quantum transport theories. Quenching of the resistance resonance by an in-plane magnetic field is observed, and the low- and high-field magnetoresistance are explained by considering the field-induced distortions of the Fermi surface. Finally, the transport properties of structures containing double quasi-1D wires are studied. The wires are formed by laterally confining two 2DEGs using a single Schottky split-gate. Ballistic quantisation in units of 2e²/h and 4e²/h is observed in widely spaced, non-interacting wires, while closely spaced wires are used to investigate the effects of wavefunction hybridisation in reduced dimensionality. Variations in the screening of a single quasi-1D wire are revealed by compressibility measurements.

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Bao, Yunjuan, and 暴云娟. "Resonant spin Hall effect in two-dimensional electron systems." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2005. http://hub.hku.hk/bib/B34627819.

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Baenninger, Matthias. "Interaction effects and disorder in two-dimensional electron systems." Thesis, University of Cambridge, 2008. https://www.repository.cam.ac.uk/handle/1810/280142.

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This thesis reports an experimental study of transport on a mesoscopic lengthscale in the localised regime of two-dimensional electron systems (2DES) with varying disorder. Devices with dimensions of a few microns were fabricated from modulation doped GaAs/ AlGaAs heterostructures, where the strength of disorder was tuned by changing the width of the undoped spacer layer separating the 2DES from the charged dopants, which are the main source of disorder in these systems. The main motivation of the experiments was to study the interplay between electron-electron interactions and short-range disorder at low electron densities, while avoiding the impact of long-range charge inhomogeneities that are usually present in this regime. Indeed, several new observations have been achieved with this approach: Chapter 5 reports an universal behaviour of hopping magnetoresistance, with evidence of the average hopping distance being equal to the average electronelectron separation, and a quantisation of the hopping prefactor in units of the quantum of resistance h/e2. Chapter 6 discusses the temperature dependence of resistance. The main result is an apparent temperature driven metal-to-insulator transition with a crossover from activated transport at high temperatures to metallic transport at low temperatures. This observation persists to resistivities of several hundred times the quantum of resistance. In chapter 7 a new kind of resistance oscillations is reported, which appear as a function of electron density when a strong perpendicular magnetic field is applied. A strongly amplified pick-up of the Shubnikov-de Haas oscillation and a modification of t he quantum Hall effect are reported in chapter 8. Furthermore, a new technique for measuring the electron density in mesoscopic 2DES is presented in chapter 4.

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Subbiah, Suresh. "Macroscopic dressing of electron states in a two dimensional ballistic electron waveguide /." Full text (PDF) from UMI/Dissertation Abstracts International, 2000. http://wwwlib.umi.com/cr/utexas/fullcit?p3004381.

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Shaukat, Muhammad Usman. "Microwave and terahertz studies of the electron dynamics in two-dimensional electron systems." Thesis, University of Leeds, 2012. http://etheses.whiterose.ac.uk/3410/.

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This thesis reports the design and implementation of different experimental systems to study electron-dynamics in two-dimensional electron gases (2DEGs) at microwave and terahertz (THz) frequencies. Both time and frequency domain techniques have been used to study the transport properties of electrons in GaAs/Al- GaAs heterostructures. First of all a free-space time-domain spectroscopy (TDS) polarisation sensitive detection system capable of measuring the full polarization state of a propagating THz electric field has been demonstrated. Polarization- sensitive systems provide additional information on material characterization that is inaccessible through conventional techniques. The polarization sensitivity of the THz-TDS system has been evaluated using a 0.5mm thick LiNbO3 sample. The ability to measure the rotation in polarisation caused by a sample allows measurement of direction-dependent material properties, such as the electrical conductivity of low-dimensional semiconductors displaying the quantum Hall effect, or the di-electric permitivity in anisotropic material systems. The newly designed system has been used to evaluate the magneto-conductivity of a 2DEG up to 1.4 THz and 6T. Two separate continuous-wave polarisation-sensitive free-space systems, operating at microwave and THz frequencies, have also been demonstrated. Both systems have been used to study the magneto-conductivity of a 2DEG at microwave (75{110 GHz) and THz (2.6{3.2 THz) frequencies up to a magnetic field of 8T. The continuous-wave microwave system has also been used to study the phenomenon of edge magneto-plasmons in a 2DEG. The work presented in this thesis provides a valuable groundwork to study the electron-dynamics in 2DEGs over a wide range of frequencies (both microwave and THz) and for a wide range of experimental conditions.

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Nikolaev, Valentin. "Many-particle correlations in quasi-two-dimensional electron-hole systems." Thesis, University of Exeter, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.248096.

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Li, Lijun. "Transport properties of single and double two dimensional electron systems." Thesis, University of Exeter, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.269749.

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Tyson, Robin Edward. "Far-infrared optical studies of low-dimensional electron systems." Thesis, University of Cambridge, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.338068.

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Renken, Volker. "Electron confinement and quantum well states in two-dimensional magnetic systems." [S.l.] : [s.n.], 2007. http://deposit.ddb.de/cgi-bin/dokserv?idn=985573546.

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Klironomos, Filippos. "Tunneling between two dimensional electron systems in a high magnetic field and crystalline phases of a two dimensional electron system in a magnetic field." [Gainesville, Fla.] : University of Florida, 2005. http://purl.fcla.edu/fcla/etd/UFE0009801.

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Raith, Martin. "Orbital effects of transverse magnetic fields in quasi two-dimensional electron systems." kostenfrei, 2009. http://epub.uni-regensburg.de/8285/.

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Howell, D. F. "Studies of two-dimensional electron systems in semiconductors at high magnetic fields." Thesis, University of Oxford, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.253390.

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Richards, David Robert. "The electronic structure and optical properties of GaAs two-dimensional electron systems." Thesis, University of Cambridge, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.359697.

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Caldwell, Joshua D. "Investigation of electron-nuclear spin interactions in two-dimensional electron systems via magnetoresistively detected magnetic resonance." [Gainesville, Fla.] : University of Florida, 2004. http://purl.fcla.edu/fcla/etd/UFE0008397.

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Galaktionov, Evgeniy A. "Transport and thermodynamic properties of low-density two-dimensional electron and hole systems." Thesis, University of Exeter, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.439126.

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Jones, Christopher Lee. "Magnetisation and optical studies of two-dimensional electron systems in high magnetic fields." Thesis, University of Exeter, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.294472.

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Soumyanarayanan, Anjan. "A nanoscale probe of the quasiparticle band structure for two dimensional electron systems." Thesis, Massachusetts Institute of Technology, 2013. http://hdl.handle.net/1721.1/83821.

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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2013.
Page 138 blank. Cataloged from PDF version of thesis.
Includes bibliographical references (pages 121-137).
The advent of a broad class of two-dimensional (2D) electronic materials has provided avenues to create and study designer electronic quantum phases. The coexistence of superconductivity, magnetism, density waves, and other ordered phases on the surfaces and interfaces of these 2D materials are governed by interactions which can be experimentally tuned with increasing precision. This motivates the need to develop spectroscopic probes that are sensitive to these tuning parameters, with the objective of studying the electronic properties and emergence of order in these materials. In the first part of this thesis, we report on spectroscopic studies of the topological semimetal antimony (Sb). Our simultaneous observation of Landau quantization and quasiparticle interference phenomena on this material enables their quantitative reconciliation - after two decades of their study on various materials. We use these observations to establish momentum-resolved scanning tunneling microscopy (MR-STM) as a robust nanoscale band structure probe, and reconstruct the multi-component dispersion of Sb(111) surface states. We quantify surface state parameters relevant to spintronics applications, and clarify the relationship between bulk conductivity and surface state robustness. At low momentum, we find a crossover in the single particle behavior from massless Dirac to massive Rashba character - a unique signature of topological surface states. In the second part of this thesis, we report on the spectroscopic study of charge density wave (CDW) order in the dichalcogenide 2H-NbSe2 - a model system for understanding the interplay of coexisting CDW and superconducting phases. We detail the observation of a previously unknown unidirectional (stripe) CDW smoothly interfacing with the familiar triangular CDW on this material. Our low temperature measurements rule out thermal fluctuations and point to local strain as the tuning parameter for this quantum phase transition. The distinct wavelengths and tunneling spectra of the two CDWs, in conjunction with band structure calculations, enable us to resolve two longstanding debates about the anomalous spectroscopic gap and the role of Fermi surface nesting in the CDW phase of NbSe2. Our observations motivate further spectroscopic studies of the phase evolution of the CDW, and of NbSe 2 as a prototypical strong coupling density wave system in the vicinity of a quantum critical point.
by Anjan Soumyanarayanan.
Ph.D.

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Elbahrawy, Mohammed. "High field electron magnetic resonance in complex correlated spin systems." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2010. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-39380.

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In this thesis we used ESR to investigate magnetic properties of low D vandium and copper oxides in which small quantum spins are arranged in 1D chains and 2D layers. The thesis covers five different low dimensional spin systems. They turned out to be experimental reliazation of some of the most intersiting theoritical models in the field of quantum magnetism.

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Ramanayaka, Aruna N. "Magnetotransport in Two Dimensional Electron Systems Under Microwave Excitation and in Highly Oriented Pyrolytic Graphite." Digital Archive @ GSU, 2012. http://digitalarchive.gsu.edu/phy_astr_diss/54.

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This thesis consists of two parts. The first part considers the effect of microwave radiation on magnetotransport in high quality GaAs/AlGaAs heterostructure two dimensional electron systems. The effect of microwave (MW) radiation on electron temperature was studied by investigating the amplitude of the Shubnikov de Haas (SdH) oscillations in a regime where the cyclotron frequency $\omega_{c}$ and the MW angular frequency $\omega$ satisfy $2\omega \leq \omega_{c} \leq 3.5\omega$. The results indicate negligible electron heating under modest MW photoexcitation, in agreement with theoretical predictions. Next, the effect of the polarization direction of the linearly polarized MWs on the MW induced magnetoresistance oscillation amplitude was investigated. The results demonstrate the first indications of polarization dependence of MW induced magnetoresistance oscillations. In the second part, experiments on the magnetotransport of three dimensional highly oriented pyrolytic graphite (HOPG) reveal a non-zero Berry phase for HOPG. Furthermore, a novel phase relation between oscillatory magneto- and Hall- resistances was discovered from the studies of the HOPG specimen.

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Brener, Sergej. "Coulomb Drag and Jahn-Teller effect in two-dimensional electron systems in strong magnetic fields." [S.l. : s.n.], 2006. http://nbn-resolving.de/urn:nbn:de:bsz:93-opus-26345.

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Yang, Chunlei. "Studies of the spintronic systems of ferromagnetic GaMnAs and non-magnetic InGaAs/InAlAs two dimensional electron gas /." View abstract or full-text, 2005. http://library.ust.hk/cgi/db/thesis.pl?PHYS%202005%20YANG.

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Kerridge, Gregg Charles. "Magneto-optical investigations of two-dimensional electron systems in GaAs-Alâ†xGaâ†1â†-â†xAs single heterojunctions." Thesis, University of Exeter, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.286493.

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Tobita, Motoi. "Development of efficient electron correlation methods for one- and two-dimensional extended systems and their applications." [Gainesville, Fla.] : University of Florida, 2002. http://purl.fcla.edu/fcla/etd/UFE1000168.

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Thesis (Ph. D.)--University of Florida, 2002.
Title from title page of source document. Document formatted into pages; contains x, 109 p.; also contains graphics. Includes vita. Includes bibliographical references.

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Zhang, Yao. "Experimental Measurements by Antilocalization of the Interactions between Two-Dimensional Electron Systems and Magnetic Surface Species." Diss., Virginia Tech, 2014. http://hdl.handle.net/10919/49020.

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Low-temperature weak-localization (WL) and antilocalization (AL) magnetotransport measurements are sensitive to electron interference, and thus can be used as a probe of quantum states. The spin-dependent interactions between controllable surface magnetism and itinerant electrons in a non-magnetic host provide insight for spin-based technologies, magnetic data storage and quantum information processing. This dissertation studies two different host systems, an In$_{0.53}$Ga$_{0.47}$As quantum well at a distance from the surface of a heterostructure, and an accumulation layer on an InAs surface. Both the systems are two-dimensional electron systems (2DESs), and possess prominent Rashba spin-orbit interaction caused by structural inversion asymmetry, which meets the prerequisites for AL. The surface local moments influence the surrounding electrons in two ways, increasing their spin-orbit scattering, and inducing magnetic spin-flip scattering, which carries information about magnetic interactions. The two effects modify the AL signals in opposing directions: the spin-flip scattering of electrons shrinks the signal, and requires a close proximity to the species, whereas the increase of spin-orbit scattering broadens and increases the signal. Accordingly, we only observe an increase in spin-orbit scattering in the study of the interactions between ferromagnetic Co$_{0.6}$Fe$_{0.4}$ nanopillars and the relatively distant InGaAs quantum well. With these CoFe nanopillars, a decrease in spin decoherence time is observed, attributed to the spatially varying magnetic field from the local moments. A good agreement between the data and a theoretical calculation suggests that the CoFe nanopillars also generate an appreciable average magnetic field normal to the surface, of value $\sim$ 35 G. We also performed a series of comparative AL measurements to experimentally investigate the interactions and spin-exchange between InAs surface accumulation electrons and local magnetic moments of rare earth ions Sm$^{3+}$, Gd$^{3+}$, Ho$^{3+}$, of transition metal ions Ni$^{2+}$, Co$^{2+}$, and Fe$^{3+}$, and of Ni$^{2+}$-, Co$^{2+}$-, and Fe$^{3+}$-phthalocyanines deposited on the surface. The deposited species generate magnetic scattering with magnitude dependent on their electron configurations and effective moments. Particularly for Fe$^{3+}$, the significant spin-flip scattering due to the outermost 3d shell and the fairly high magnetic moments modifies the AL signal into a WL signal. Experiments indicate a temperature-independent magnetic spin-flip scattering for most of the species except for Ho$^{3+}$ and Co$^{2+}$. Ho$^{3+}$ yields electron spin-flip rates proportional to the square root of temperature, resulting from transitions between closely spaced energy levels of spin-orbit multiplets. In the case of Co$^{2+}$, either a spin crossover or a spin-glass system forms, and hence spin-flip rates transit between two saturation regions as temperature varies. Concerning the spin-orbit scattering rate, we observe an increase for all the species, and the increase is correlated with the effective electric fields produced by the species. In both 2DESs, the inelastic time is inversely proportional to temperature, consistent with phase decoherence via the Nyquist mechanism. Our method provides a controlled way to probe the quantum spin interactions of 2DESs, either in a quantum well, or on the surface of InAs.
Ph. D.

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Dai, Ji. "Low-dimensional electron systems studied by angle- and spin-resolved photoemission spectroscopy." Thesis, Université Paris-Saclay (ComUE), 2019. http://www.theses.fr/2019SACLS345.

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Les matériaux dans lesquels des interactions à plusieurs particules, un confinement de faible dimension et/ou un fort couplage spin-orbite sont présents témoignent d’une grande variété de phénomènes, mais sont encore mal compris. Des informations essentielles sur l’origine de tels phénomènes peuvent être obtenues en mesurant leur structure électronique. Cette thèse présente une étude expérimentale de la structure électronique de matériaux de faible dimension et/ou fortement corrélés présentant un intérêt fondamental actuel, en utilisant la spectroscopie par photoémission résolue en angle et en spin (ARPES et SARPES).Dans la partie introductive, je présente mon travail sur deux exemples de type "livre de texte", mais innovants, montrant comment les interactions affectent la structure de bande d'un matériau: le couplage des électrons avec des phonons dans une distribution de Debye dans un système électronique à deux dimensions (2DES) dans ZnO, semi-conducteur à oxyde à bande interdite large utilisé dans les applications photovoltaïques, et le dédoublement induit par un fort couplage spin-orbite (SOC) dans la bande de valence du ZnTe, un autre semi-conducteur important utilisé dans les dispositifs optoélectroniques. Ensuite, dans la suite de cette thèse, je discute de mes résultats originaux dans trois systèmes différents de basse dimensionnalité et d'intérêt actuel en recherche : 1.La réalisation d'un 2DES à la surface (110) de SnO₂, le premier du genre dans une structure rutile. L'ajustabilité de la densité de ses porteurs au moyen de la température ou du dépôt d'Eu, et la robustesse vis-à-vis les reconstructions de surface et l'exposition aux conditions ambiantes rendent ce 2DES prometteur pour les applications. Au moyen d'une simple réaction redox à la surface, ces travaux ont prouvé que les lacunes en oxygène pouvaient doper la bande de conduction à la surface de SnO₂, résolvant ainsi un problème longtemps débattu concernant le rôle desdites lacunes dans le dopage de type n dans SnO₂. 2.L'étude des états de surface topologiques dans M₂Te₂X (avec M = Hf, Zr ou Ti; et X = P ou As), une nouvelle famille de métaux topologiques en trois dimensions, provenant du SOC et étant protégés par la symétrie du renversement du temps. Leur structure électronique et leur texture de spin, étudiées par ARPES et SARPES, révèlent la présence de fermions de Dirac sans masse donnant naissance à des arcs de nœuds de Dirac. 3.L'étude du matériau YbNi₄P₂ à fermions lourds quasi unidimensionnel, qui présente une transition de phase quantique de second ordre d’une phase ferromagnétique à une phase paramagnétique de liquide de Fermi lors de la substitution partielle du phosphore par l'arséniure. Une telle transition ne devrait se produire que dans les systèmes zéro ou unidimensionnels, mais la mesure directe de la structure électronique des matériaux ferromagnétiques quantiques critiques faisait jusqu'à présent défaut. Grâce à une préparation et nettoyage méticuleux in situ de la surface des monocristaux YbNi₄P₂, qui sont impossibles à cliver, leur structure électronique a été mesurée avec succès au moyen de l'ARPES, dévoilant ainsi le caractère quasi-1D, nécessaire à la compréhension de la criticité quantique ferromagnétique, dans YbNi₄P₂. Le protocole utilisé pour rendre ce matériau accessible à l'ARPES peut être facilement généralisé à d'autres matériaux exotiques dépourvus de plan de clivage
Materials in which many-body interactions, low-dimensional confinement, and/or strong spin-orbit coupling are present show a rich variety of phenomena, but are still poorly understood. Essential information about the origin of such phenomena can be obtained by measuring their electronic structure. This thesis presents an experimental study of the electronic structure of some low-dimensional and/or strongly correlated materials of current fundamental interest, using angle- and spin-resolved photoemission spectroscopy (ARPES and SARPES). In the introductory part, I present my work on two innovative textbook examples showing how interactions affect the band structure of a material: the coupling of electrons with phonons in a Debye distribution in a two-dimensional electron system (2DES) in ZnO, a wide-band-gap oxide semiconductor used in photovoltaic applications, and the splitting induced by strong spin-orbit coupling (SOC) in the bulk valence band of ZnTe, another important semiconductor used in optoelectronic devices. Then, in the rest of this thesis, I discuss my original results in three different low-dimensional systems of current interest: 1.The realisation of a 2DES at the (110) surface of SnO₂, the first of its kind in a rutile structure. Tunability of its carrier density by means of temperature or Eu deposition and robustness against surface reconstructions and exposure to ambient conditions make this 2DES promising for applications. By means of a simple redox reaction on the surface, this work has proven that oxygen vacancies can dope the conduction band minimum at the surface of SnO₂, solving a long-debated issue about their role in n-type doping in SnO₂. 2.The study of topological surface states in M₂Te₂X (with M = Hf, Zr, or Ti; and X = P or As), a new family of three-dimensional topological metals, originating from SOC and being protected by time-reversal symmetry. Their electronic structure and spin texture, studied by ARPES and SARPES, reveal the presence of massless Dirac fermions giving rise to Dirac-node arcs. 3.The investigation of the quasi-one-dimensional heavy-fermion material YbNi₄P₂, which presents a second-order quantum phase transition from a ferromagnetic to a paramagnetic phase upon partial substitution of phosphorous by arsenide. Such a transition is expected to occur only in zero- or one-dimensional systems, but a direct measurement of the electronic structure of ferromagnetic quantum-critical materials was missing so far. By careful in-situ preparation and cleaning of the surface of YbNi₄P₂ single crystals, which are impossible to cleave, their electronic structure has been successfully measured by ARPES, thus effectively unveiling the quasi-one-dimensionality of YbNi₄P₂. Moreover, the protocol used to make this material accessible to ARPES can be readily generalised to other exotic materials lacking a cleavage plane

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Höpfner, Philipp Alexander [Verfasser], and Ralph [Akademischer Betreuer] Claessen. "Two-Dimensional Electron Systems at Surfaces — Spin-Orbit Interaction and Electronic Correlations / Philipp Alexander Höpfner. Betreuer: Ralph Claessen." Würzburg : Universitätsbibliothek der Universität Würzburg, 2013. http://d-nb.info/1037093488/34.

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Dodoo-Amoo, Nii Amoo. "The nature of critical phenomena of the scaling theory of quantum Hall transitions in two-dimensional electron systems." Thesis, University of Leeds, 2013. http://etheses.whiterose.ac.uk/4764/.

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The nature of the transitions that occur between the quantized plateaux observed in the quantum Hall effect (QHE) have been classified as second order quantum phase transitions. These transitions occur between the localized and the extended states found within a Landau level band of energies. The theory of the critical phenomena associated with these quantum Hall transitions (QHTs) predicts a universal behaviour irrespective of any microscopic detail of the two-dimensional system (2DES) within which they are observed such as carrier concentration or mobility. This proposed universality of QHTs can be verified by measuring the value of certain critical exponents governing the transitions. If valid, these critical exponents should be measured as a universal constant in all instances. This thesis investigates the universality of QHTs using a finite-size scaling theory and attempts to address disagreements that exist in the literature on the critical exponents associated with QHTs. The scaling theory of QHTs presented here involves experimental studies based on varying either the temperature of the 2DES or the frequency of the applied electric field. It was found that the critical exponents of QHTs are not universal across all systems investigated. It is shown that changing the nature of disorder within the system influences the value of the critical exponent measured. In general, it was found that the experimental observation of quantum criticality, as expected from the critical phenomena theory of QHTs, depends on the competition between three key length scales characterizing the 2DES; the size of system, the phase coherent length and the typical size of the electron clusters forming within the system. A study on the limit of the observation of the QHE is also undertaken in the millimetre wave regime. It was found that localization within the 2DES, and as a result the QHE, is destroyed at frequencies below the millimetre wave regime for a GaAs based 2DES.

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Lynass, Mark Ronald. "Novel physics in two dimensional charge carrier structures : anisotropc magneto-transport at Landau-level crossings in aluminium arsenide quantum wells and coupled two dimensional electron and hole systems." Thesis, University of Bath, 2006. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.428382.

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Kärcher, Daniel [Verfasser], and Bernhard [Akademischer Betreuer] Keimer. "Transport in two-dimensional electron systems in ZnO under the influence of microwave radiation / Daniel Kärcher. Betreuer: Bernhard Keimer." Stuttgart : Universitätsbibliothek der Universität Stuttgart, 2016. http://d-nb.info/1084636115/34.

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Brasse, Matthias [Verfasser], Dirk [Akademischer Betreuer] Grundler, and PETER [Akademischer Betreuer] BOENI. "Magnetization of correlated electron systems: MnSi thin films, CrB2 single crystals and two-dimensional electron systems in MgZnO/ZnO / Matthias Brasse. Gutachter: Dirk Grundler ; Peter Böni. Betreuer: Dirk Grundler." München : Universitätsbibliothek der TU München, 2014. http://d-nb.info/1047883554/34.

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Griesbeck, Michael [Verfasser], and Christian [Akademischer Betreuer] Schüller. "Spin dynamics in high-mobility two-dimensional electron systems embedded in GaAs/AlGaAs quantum wells / Michael Griesbeck. Betreuer: Christian Schüller." Regensburg : Universitätsbibliothek Regensburg, 2012. http://d-nb.info/1028958951/34.

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Rösch, Matthias. "Kopplung von kollektiven Anregungen in einlagigen und doppellagigen quasi-zweidimensionalen Elektronensystemen Coupling of collective excitations in monolayer and bilayer quasi two-dimensional electron systems /." [S.l. : s.n.], 2004. http://deposit.ddb.de/cgi-bin/dokserv?idn=974407658.

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Schmalzbauer, Michael [Verfasser], Dominique [Akademischer Betreuer] Bougeard, Dieter [Akademischer Betreuer] Weiss, Milena [Akademischer Betreuer] Grifoni, and Karsten [Akademischer Betreuer] Rincke. "Heterostructure design of Si/SiGe two-dimensional electron systems for field-effect devices / Michael Schmalzbauer. Betreuer: Dominique Bougeard ; Dieter Weiss ; Milena Grifoni ; Karsten Rincke." Regensburg : Universitätsbibliothek Regensburg, 2015. http://d-nb.info/1070423521/34.

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Stegmann, Thomas [Verfasser], Dietrich E. [Akademischer Betreuer] Wolf, Orsolya [Akademischer Betreuer] Ujsághy, and Jürgen [Akademischer Betreuer] König. "Quantum transport in nanostructures : From the effects of decoherence on localization to magnetotransport in two-dimensional electron systems / Thomas Stegmann. Gutachter: Orsolya Ujsághy ; Jürgen König. Betreuer: Dietrich E. Wolf." Duisburg, 2014. http://d-nb.info/1060631873/34.

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Peters, John Archibald. "Ballistic Magnetotransport and Spin-Orbit Interaction InSb and InAs Quantum Wells." Ohio University / OhioLINK, 2006. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1143487911.

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Chickering, William Elbridge. "Thermopower in Two-Dimensional Electron Systems." Thesis, 2016. https://thesis.library.caltech.edu/9320/2/thesis.pdf.

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The subject of this thesis is the measurement and interpretation of thermopower in high-mobility two-dimensional electron systems (2DESs). These 2DESs are realized within state-of-the-art GaAs/AlGaAs heterostructures that are cooled to temperatures as low as T = 20 mK. Much of this work takes place within strong magnetic fields where the single-particle density of states quantizes into discrete Landau levels (LLs), a regime best known for the quantum Hall effect (QHE). In addition, we review a novel hot-electron technique for measuring thermopower of 2DESs that dramatically reduces the influence of phonon drag.

Early chapters concentrate on experimental materials and methods. A brief overview of GaAs/AlGaAs heterostructures and device fabrication is followed by details of our cryogenic setup. Next, we provide a primer on thermopower that focuses on 2DESs at low temperatures. We then review our experimental devices, temperature calibration methods, as well as measurement circuits and protocols.

Latter chapters focus on the physics and thermopower results in the QHE regime. After reviewing the basic phenomena associated with the QHE, we discuss thermopower in this regime. Emphasis is given to the relationship between diffusion thermopower and entropy. Experimental results demonstrate this relationship persists well into the fractional quantum Hall (FQH) regime.

Several experimental results are reviewed. Unprecedented observations of the diffusion thermopower of a high-mobility 2DES at temperatures as high as T = 2 K are achieved using our hot-electron technique. The composite fermion (CF) effective mass is extracted from measurements of thermopower at LL filling factor ν = 3/2. The thermopower versus magnetic field in the FQH regime is shown to be qualitatively consistent with a simple entropic model of CFs. The thermopower at ν = 5/2 is shown to be quantitatively consistent with the presence of non-Abelian anyons. An abrupt collapse of thermopower is observed at the onset of the reentrant integer quantum Hall effect (RIQHE). And the thermopower at temperatures just above the RIQHE transition suggests the existence of an unconventional conducting phase.

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Huang, Jian-Zhe, and 黃健哲. "transport in two-dimensional GaN electron systems." Thesis, 2007. http://ndltd.ncl.edu.tw/handle/05735658938187878178.

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碩士
國立臺灣大學
物理研究所
95
Abstract This thesis describes the measurements on the low-temperature electron transport properties in a two-dimensional GaN electron system. This thesis consists of the following two parts: 1. Electron-electron interaction in a perpendicular magnetic field: We report on experimental studies of Al0.15Ga0.85N/GaN high electron mobility transistor (HEMT) structures grown on p-type Si (111) substrates. By introducing an ultra thin SiN layer during the crystal growth, the Hall mobility of the HEMT structure can be greatly enhanced (>3 times). This SiN treatment technique also allows the observation of Shubnikov-de Haas oscillations which is not possible in the untreated HEMT structure. Our experimental results pave way to integration of AlxGa1-xN/GaN HEMT structures with the mature Si technology in industry. 2. Weak localization effect in a perpendicular magnetic field: We can illustrate weak localization by the constructive interference of wave functions which back to the origin after transmitting along time-reversed paths. Electrons will be localized by weal localization. The dephasing time , which determines the time scale kept the constructive interference of wave functions, limits the length of the time-reversed paths. It is known that weak localization causes a negative magnetoresistivity.

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Huang, Jian-Zhe. "transport in two-dimensional GaN electron systems." 2007. http://www.cetd.com.tw/ec/thesisdetail.aspx?etdun=U0001-0307200716200600.

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Huang, Tsai-Yu, and 黃才育. "Transport properties in two-dimensional GaAs electron systems." Thesis, 2005. http://ndltd.ncl.edu.tw/handle/09048648273325713081.

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博士
國立臺灣大學
物理研究所
93
We have investigated the low-temperature magnetotransport measurements in two-dimensional GaAs electron systems. This dissertation consists of the following four topics. 1. On the low-field insulator-quantum Hall transitions We studied the insulator-quantum Hall conductor transition which separates the low-field insulator from the quantum Hall state of the filling factor /nu=4 on a gated two-dimensional GaAs electron system containing self-assembled InAs quantum dots. To enter the /nu=4 quantum Hall state directly from the low-field insulator, the system undergoes a crossover from the low-field localization to Landau quantization. The crossover, in fact, covers a wide range with respect to the magnetic field rather than only a small region near the critical point of the insulator-quantum Hall conductor transition. 2. On the various Hall insulators We have studied the various Hall insulators (HIs) in a gated two-dimensional GaAs electron system containing self-assembled InAs quantum dots. It is shown that the quantized HI is not necessarily accompanied by the insulator-quantum Hall (I-QH) transition. From our study, the semicircle law can become invalid in the QH liquid so that the quantized Hall plateau is absent at the I-QH transition. The appearance and breakdown of the semicircle law in the insulating phase can be both observed when the QH liquid is destroyed by disorder. 3. Direct measurement of the spin gaps in a gated two-dimensional GaAs electron gas We have investigated magneto transport in gated GaAs two-dimensional electron gases. From the evolution of spin-split Landau levels (LLs) in the Energy (E)-magnetic field (B) plane, we can perform direct measurements of the spin gap for different LLs. The measured g-factor is greatly enhanced over its bulk value in GaAs (0.44) due to electron-electron (e-e) interactions. As the LL index decreases, the g-factor increases, suggesting that the strength of e-e interactions increases as the number of occupied LL decreases. Moreover, the g-factor determined from the conventional activation energy studies is ~ 2.5 times smaller than that deduced from the direct measurements. 4. “Mobility gap” of a spin-split GaAs two- dimensional electron system We have performed magnetotransport measurements of the electron g-factor in a two-dimensional GaAs electron gas. In order to obtain the spin gap△S, we measure the spin-split longitudinal resistivity minimum which shows an activated behavior. From the spin gaps at different odd filling factors, we can obtain the effective g-factor which is greatly enhanced over its bare value (0.44) in GaAs. This enhancement is due to many-body electron-electron interactions. Our experimental results provide compelling evidence that conventional activation energy studies yield a “mobility gap” which can be very different from the real spin gap in the energy spectrum.

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Dissertations / Theses on the topic 'Two Dimensional Electron Systems (2DES)'

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