Dissertations / Theses on the topic 'Quantum phenomenon- Two dimensional electron gas'
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1
Evaldsson, Martin. "Quantum transport and spin effects in lateral semiconductor nanostructures and graphene." Doctoral thesis, Norrköping : Department of Science and technology, Linköping University, 2008. http://www.bibl.liu.se/liupubl/disp/disp2008/tek1202s.pdf.
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Paul, Jagannath. "Coherent Response of Two Dimensional Electron Gas probed by Two Dimensional Fourier Transform Spectroscopy." Scholar Commons, 2017. http://scholarcommons.usf.edu/etd/6738.
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Advent of ultrashort lasers made it possible to probe various scattering phenomena in materials that occur in a time scale on the order of few femtoseconds to several tens of picoseconds. Nonlinear optical spectroscopy techniques, such as pump-probe, transient four wave mixing (TFWM), etc., are very common to study the carrier dynamics in various material systems. In time domain, the transient FWM uses several ultrashort pulses separated by time delays to obtain the information of dephasing and population relaxation times, which are very important parameters that govern the carrier dynamics of materials. A recently developed multidimensional nonlinear optical spectroscopy is an enhanced version of TFWM which keeps track of two time delays simultaneously and correlate them in the frequency domain with the aid of Fourier transform in a two dimensional map. Using this technique, the nonlinear complex signal field is characterized both in amplitude and phase. Furthermore, this technique allows us to identify the coupling between resonances which are rather difficult to interpret from time domain measurements. This work focuses on the study of the coherent response of a two dimensional electron gas formed in a modulation doped GaAs/AlGaAs quantum well both at zero and at high magnetic fields.
In modulation doped quantum wells, the excitons are formed as a result of the inter- actions of the charged holes with the electrons at the Fermi edge in the conduction band, leading to the formation of Mahan excitons, which is also referred to as Fermi edge singularity (FES). Polarization and temperature dependent rephasing 2DFT spectra in combination with TI-FWM measurements, provides insight into the dephasing mechanism of the heavy hole (HH) Mahan exciton. In addition to that strong quantum coherence between the HH and LH Mahan excitons is observed, which is rather surprising at this high doping concentration. The binding energy of Mahan excitons is expected to be greatly reduced and any quantum coherence be destroyed as a result of the screening and electron-electron interactions. Such correlations are revealed by the dominating cross-diagonal peaks in both one-quantum and two-quantum 2DFT spectra. Theoretical simulations based on the optical Bloch Equations (OBE) where many-body effects are included phenomenologically, corroborate the experimental results. Time-dependent density functional theory (TD-DFT) calculations provide insight into the underlying physics and attribute the observed strong quantum coherence to a significantly reduced screening length and collective excitations of the many-electron system. Furthermore, in semiconductors under the application of magnetic field, the energy states in conduction and valence bands become quantized and Landau levels are formed. We observe optical excitation originating from different Landau levels in the absorption spectra in an undoped and a modulation doped quantum wells. 2DFT measurements in magnetic field up to 25 Tesla have been performed and the spectra reveal distinct difference in the line shapes in the two samples. In addition, strong coherent coupling between landau levels is observed in the undoped sample. In order to gain deeper understanding of the observations, the experimental results are further supported with TD-DFT calculation.
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Magyar, Peter. "Quelques aspects du transport électronique bidimensionnel : études théoriques en champ magnétique faible et fort." Université Joseph Fourier (Grenoble ; 1971-2015), 1997. http://www.theses.fr/1997GRE10025.
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Ce travail presente deux etudes de transport des systemes d'electrons bidimensionnels dans les limites du champ magnetique faible et fort. Premierement, nous avons analyse l'utilite d'une nouvelle approche theorique, destinee a calculer la mobilite limitee par (a) des impuretes ionisees ou par (b) des dislocations. Ce traitement a pour but d'ameliorer la description de l'effet d'ecran. Notre etude a revele que la methode ne represente que des avantages tres limites dans le cas (a), tandis que dans le cas (b) les courbes calculees en fonction de la temperature et de la largeur du puits sont en accord avec les mesures. Deuxiemement, l'effet hall quantique entier a ete etudie dans un potentiel de desordre modele. Le systeme a ete couple a un bain thermique. A partir de l'etat stationnaire d'une equation de boltzmann quantique, les conductivites de hall et dissipative ont ete obtenues en fonction de la temperature pour tous les facteurs de remplissage. Nos resultats sont en accord qualitatif avec les experiences. En outre, ils montrent que, du aux phonons, les plateaux quantifies de la conductivite de hall sont sensiblement plus larges a tres basses temperatures que ceux de la conductivite dissipative.
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De, Liberato Simone. "Cavity quantum electrodynamics and intersubband polaritonics of a two dimensional electron gas." Phd thesis, Université Paris-Diderot - Paris VII, 2009. http://tel.archives-ouvertes.fr/tel-00421386.
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L'électrodynamique quantique en cavité, c'est-à-dire l'étude du couplage lumière-matière en géométries confinées, a permis d'observer, grâce à des cavités de plus en plus performantes, le régime de couplage fort lumière-matière.Dans ce régime, le temps de vie d'un photon est plus long que le temps caractéristique de l'interaction avec la matière ; un seul photon subit donc plusieurs cycles d'absorption et de réémission avant de s'échapper de la cavité.
Les premières expériences dans ce régime, effectuées avec des atomes dans des cavités supraconductrices, ont été suivies par des réalisations en matière condensée, utilisant des excitons dans des microcavités planaires, des boites de Cooper couplées à des résonateurs unidimensionnels ou bien des transitions intersousbandes dans des puits quantiques dopés, couplées à un mode de microcavité. Le couplage fort dans ce dernier système donne naissance à des excitations mixtes, moitié lumière et moitié matière, nommées polaritons intersousbandes.
Ma thèse s'attache à plusieurs aspects de la physique de ces excitations, qui se caractérisent par la force extrême du couplage, qui a poussé les chercheurs à introduire le terme couplage ultra-fort.
Dans la première partie de ma thèse, après avoir donné un aperçu général des différents concepts théoriques engagés, j'étudie les conséquences de ce couplage ultra-fort en présence d'une modulation externe appliquée au système. Je montre, en utilisant une théorie de Langevin quantique, qu'une radiation peut être émise à partir du vide, effet qui rappelle de près l'effet Casimir dynamique. L'intensité de cette radiation est assez forte pour pouvoir être mesurée et je reporte ici les résultats de deux expériences préliminaires menées en vue de l'observation d'un tel effet, auxquelles j'ai participé pour la partie théorique.
J'étudie ensuite la manière dont le couplage fort lumière-matière peut influencer le transport électronique et les expériences d'électroluminescence. Dans ce but j'ai développé des méthodes analytiques et numériques que j'ai exploitées pour montrer qu'il est possible d'augmenter grandement l'efficacité quantique des LEDs basées sur des transitions intersousbandes. J'ai aussi donné une première preuve d'extension de l'effet Purcell au régime de couplage fort.
Enfin, dans ma dernière partie, j'ai développé la théorie du scattering stimulé entre polaritons intersousbandes dû au couplage avec des phonons optiques. Je montre que ce mécanisme peut être exploité afin d'obtenir des lasers sans inversion de population avec un seuil extrêmement bas.
5
Bowman, John V. "Transport in a confined two-dimensional electron gas with longitudinal potential variations." Virtual Press, 1995. http://liblink.bsu.edu/uhtbin/catkey/958798.
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Since the discovery of conductance quantization within a nanostnucture, investigations have sought out causes to conductance fluctuations beyond the established plateaus. The focus of this work is to show the fundamental effects upon conductance due to longitudinal potentials and double quantum boxes when confined by hardwall boundaries. A theoretical model based upon a tight-binding recursive tureen's function methodology was modified to incorporate potential barrier variations. A qualitative evaluation, as well as, explanation of the model's results and limitations is discussed.Department of Physics and Astronomy
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Yakymenko, Ivan. "Modelling of injection of electrons by low-dimensional nanowire into a reservoir." Thesis, Linköpings universitet, Teoretisk Fysik, 2018. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-145659.
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High-mobility two-dimensional electron gas (2DEG) which resides at the interface between GaAs and AlGaAs layered semiconductors has been used experimentally and theoretically to study ballistic electron transport. The present project is motivated by recent experiments in magnetic electron focusing. The proposed device consists of two quantum point contacts (QPCs) serving as electron injector and detector which are placed in the same semiconductor GaAs/AlGaAs heterostructure. This thesis is focused on the theoretical study of electron flow coming from the injector QPC (a short quantum wire) and going into an open two-dimensional (2D) reservoir. The transport is considered for non-interacting electrons at different transmission regimes using the mode-matching technique. The proposed mode-matching technique has been implemented numerically using Matlab software. Electron flow through the quantum wire with rectangular, conical and rounded openings has been studied with and without an applied electric bias. We have found that the geometry of the opening does not play a crucial role for the electron flow propagation while the conical opening allows the electrons to travel longer distances into the 2D reservoir. When electric bias is applied, the electron flow also penetrates farther into the 2D region. The results of this study can be applied in designing magnetic focusing devices.
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Cipiloglu, Mustafa Ali. "Thermoelectric Effects In Mesoscopic Physics." Phd thesis, METU, 2004. http://etd.lib.metu.edu.tr/upload/12604753/index.pdf.
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The electrical and thermal conductance and the Seebeck coefficient are calculated for one-dimensional systems, and their behavior as a function of temperature and chemical potential is investigated. It is shown that the conductances are proportional to an average of the transmission probability around the Fermi level with the average taken for the thermal conductance being over a wider range. This has the effect of creating less well-defined plateaus for thermal-conductance quantization experiments.
For weak non-linearities, the charge and entropy currents across a quantum point contact are expanded as a series in powers of the applied bias voltage and the temperature difference. After that, the expansions of the Seebeck voltage in temperature difference and the Peltier heat in current are obtained. Also, it is shown that the linear thermal conductance of a quantum point contact displays a half-plateau structure, almost flat regions appearing around half-integer multiples of the conductance quantum. This structure is investigated for the saddle-potential model.
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Szewc, Wojciech. "Theory and simulation of scanning gate microscopy : applied to the investigation of transport in quantum point contacts." Phd thesis, Université de Strasbourg, 2013. http://tel.archives-ouvertes.fr/tel-00876522.
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This work is concerned with the theoretical description of the Scanning Gate Microscopy (SGM) in general and with solving particular models of the quantum point contact (QPC) nanostructure, analytically and numerically. SGM is an experimental technique, which measures the conductance of a nanostructure, while a charged AFM tip is scanned above its surface. It gives many interesting results, such as lobed and branched images, interference fringes and a chequerboard pattern. A generally applicable theory, allowing for unambiguous interpretation of the results, is still missing. Using the Lippman-Schwinger scattering theory, we have developed a perturbative description of non-invasive SGM signal. First and second order expressions are given, pertaining to the ramp- and plateau-regions of the conductance curve. The maps of time-reversal invariant (TRI) systems, tuned to the lowest conductance plateau, are related to the Fermi-energy charge density. In a TRI system with a four-fold spatial symmetry and very wide leads, the map is also related to the current density, on any plateau. We present and discuss the maps calculated for two analytically solvable models of the QPC and maps obtained numerically, with Recursive Green Function method, pointing to the experimental features they reproduce and to the fundamental difficulties in obtaining good plateau tuning which they reveal.
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Kunc, Jan. "Gaz électronique bidimensionnel de haute mobilité dans des puits quantiques de CdTe : études en champ magnétique intense." Phd thesis, Université de Grenoble, 2011. http://tel.archives-ouvertes.fr/tel-00586639.
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Une étude expérimentale de gaz d'électrons bidimensionnel confinés dans des puits quantiques de CdTe et de CdMnTe est présentée. L'analyse de données est soutenue par des calculs numériques de la structure de bande des états confinés, utilisant l'approximation de densité locale et de fonction enveloppe. Un calcul de type k.p a été utilisé pour justifier l'approximation parabolique appliquée pour les bandes valence. Les échantillons ont été caractérisés par spectroscopie Raman et par spectroscopie d'absorption de la résonance cyclotron infrarouge. Le magnéto-transport à bas champ est dominé par la contribution semi-classique de Drude et révèle trois contributions plus faibles, qui sont la localisation faible, l'interaction électron-électron et les oscillations Shubnikov-De Haas. La contribution des interactions électron-électron est expliquée dans un modèle semi-classique à trajectoire circulaire. La forme des niveaux de Landau, leurs élargissement, les temps de vie transport et quantique de la diffusion et le mécanisme (long-portée) de la diffusion dominant ont été déterminés. Le magnéto-transport sous champs magnétiques intenses révèle la présence d'états Hall quantique fractionnaires dans les niveaux de Landau N=0 et N=1. Nous avons montré, que les états 5/3 et 4/3 étaient complètement polarisés en spin, en accord avec l'approche des fermions composites pour l'effet Hall quantique fractionnaire. La forme de la photoluminescence à champ magnétique nul et son évolution avec la température sont décrites par un modèle analytique simple. La dépendance en champ magnétique et en température de la photoluminescence indique que le gap de spin est amplifié dans les niveaux de landau entièrement occupés. Ces effets multi-corps de l'amplification du gap du spin ont été décrits avec succès par un modèle numérique simple. L'intensité de la photoluminescence a mise en évidence l'importance des processus non-radiatifs pendant la recombinaison, la dégénérescence des niveaux de Landau, leur taux d'occupation, les règles de sélection et l'influence de l'écrantage. Le mécanisme de la relaxation parallèle de spin d'électron et de trou a été identifié et attribué au mécanisme Bir-Aharonov-Pikus, assistée par les phonons acoustiques. Les spectres de photoluminescence d'excitation reflètent la densité des états caractéristique des systèmes bidimensionnels. Les résonances excitoniques, qui sont observées aux bords des sous-bandes électriques inoccupées, illustrent l'importance de l'écrantage et des champs électriques intrinsèques dans les puits asymétriquement dopés.
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Lovisa, Stephane. "Propriétés optiques de puits quantiques de CdTe contenant un gaz d'électrons bidimensionnel." Université Joseph Fourier (Grenoble ; 1971-2015), 1998. http://www.theses.fr/1998GRE10099.
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Les proprietes optiques et magneto-optiques de puits quantiques de cdte contenant un gaz d'electrons bidimensionnel ont ete etudiees par spectroscopie d'absorption et d'emission, en fonction de la concentration des electrons. Une premiere etude par spectroscopie d'emission, effectuee sur une serie d'echantillons a densite electronique fixe, a montre que la masse effective des electrons depend fortement de leur concentration. Des calculs tenant compte des effets a n corps expliquent bien cette renormalisation de masse. Par ailleurs, des dispositifs schottky ont ete realises qui permettent de varier continument la densite du gaz d'electrons dans un puits quantique de cdte a l'aide d'une tension inverse appliquee. L'etude des proprietes de magneto-absorption en fonction de la densite a mene a une modelisation de la force d'oscillateur de l'exciton charge negativement ou trion. Les spectres d'absorption obtenus en champ nul sur des echantillons possedant des desordres electrostatiques differents ont montre l'apparition de zones isolantes pour une valeur du rapport de la densite d'electrons et du parametre de desordre exactement egale a celle prevue par la theorie recente d'efros. La transition optique temoignant de l'apparition de ces zones est attribuee a un trion a n corps.
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Mondal, Puja. "Quantum transport in two-dimensional electron gas with non-relativistic and relativistic dispersion." Thesis, 2018. http://localhost:8080/iit/handle/2074/7766.
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(9142649), Dohyung Ro. "MULTI-ELECTRON BUBBLE PHASES." Thesis, 2020.
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Strong electronic correlations in many-body systems are cradles of new physics. They give birth to novel collective states hosting emergent quasiparticles as well as intriguing geometrical charge patterns. Two-dimensional electron gas in GaAs/AlGaAs under perpendicular magnetic field is one of the most well-known hosts in condensed matter physics where a plethora of the collective states appear. In the strong magnetic field regime, strong Coulomb interactions among the electrons create emergent quasiparticles, i.e. composite fermions and Cooper-paired composite fermions. In the weak magnetic field regime, modified Coulomb interactions drive electron solid phases having geometrical charge patterns in the shape of stripes and bubbles and lower the spatial symmetry of the states.
The fascinating charge order in bubble geometry is the electron bubble phase predicted first by the Hartree-Fock theory. In a bubble phase, certain number of electrons cluster as an entity called bubble and the bubbles order into a crystal of triangular lattice. In addition to the Hartree-Fock theory, the density matrix renormalization group and the exact diagonalization methods further support the formation of electronic bubbles.
Reentrant integer quantum Hall states are commonly accepted as the manifestations of the bubble phases in transport experiment. Soon after the first prediction of the Hartree-Fock theory, the reentrant integer quantum Hall states were observed in the third and higher Landau levels. Since then, the association to the bubble phases has been tested with different experimental techniques for decades.
Although the experimental results from different methods support the bubble phase picture of the reentrant integer quantum Hall states, the electron confinement under the quantum well structure hindered direct scanning of bubble morphology. Thus none of the experiments could showcase the bubble morphology of the reentrant integer quantum Hall states. Meanwhile, a significant discrepancy still remained in between the bubble theories and the experiments. Even though the bubble theories predict the proliferation of bubble phases with increasing orbital index, none of the experiments could observe multiple reentrant integer quantum Hall states in a high Landau level, which signify the multiple bubble formation. Therefore, the proliferation of bubble phases with increasing Landau level index was pessimistic.
In this Dissertation, I present my research on solving this discrepancy. In chapter 4, we performed a magnetotransport measurement of reentrant integer quantum Hall states in the third and higher Landau levels at various different temperatures. Then, we scrutinized how each of the reentrant integer quantum Hall states develops with the gradual increase of the temperature. As a result, we observed multiple reentrant integer quantum Hall states in the fourth Landau level which are associated with the two- and three-electron bubble phases. This result strongly supports the bubble phase picture of the reentrant integer quantum Hall states by confirming the possibility of the proliferation of bubble phases in high Landau levels.
In chapter 5, I analyzed the energetics of newly resolved two- and three-electron bubble phases in the fourth Landau level as well as those of two-electron bubble phases in the third Landau level. Here, I first found, in the fourth Landau level, the three-electron bubbles are more stable than the two-electron bubbles indicating that the multi-electron bubbles with higher electron number are more stable within a Landau level. Secondly, I found distinct energetic features of two- and three-electron bubble phases which are independent of Landau level index throughout the third and the fourth Landau levels. These results highlight the effect of the number of electrons per bubble on the energetics of multi-electron bubble phases and are expected to contribute on improving the existing Hartree-Fock theories.
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(7023347), Vidhi Shingla. "Magnetotransport Studies of Diverse Electron Solids in a Two-Dimensional Electron Gas." Thesis, 2019.
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The two dimensional electron gas subjected to a perpendicular magnetic field
is a model system that supports a variety of electronic phases. Perhaps the most
well-known are the fractional quantum Hall states, but in recent years there has
been an upsurge of interest in the charge ordered phases commonly referred to as
electron solids. These solids are a consequence of electron-electron interactions in a
magnetic field. While some solid phases form in the lowest Landau level, the charged
ordered phases are most abundant in the higher Landau levels. Examples of such
phases include the Wigner solids, electronic bubble phases and stripe or nematic
phases. Open questions surround the exact role of disorder, confinement potential,
temperature and the Landau level index in determining the stability and competition
of these phases with other ground states. The interface of GaAs/AlGaAs remains the cleanest host for the two-dimensional
electron gas due to the extremely high quality of materials available and the advancement in molecular beam epitaxy growth techniques. As a result, exceptionally high
electron mobilities in this system have been instrumental in the discovery of numerous
electron solids.
In this Thesis, I discuss the discovery and properties of several electron solids
that develop in such state-of-the-art two dimensional electron gases. These electron
solids often develop at ultra low temperatures, in the milliKelvin temperature range.
After an introduction to the physics of the quantum Hall effect in two dimensions, in
chapter 3, I discuss electron solids developing in the N=1 Landau level. While these
solids have been known for some time, details of the competition of these phases
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with the nearby fractional quantum Hall states remains elusive. A number of reports
observe new fractional quantum Hall states at filling factors where electron solids
are found in other experiments. We undertook a systematic study to answer some
of these unsettled questions. We see evidence for incipient fractional quantum Hall
states at 2+2/7 and 2+5/7 at intermediate temperatures which are overtaken by
the electronic bubble phases at lower temperatures. Several missing fractional states
including those at filling factors 2+3/5, 2+3/7, 2+4/9 highlight the relative stability
of the electronic solids called the bubble phases in the vicinity in our sample.
In chapter 4, I discuss a newly seen electron crystal which manifests itself in
transport measurements as a reentrant integer quantum Hall state. Reentrant integer
behavior is common in high Landau levels, but so far it was not observed in the
lowest Landau level in narrow quantum well samples. In contrast to high Landau
levels, where such reentrant integer behavior was associated with electronic bubbles,
we believe that the same signature in the N=0 Landau level is due to an electronic
Wigner crystal. The filling factors at which we observe such reentrance reveal that
it is a crystal of holes, rather than electrons. The discovery of this reentrant integer
state paints a complex picture of the interplay of the Wigner crystal and fractional
quantum Hall states.
Finally, in chapter 5, I discuss the observation of a novel phenomenon, that of
reentrant fractional quantum Hall effect. In the lowest Landau level, we observe a
fractional quantum Hall state, but as the field is increased, we see a deviation and
then a return to quantization in the Hall resistance. Such a behavior indicates a novel
electron solid. In contrast to the collective localization of electrons evidenced by the
reentrant integer quantum Hall effect, such reentrance to a fractional Hall resistance
clearly points to the involvement of composite fermion quasiparticles. This property thus distinguishes the ground state we observed as a solid formed of composite
fermions. Such a solid phase is evidence for exotic electron-electron correlations at
play which are clearly different from those in the traditional Wigner solid of electrons.
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Huang, Yen-Hao, and 黃彥豪. "Quantum point contact and quantum dot fabrication and measurement in two-dimensional electron gas." Thesis, 2017. http://ndltd.ncl.edu.tw/handle/ymqk3h.
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碩士國立中央大學
物理學系
106
In this study, we used GaAs/AlGaAs heterostructure and monolayer MoS2 to construct two dimensional electron gas. We tried to fabricate double quantum dot structure on GaAs/AlGaAs heterostructure and measure the coplanar waveguide resonator coupling the two level system which consist of double quantum dot or not. This topic is related to circuit quantum electrodynamics. The recently progress is still in fabrication. Resonator and double quantum dot system completed separately. We must solve alignment fail and lift-off fail before measuring. On the other hand, we used a pair of the top-gate to fabricated quantum point contact device on monolayer MoS2 two dimensional electron gas. Top-gate vertex spacing is the width of the quantum point contact device channel. Top-gate line-width is the length of the quantum point contact device channel. In order to drive electrons out of the two dimensional electron gas near the top-gate range at -10V, we design the top-gate spacing below 100 nm. The electron beam lithography made the width of the top-gate line as narrow is about 60 nm, we used solid polymer electrolyte gate upgrade the carrier density on MoS2 surface so that the mean free path of electrons in the system can exceed 60 nm. We used current annealing to reduce the contact resistance in device, making smaller resistance in measurement. In order to reduce the thermal fluctuation, the measurement temperature was 50mK. We observed similar conductance quantization by constant current measurement.
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Chen, Min-Yin, and 陳明印. "Electronic Proporties of Two-Dimensional Electron Gas in AlGaSb/ InAs Quantum Wells." Thesis, 1995. http://ndltd.ncl.edu.tw/handle/79307535189160880677.
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碩士國立中山大學
物理研究所
83
The main purpose of this thesis is to explore the properties of electrons in the two-dimension electron gas(2DEG)and super- lattice. In the firt part, we introduce a scheme of computing the band structure:the tight-binding method,to comput that of type-I GaAs/AlAs semi-conductor superlattice by the adoption of sp3s* model,and the direct-indirect transitions between the energy gaps of different thickness,as the theoretic groundwork of our analyzing experiment.In the second part,the two- dimension electron gas(2DEG)experiment.We next explore the electrons properties in the quantum well of AlGaSb/InAs at low- temperature 1.5K, which is varied with the external magnetic field. We used Shubnikov-de Haas(SdH)measurements and Quantum Hall effect(QHE) measurements as our experiment approach,and found that there was a negative persistent photoconductivity( NPPC)effect after the illumination(red LED;635nm)at low- temperature(1.5K).By illuminat- ing the sample at low- temperature,the carrier concentration of the two-dimension electron gas in the InAs well was decreasing.
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Chao-Ping, Huang. "Transport in a GaAs two-dimensional electron gas containing self-assembled InAs quantum dots." 2006. http://www.cetd.com.tw/ec/thesisdetail.aspx?etdun=U0001-0906200611070900.
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Huang, Chao-Ping, and 黃兆平. "Transport in a GaAs two-dimensional electron gas containing self-assembled InAs quantum dots." Thesis, 2006. http://ndltd.ncl.edu.tw/handle/29449273137822310020.
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碩士國立臺灣大學
物理研究所
94
The thesis describes the low temperature magnetotransport properties in a GaAs two-dimensional electron gas containing self-assembled InAs quantum dots. This thesis comprises the following two parts. 1. Localization, Landau quantization, and insulator-quantum Hall transition at low magnetic fields We present a magnetotransport study of a two-dimensional electron system. Shubnikov-de Haas (SdH) oscillations are observed in the insulator, indicating that Landau quantization can modulate the density of states without causing the formation of the QH liquid. With increasing the perpendicular magnetic field, from our study the insulator-quantum Hall (I-QH) transition from the low-B insulator to a filling factor ν which is greater than or equal to 3 QH state does occur as Landau bands become well-separated while the Hall and longitudinal resistivities may be different at the transition point. We can estimate the quantum mobility according to the expression of SdH oscillations. At the transition point, the relation μB ~ 1 is still valid, which represents a necessity of well-separated Landau bands in the energy spectrum. 2. Temperature driven flow lines and “phase” transitions in a GaAs two-dimensional electron system containing InAs quantum dots We have constructed temperature driven flow lines from the longitudinal and transverse conductivities (σxx , σxy) in a gated two-dimensional GaAs electron gas containing InAs quantum dots, which allowed us to study the phase transition in our system. The flow lines could be realized theoretically from the effect of the renormalization-group functions. In the spin-degenerate regime, the separatrix σxy = me2/h ( m is an odd integer) separates a quantum Hall state from another. Moreover, the merging into these quantum Hall states with decreasing temperature appears to deviate from a semicircle relation. This could be due to the fact that the presence of inelastic scatterings (electron-phonon, electron-electron, etc.) is known to affect the localization effect.
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杜冠誼. "QUANTUM SCATTERING FROM A CIRCULAR DISK IN A CUBIC-K DRESSELHAUS-TYPE TWO DIMENSIONAL ELECTRON GAS." Thesis, 2009. http://ndltd.ncl.edu.tw/handle/29282360735395454792.
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碩士國立交通大學
電子物理系所
97
This thesis work has devoted to the study of spin-dependent scattering e®ects from a circular-disk microscopic structure with Dresselhaus-type spin-orbit coupling. The Dres- selhaus spin-orbit coupling considered here includes both contributions terms for one is linear-k dependence and the other is cubic-k dependence. Based on the method of partial waves, the complete scattering wave function in a circular scattering region can be rigorously derived and obtained. Through investigating their spatial-resolved scattering behaviors from linear and cubic Dresselhaus-type SOI disk under the electron plane wave incidence, di®erent DSOI contributions can be appar- ently discerned, and their corresponding detail energy dispersion relationships as well. In our ‾ndings: for linear-k Dresselhaus case, the spin density and probability density distri- butions own their spatial symmetry pro‾le, which is featured independence of the plane wave incident angle. On the contrary, strong incident angle dependence is manifested for the case of cubic-k Dresselhaus spin-orbit interaction. In particular , for incidence plane wave in some characteristic angle, we can ‾nd similar spin density responses between cubic-k Dresselhaus case and linear-k Dresselhaus case.
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Yao, Wen-Jiaw, and 姚文醮. "Transport Studies of Two-Dimensional Electron Gas in AlGaN/GaN Quantum Well at Low Temperature and High Magnetic Field." Thesis, 2003. http://ndltd.ncl.edu.tw/handle/99767580470644091400.
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碩士國立中山大學
物理學系研究所
91
We have studied the electronic properties of AlxGa1-xN/GaN heterostructures by using Shubnikov–de Haas(SdH) measurement. Two SdH oscillations were detected on the samples of x=0.35 and 0.31, due to the population of the first two subbands with the energy separations of 128 and 109 meV, respectively. For the sample of x=0.25, two SdH oscillations beat each other, probably due to a finite zero-field spin splitting. The spin-splitting energy is equal to 9.0 meV. The samples also showed a persistent photoconductivity effect after illuminating by blue light-emitting diode. For the part of experiment , we installed a "Regulator" on low temperature and high magnetic field system, in order to control the temperature of sample from 0.3K to 10K accurately. For the convenience of SdH measurements at different tempertures.
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Ho, Pei-Chi, and 何珮綺. "Transport studies of two-dimensional electron gas in GaAs/Al0.3Ga0.7As double quantum well at low temperature and high magnetic field." Thesis, 2003. http://ndltd.ncl.edu.tw/handle/44225399162143297287.
Full textAbstract:
碩士國立中山大學
物理學系研究所
91
The two-dimensional electron system in strongly coupled GaAs/Al0.3Ga0.7As double quantum wells has been studied by Shubnikov-de Haas（SdH）measurements. The degenerate subbands of the double quantum wells are lift into two subbands with a symmetric or anti- symmetric z-direction wave function. We observed that the SdH oscillation due to the second subband of anti-symmetric wave function. The effective mass of the second subband is linearly dependent on the magnetic field range from 0.680 T to 1.964 T. The mass enhancement for the double-quantum-well with an equal well thickness is greater than that with an unequal one. This mass enhancement is attributed to the electron-electron interaction.
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Aamir, Mohammed Ali. "Impact of Disorder and Topology in Two Dimensional Systems at Low Carrier Densities." Thesis, 2016. http://etd.iisc.ac.in/handle/2005/3118.
Full textAbstract:
Two dimensional (2D) systems with low carrier density is an outstanding platform for studying a wide spectrum of physics. These include both classical and quantum effects, arising from disorder, Coulomb interactions and even non-trivial topological properties of band-structure. In this thesis, we have explored the physics at low carrier number density in GaAs/AlGaAs heterostructure and bilayer graphene, by investigating in a larger phase space using a variety of electrical measurement tools.
A two-dimensional electron system (2DES) formed in a GaAs/AlGaAs heterostructure offers an avenue to build a variety of mesoscopic devices, primarily because its surface gates can very effectively control its carrier density profile. In the first half of the thesis, we study the relevance of disorder in two kinds of devices made in a 2DES. A very strong negative gate voltage not only reduces the carrier density of the 2DES, but also drives it to a disordered state. In this state, we explore a new direction in parameter space by increasing in-plane electric field and investigating its magneto-resistance (MR). At sufficiently strong gate voltage and source-drain bias, we discover a remarkably linear MR. Its enormous magnitude and weak temperature dependence indicate that this is a classical effect of disorder. In another study, we examine a specially designed dual-gated device that can induce low number density in a periodic pattern. By applying appropriate gate voltages, we demonstrate the formation of an electrostatically tunable quantum dot lattice and study the impact of disorder on it. This work is important in paving way for solid state based platform for experimental simulations of artificial solids.
The most striking property of bilayer graphene is the ability to open its band gap by a perpendicular electric field, giving the prospects of enabling a large set of de-vice applications. However, despite a band gap, a number of transport mechanisms are still active at very low densities that range from hopping transport through bulk to topologically protected 1D transport at the edges or along 1D crystal dislocations. In the second half of the thesis, we have used higher order statistical moment of resistance/conductance fluctuations, namely the variance of the fluctuations, to complement averaged resistance/conductance, and study and infer the dominant transport mechanism at low densities in a gapped bilayer graphene. Our results show possible evidence of percolative transport and topologically protected edge transport at different ranges of low number densities. We also explore the same phase space by studying its mesoscopic conductance fluctuations at very low temperatures. This is the first of its kind systematic experiment in a dual-gated bilayer graphene device. Its conductance fluctuations have several anomalous features suggesting non-universal behaviour which is at odds with conventional disordered systems.
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Aamir, Mohammed Ali. "Impact of Disorder and Topology in Two Dimensional Systems at Low Carrier Densities." Thesis, 2016. http://hdl.handle.net/2005/3118.
Full textAbstract:
Two dimensional (2D) systems with low carrier density is an outstanding platform for studying a wide spectrum of physics. These include both classical and quantum effects, arising from disorder, Coulomb interactions and even non-trivial topological properties of band-structure. In this thesis, we have explored the physics at low carrier number density in GaAs/AlGaAs heterostructure and bilayer graphene, by investigating in a larger phase space using a variety of electrical measurement tools.
A two-dimensional electron system (2DES) formed in a GaAs/AlGaAs heterostructure offers an avenue to build a variety of mesoscopic devices, primarily because its surface gates can very effectively control its carrier density profile. In the first half of the thesis, we study the relevance of disorder in two kinds of devices made in a 2DES. A very strong negative gate voltage not only reduces the carrier density of the 2DES, but also drives it to a disordered state. In this state, we explore a new direction in parameter space by increasing in-plane electric field and investigating its magneto-resistance (MR). At sufficiently strong gate voltage and source-drain bias, we discover a remarkably linear MR. Its enormous magnitude and weak temperature dependence indicate that this is a classical effect of disorder. In another study, we examine a specially designed dual-gated device that can induce low number density in a periodic pattern. By applying appropriate gate voltages, we demonstrate the formation of an electrostatically tunable quantum dot lattice and study the impact of disorder on it. This work is important in paving way for solid state based platform for experimental simulations of artificial solids.
The most striking property of bilayer graphene is the ability to open its band gap by a perpendicular electric field, giving the prospects of enabling a large set of de-vice applications. However, despite a band gap, a number of transport mechanisms are still active at very low densities that range from hopping transport through bulk to topologically protected 1D transport at the edges or along 1D crystal dislocations. In the second half of the thesis, we have used higher order statistical moment of resistance/conductance fluctuations, namely the variance of the fluctuations, to complement averaged resistance/conductance, and study and infer the dominant transport mechanism at low densities in a gapped bilayer graphene. Our results show possible evidence of percolative transport and topologically protected edge transport at different ranges of low number densities. We also explore the same phase space by studying its mesoscopic conductance fluctuations at very low temperatures. This is the first of its kind systematic experiment in a dual-gated bilayer graphene device. Its conductance fluctuations have several anomalous features suggesting non-universal behaviour which is at odds with conventional disordered systems.
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"Polarization Effects in Group III-Nitride Materials and Devices." Doctoral diss., 2012. http://hdl.handle.net/2286/R.I.14643.
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abstract: Group III-nitride semiconductors have wide application in optoelectronic devices. Spontaneous and piezoelectric polarization effects have been found to be critical for electric and optical properties of group III-nitrides. In this dissertation, firstly, the crystal orientation dependence of the polarization is calculated and in-plane polarization is revealed. The in-plane polarization is sensitive to the lateral characteristic dimension determined by the microstructure. Specific semi-polar plane growth is suggested for reducing quantum-confined Stark effect. The macroscopic electrostatic field from the polarization discontinuity in the heterostructures is discussed, b ased on that, the band diagram of InGaN/GaN quantum well/barrier and AlGaN/GaN heterojunction is obtained from the self-consistent solution of Schrodinger and Poisson equations. New device design such as triangular quantum well with the quenched polarization field is proposed. Electron holography in the transmission electron microscopy is used to examine the electrostatic potential under polarization effects. The measured potential energy profiles of heterostructure are compared with the band simulation, and evidences of two-dimensional hole gas (2DHG) in a wurtzite AlGaN/ AlN/ GaN superlattice, as well as quasi two-dimensional electron gas (2DEG) in a zinc-blende AlGaN/GaN are found. The large polarization discontinuity of AlN/GaN is the main source of the 2DHG of wurtzite nitrides, while the impurity introduced during the growth of AlGaN layer provides the donor states that to a great extent balance the free electrons in zinc-blende nitrides. It is also found that the quasi-2DEG concentration in zinc-blende AlGaN/GaN is about one order of magnitude lower than the wurtzite AlGaN/GaN, due to the absence of polarization. Finally, the InAlN/GaN lattice-matched epitaxy, which ideally has a zero piezoelectric polarization and strong spontaneous polarization, is experimentally studied. The breakdown in compositional homogeneity is triggered by threading dislocations with a screw component propagating from the GaN underlayer, which tend to open up into V-grooves at a certain thickness of the InxAl1-xN layer. The V-grooves coalesce at 200 nm and are filled with material that exhibits a significant drop in indium content and a broad luminescence peak. The structural breakdown is due to heterogeneous nucleation and growth at the facets of the V-grooves.Dissertation/Thesis
Ph.D. Physics 2012
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z, Drozdowa Byszewski Marcin. "Propriétés optiques d'un gaz d'électrons bidimensionnel soumis à un champ magnétique." Phd thesis, 2005. http://tel.archives-ouvertes.fr/tel-00010691.
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Les propriétés d'un gaz électronique bidimensionnel soumis à champs magnétiques intenses et à bas champs magnétiques sont étudiés par la spectroscopie optique: l'effet Hall quantique fractionnaire (FQHE) par photoluminescence et diffusion inélastique de la lumière, puis un nouvel effet oscillatoire de la résistance induit par micro-ondes (MIROs) par transport et absorption des micro-ondes. Les effets des interactions entre électrons du 2DEG sont à l'origine de FQHE. Jusqu'à maintenant, les expériences d'optiques n'ont pas permis les études des interactions entre électrons sur toute la gamme de fractions. Les fractions 1/3, 2/5, 3/7, 3/5, 2/3 et 1 sont clairement observées dans les spectres non traités et montrent une symétrie autour du facteur de remplissage 1/2. La symétrie des fermions composites ets observée dans les spectres. A bas champ magnétique, sous irradiation micro-onde, les propriétés de transport s'écartent nettement des oscillations bien connues de Shubnikov - de Haas pour évoluer vers une série d'états de résistance zéro. Les résultats des mesures d'absorption des micro-ondes sont présentés pour deux échantillons. L'échantillon de basse mobilité montre seulement une absorption autour de la résonance cyclotron (CR). L' échantillon de haute mobilité montre aussi des signaux d'absorption aux harmoniques de la CR. Les mesures ont permis d'inférer l'existence de deux processus d'absorption différents et séparés. L'absorption non résonante est mieux visible en transport et observée comme MIROs, et l'absorption résonante, mieux observée dans les mesures d'absorption, suit probablement les règles de polarisation de résonance cyclotron.
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Kunc, Jan. "Dvourozměrný elektronový plyn v kvantových jamách CdTE: studie ve vysokých magnetických polích." Doctoral thesis, 2011. http://www.nusl.cz/ntk/nusl-296138.
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KurHigh mobility two-dimensional electron gas in CdTe quantum wells: High magnetic field studies Experimental studies of two-dimensional electron gases confined in CdTe and CdMnTe quantum wells are presented. The data analysis is supported by numerical calcula- tions of the band structure of confined states, using the local density and envelope func- tion approximations. Four by four, k.p calculations have been performed to justify the parabolic approximation of valence bands. Samples were characterized by Raman scatter- ing spectroscopy and far infrared cyclotron resonance absorption measurements. Low-field magneto-transport shows the dominant contribution of the semi-classical Drude conduc- tivity and three orders of magnitude weaker contributions of weak localization, electron- electron interaction and Shubnikov-de Haas oscillations. The contribution of electron- electron interactions is explained within a semi-classical model of circling electrons. The shape of Landau levels, broadening, transport and quantum lifetimes and dominant long- range scattering mechanism have been determined. High-field magneto-transport displays fractional quantum Hall states at Landau levels N = 0 and N = 1. The ground states 5/3 and 4/3 have been determined to be fully spin polarized, in agreement with the approach of composite...