Dissertations / Theses: 'Quantum point contacts'

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Ren, Yuan. "Spin effects in quantum point contacts." Thesis, University of British Columbia, 2011. http://hdl.handle.net/2429/37736.

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Quantum point contacts (QPCs) are narrow constrictions between large reservoirs of two-dimensional electron gas, with conductance quantized in units of G=2e²/h at zero magnetic field. Despite decades of investigation, some conductance features of QPCs remain mysterious, such as an extra conductance plateau at 0.7(2e²/h) (0.7 structure) and a zero-bias peak (ZBP) in nonlinear conductance. In this thesis, we present experimental studies of transport anomalies in QPCs, aiming at shedding more light on these features. Conductance measurements are performed for ZBPs in a much wider range than in most previous work, focused especially on the low- and high-conductance regimes. The Kondo model and a model of subband motion are compared with experimental results, but both of them fall short of explaining the data. The subband-motion model is not spin-dependent, so it conflicts with the spin-related nature of ZBPs as confirmed by measurements of nuclear spin polarization in QPCs in an in-plane magnetic field. However, the motion of subbands and the spin dependence of these motions are clearly shown by thermopower spectroscopy. These results may help understand the origin of ZBPs and 0.7 structure.

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Heyder, Jan. "The 0.7 anomaly in quantum point contacts." Diss., Ludwig-Maximilians-Universität München, 2014. http://nbn-resolving.de/urn:nbn:de:bvb:19-178912.

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This thesis aims at shedding light on the microscopic origin of a phenomenon in the field of semiconductor nanostructures, which occurs in transport through a short and narrow quasi one-dimensional constriction, the quantum point contact (QPC). Unlike the stepwise increase of linear conductance of a QPC as function of its width in units of the quantum GQ, which is well understood and was predicted already in the 1950s, an additional shoulder-like step at 0.7xG_Q raises questions since its discovery in 1996 : the 0.7 anomaly. Subsequent experimental investigations revealed a plethora of accompanying features of this fascinating structure. Most famously these include a strong reduction of conductance in the sub-open regime of a QPC as function of external parameters such as magnetic field, temperature or bias voltage. While it is agreed upon that the 0,7 anomaly arises from electron-electron interactions, the high number of theoretical attempts at an explanation indicates that the detailed microscopic origin of the peculiar shoulder is still subject to controversal discussions. In particular no theory seems to describe the whole variety of signatures of the 0.7 anomaly sufficiently. Here, we present a microscopic model that qualifies to meet this requirement. We model the effective barrier of the lowest transport mode of a QPC by a one-dimensional parabolic potential with short-ranged Coulomb interactions. By systematic analysis of experimental data we show that a parabolic barrier approximates the actual barrier shape of the QPC adequately well. In order to understand the physics of a QPC in detail, we put emphasis on the noninteracting properties of our model; we find a pronounced maximum in the local density of states in the vicinity of the barrier center at energies just above the potential. Importantly, this "van Hove ridge", which can be associated with slow electrons above the barrier coincides with the chemical potential if the QPC is tuned to be sub-open. Here, it causes an enhancement of backscattering at finite interactions and a subsequent anomalous reduction of conductance. In case of a magnetic field the underlying mechanism for this reduction is an interaction-enhanced local depopulation of the disfavoured spin species' subband; at finite excitation energies the reduction is a consequence of an interaction-enhanced inelastic backstattering probability. Hence, the interplay of van Hove ridge and electron-electron interactions provides a natural explanation for the appearance of the 0.7 anomaly and its various features. We calculate properties of our interacting one-dimensional QPC model using two methods: A specially developed approximation scheme within the functional renormalization group (fRG) provides reliable results for the magnetic field dependence of the 0.7 anomaly at zero temperature. At finite temperature and finite bias voltage we rely on second order perturbation theory in the interaction (SOPT). Since SOPT's validity is restricted to weaker interaction strength, where calculations clearly show the right trend but not yet the full manifestation of the 0.7 anomaly, we are currently setting up an extension of our fRG approach within the Keldysh formalism, which will allow us to also explore finite excitation energies.

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3

Liu, Tai-Min. "Electronic Interactions in Semiconductor Quantum Dots and Quantum Point Contacts." University of Cincinnati / OhioLINK, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1311773375.

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Keyser, Ulrich Felix. "Nanolithography with an atomic force microscope quantum point contacts, quantum dots, and quantum rings /." [S.l. : s.n.], 2002. http://deposit.ddb.de/cgi-bin/dokserv?idn=966282337.

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Gustafsson, Alexander. "Electron transport in quantum point contacts : A theoretical study." Thesis, Linnéuniversitetet, Institutionen för datavetenskap, fysik och matematik, DFM, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:lnu:diva-10771.

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Electron transport in mesoscopic systems, such as quantum point contacts and Aharonov-Bohm rings are investigated numerically in a tight-binding language with a recursive Green's function algorithm. The simulation reveals among other things the quantized nature of the conductance in point contacts, the Hall conductance, the decreasing sensitivity to scattering impurities in a magnetic field, and the periodic magnetoconductance in an Aharonov-Bohm ring. Furthermore, the probability density distributions for some different setups are mapped, making the transmission coefficients, the quantum Hall effect, and the cyclotron radius visible, where the latter indicates the correspondance between quantum mechanics and classical physics on the mesoscopic scale.

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Moore, Lindsay Shannon. "Novel devices for measuring interactions in quantum point contacts /." May be available electronically:, 2009. http://proquest.umi.com/login?COPT=REJTPTU1MTUmSU5UPTAmVkVSPTI=&clientId=12498.

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Hadji-Ristic, Daniel Ilan. "Thermo-electric and transport properties of etched quantum point contacts." Thesis, Royal Holloway, University of London, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.444164.

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Bauer, Florian. "Microscopic Origin of the 0.7-Anomaly in Quantum Point Contacts." Diss., Ludwig-Maximilians-Universität München, 2014. http://nbn-resolving.de/urn:nbn:de:bvb:19-178928.

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A Quantum point contact (QPC) is a one dimensional constriction, separating two extended electron systems allowing transport between them only though a short and narrow channel. The linear conductance of QPCs is quantized in units of the conductance quantum G_Q=2e^2/h, where e is the electron charge and h is Planck's constant. Thus the conductance shows a staircase when plotted as a function of gate-voltage which defines the width of the channel. In addition measured curves show a shoulder-like step around 0.7G_Q. In this regime QPCs show anomalous behaviour in quantities like electrical or thermal conductance, noise, and thermopower, as a function of external parameters such as temperature, magnetic field, or applied voltage. These phenomena, collectively known as the 0.7-anomaly in QPCs are subject of controversial discussion. This thesis offers a detailed description of QPCs in the parameter regime of the 0.7-anomaly. A model is presented which reproduces the phenomenology of the 0.7-anomaly. We give an intuitive picture and a detailed description of the microscopic mechanism leading to the anomalous behavior. Further, we offer detailed predictions for the behavior of the 0.7-anomaly in the presence of spin-orbit interactions. Our best theoretical results were achieved using an approximation scheme within the functional renormalization group (fRG) which we developed to treat inhomogeneous interacting fermi systems. This scheme, called the coupled ladder approximation (CLA), allows the flow of the two-particle vertex to be incorporated even if the number of interacting sites N, is large, by reducing the number of independent variables which represent the two-particle vertex from O(N^4) to O (N^2).

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Freudenfeld, Jaan [Verfasser]. "Coupling Quantum Point Contacts via Ballistic Electron Optics / Jaan Freudenfeld." Berlin : Freie Universität Berlin, 2021. http://d-nb.info/123127607X/34.

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Jones, Alexander M. "Onset of Spin Polarization in Four-Gate Quantum Point Contacts." University of Cincinnati / OhioLINK, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1485188708345005.

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Donehoo, Brandon. "A superconducting investigation of nanoscale mechanics in niobium quantum point contacts." Diss., Atlanta, Ga. : Georgia Institute of Technology, 2008. http://hdl.handle.net/1853/24784.

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Thesis (Ph.D.)--Physics, Georgia Institute of Technology, 2008.
Committee Chair: Alexei Marchenkov; Committee Member: Bruno Frazier; Committee Member: Dragomir Davidovic; Committee Member: Markus Kindermann; Committee Member: Phillip First

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Dutta, Maitreya. "Hysteresis in the Conductance of Quantum Point Contacts with In-Plane Side Gates." University of Cincinnati / OhioLINK, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1396530257.

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RAHMAN, S. M. SAYDUR. "Spontaneous Spin Polarization due to Lateral Spin Orbit Coupling in InAs Quantum Point Contacts." University of Cincinnati / OhioLINK, 2007. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1196103387.

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14

Bretheau, L. "LOCALIZED EXCITATIONS IN SUPERCONDUCTING POINT CONTACTS: PROBING THE ANDREEV DOUBLET." Phd thesis, Ecole Polytechnique X, 2013. http://tel.archives-ouvertes.fr/tel-00772851.

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L'effet Josephson décrit le couplage cohérent entre supraconducteurs et le supercourant qui en résulte. D'un point de vue microscopique, il découle de l'existence d'états de quasiparticules discrets, localisés au niveau du lien faible, les états liés d'Andreev. Ils viennent en doublets dans chaque canal de conduction du lien faible, avec des énergies et supercourants opposés. Chaque doublet d'Andreev donne lieu à quatre états: l'état fondamental \left|-\right\rangle et l'état excité \left|+\right\rangle , avec une parité paire, et les états excités impairs \left|\uparrow\right\rangle et \left|\downarrow\right\rangle . Est-il possible d'exciter les doublets Andreev? Cette thèse décrit deux séries d'expériences conçues pour répondre à cette question en utilisant l'élément Josephson le plus élémentaire, un contact atomique entre deux électrodes supraconductrices. Dans une première expérience, nous avons observé et caractérisé les états excités impairs \left|\uparrow\right\rangle et \left|\downarrow\right\rangle . Comme attendu pour un système dégénéré en spin, ils ne portent pas de supercourant. Dans cette expérience, l'excitation n'était pas contrôlée mais dû au piégeage spontanée de quasi-particules parasites dans l'un des états d'Andreev. Sous certaines conditions, le temps de vie mesuré de ces états impairs peut dépasser 100 µs. La deuxième expérience est une spectroscopie photonique des états d'Andreev. Elle a été effectuée en utilisant une junction Josephson en tant qu'émetteur et détecteur microonde. Les transitions d'Andreev observées correspondent à des excitations depuis l'état fondamental \left|-\right\rangle vers l'état excité paire \left|+\right\rangle , et sont bien décrites par notre modèle quantique. Ce résultat ouvre la voie à la manipulation cohérente de ce système à deux niveaux. L'observation directe de l'état excité d'Andreev, soit par injection de quasiparticules soit par absorption de photons, conforte la théorie mésoscopique de l'effet Josephson. Cela démontre que, en plus de la différence de phase, chaque canal d'un lien faible Josephson possède un degré de liberté fermionique interne similaire à un spin un-demi.

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15

Bauer, Florian [Verfasser], and Delft Jan [Akademischer Betreuer] von. "Microscopic Origin of the 0.7-Anomaly in Quantum Point Contacts / Florian Bauer. Betreuer: Jan von Delft." München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2014. http://d-nb.info/1067399690/34.

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Zimmermann, Katrin. "Contacts ponctuels quantiques dans le graphène de haute mobilité." Thesis, Université Grenoble Alpes (ComUE), 2016. http://www.theses.fr/2016GREAY008/document.

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Dans le régime de l'effet Hall quantique, les porteurs de charge se propagent le long de canaux unidimensionnels situés au bords d'un gaz d'électron bidimensionel (2D electron gas, 2DEG). Un contact ponctuel quantique (quantum point contact, QPC) - une constriction étroite confinant spatialement le gaz électronique - permet de contrôler la transmission de ces canaux de bords. Dans un 2DEG conventionnel, une tension négative appliquée sur les grilles électrostatiques du QPC engendre la déplétion locale du gaz électronique sous la grille, forçant les électrons à se propager au travers de la constriction. Cependant, dans le graphène, du fait de l'absence de bande interdite, une tension négative provoque la transition continue du dopage d'électrons à trous. Dans le régime de l'effet Hall quantique, électrons et trous se propagent le long de l'interface p-n dans la même direction, et la diffusion inélastique induit un transfert de charge et du mélange entre eux.Au cours de cette thèse, nous avons fabriqué des dispositifs à base de graphène encapsulé dans deux feuillets de hBN, et munis de grilles électrostatiques définissant un QPC. Nous avons étudié l'effet du QPC sur la propagation des canaux de bords entiers et fractionnaires de l'effet Hall quantique, et sur le mélange entre eux. Dans l'effet Hall quantique, nous avons démontré que les canaux entiers et fractionnaires peuvent être contrôlés et sélectivement transmis au travers de la constriction. Du fait de la haute mobilité de nos structures, et de la levée de dégénérescence complète des niveaux de Landau qui en résulte à fort champ magnétique, l'équilibrage à l'interface p-n est réduit aux sous-niveaux de même spin et au niveau de Landau N=0.Un QPC dans le régime de l'effet Hall quantique constitue également un système idéal pour l'étude de l'effet tunnel des porteurs de charge entre canaux de bords fractionnaires, unidimensionnels et fortement corrélés, se propageant dans des directions opposées, décrits par la théorie de Tomonaga-Luttinger. Nous avons étudié l'effet tunnel entre canaux de bords fractionnaires dans notre structure muni un QPC, en nous concentrant sur l'état fractionnaire 7/3 et la dépendance en température de ses propriétés tunnels
In the quantum Hall regime, the charge carriers are conducted within one-dimensional channels propagating at the edge of a two-dimensional electron gas (2DEG). A quantum point contact (QPC) – a narrow constriction confining spatially electron transport – can control the transmission of these quantum Hall edge channels. In conventional 2DEG systems, a negative voltage applied on the electrostatic split gates depletes locally the electrons underneath them forcing the electrons to pass through the constriction. In contrast, due to the absence of a band gap in graphene, a negative gate voltage induces a continuous shift of the doping from electrons to holes. In the quantum Hall regime, electron and hole edge channels propagate along the pn-interface in the same direction while inelastic scattering induces charge transfer and mixing between them.In this PhD thesis, we have fabricated ballistic graphene devices made by van der Waals stacking of hBN/Gr/hBN heterostructures, and equipped with split gates forming a quantum point contact (QPC) constriction. We have studied the effect of the QPC on the propagation of integer and fractional quantum Hall edge channels and the mixing among them. In the quantum Hall regime, we demonstrate that the integer and fractional quantum Hall edge channels can be controlled and selectively transmitted by the QPC. Due to the high mobility of our devices and the resultant full lifting of the degeneracies of the Landau levels in strong magnetic field, equilibration at the pn-interface is restricted to sublevels of identical spins of the N=0 Landau level.A QPC in the quantum Hall regime offers also an ideal system to study the tunnelling of charge carriers between counter-propagating fractional edge channels of highly correlated, one-dimensional fermions described by the theory of Tomonaga-Luttinger. We study the tunnelling between fractional quantum Hall edge channels in our QPC device in graphene and focus on the 7/3-fractional state to explore the temperature dependence of tunnelling characteristics

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Dai, Zhenting. "Coherent and Dissipative Transport in Metallic Atomic-Size Contacts." Diss., Georgia Institute of Technology, 2006. http://hdl.handle.net/1853/19880.

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Thin-film niobium mechanically controlled break junctions and resistively shunted niobium mechanically-controlled break junctions were developed and successfully microfabricated. Using these devices, high-stability atomic size contacts were routinely produced and investigated both in the normal and superconducting states. Investigations of the two-level conductance fluctuations in the smallest contacts allowed the calculation of their specific atomic structure. Embedding resistive shunts close to the superconducting atomic-sized junctions affected the coherence of the electronic transport. Finally, point contact spectroscopy measurements provide evidence of the interaction of conduction electrons with the mechanical degrees of freedom of the atomic-size niobium contacts.

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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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Sloggett, Clare Physics Faculty of Science UNSW. "Electron correlations in mesoscopic systems." Awarded by:University of New South Wales. School of Physics, 2007. http://handle.unsw.edu.au/1959.4/31875.

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This thesis deals with electron correlation effects within low-dimensional, mesoscopic systems. We study phenomena within two different types of system in which correlations play an important role. The first involves the spectra and spin structure of small symmetric quantum dots, or "eartificial atoms"e. The second is the "e0.7 structure"e, a well-known but mysterious anomalous conductance plateau which occurs in the conductance profile of a quantum point contact. Artificial atoms are manufactured mesoscopic devices: quantum dots which resemble real atoms in that their symmetry gives them a "eshell structure"e. We examine two-dimensional circular artificial atoms numerically, using restricted and unrestricted Hartree-Fock simulation. We go beyond the mean-field approximation by direct calculation of second-order correlation terms; a method which works well for real atoms but to our knowledge has not been used before for quantum dots. We examine the spectra and spin structure of such dots and find, contrary to previous theoretical mean-field studies, that Hund's rule is not followed. We also find, in agreement with previous numerical studies, that the shell structure is fragile with respect to a simple elliptical deformation. The 0.7 structure appears in the conductance of a quantum point contact. The conductance through a ballistic quantum point contact is quantised in units of 2e^2/h. On the lowest conductance step, an anomalous narrow conductance plateau at about G = 0.7 x 2e^2/h is known to exist, which cannot be explained in the non-interacting picture. Based on suggestive numerical results, we model conductance through the lowest channel of a quantum point contact analytically. The model is based on the screening of the electron-electron interaction outside the QPC, and our observation that the wavefunctions at the Fermi level are peaked within the QPC. We use a kinetic equation approach, with perturbative account of electron-electron backscattering, to demonstrate that these simple features lead to the existence of a 0.7-like structure in the conductance. The behaviour of this structure reproduces experimentally observed features of the 0.7 structure, including the temperature dependence and the behaviour under applied in-plane magnetic fields.

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Schulz, Leonhard Ferdinand [Verfasser], and Klaus [Akademischer Betreuer] Klitzing. "Parallel arrangements of quantum dots and quantum point contacts in high magnetic fields : periodic conductance modulations with magnetic flux change / Leonhard Ferdinand Schulz. Betreuer: Klaus Klitzing." Stuttgart : Universitätsbibliothek der Universität Stuttgart, 2015. http://d-nb.info/1065235798/34.

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Heyder, Jan [Verfasser], and Jan von [Akademischer Betreuer] Delft. "The 0.7 anomaly in quantum point contacts : a microscopic model for the first conductance step / Jan Heyder. Betreuer: Jan von Delft." München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2014. http://d-nb.info/1066206635/34.

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Weidinger, Lukas [Verfasser], and Jan von [Akademischer Betreuer] Delft. "Finite-ranged interactions and multiband effects in quantum point contacts : a functional renormalization group study / Lukas Weidinger ; Betreuer: Jan von Delft." München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2021. http://d-nb.info/1233201271/34.

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Ngo, Anh T. "Spin-orbit Effects and Electronic Transport in Nanostructures." Ohio University / OhioLINK, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1292260134.

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Percebois, Gaëtan. "Quantum transport in two-dimensional systems : artificial intelligence applied to material science." Electronic Thesis or Diss., Strasbourg, 2023. http://www.theses.fr/2023STRAE033.

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Les hétérostructures représentent l'un des dispositifs les plus largement utilisés pour générer un gaz d'électrons bidimensionnel (2DEG). Cependant, les propriétés exactes de transport des électrons au sein de tels dispositifs ne sont pas complètement contrôlables et prévisibles. Cela est principalement dû à la présence de dopants ionisés, nécessaires à la création du gaz d’électrons, qui impactent le mouvement des électrons par le biais d'interactions coulombiennes, entraînant ainsi un potentiel de désordre. Dans cette étude, nous introduisons une méthode pour déterminer ce désordre en utilisant des données de transport locales obtenues à partir d'expériences de microscopie à grille locale (SGM). La correspondance entre les données capturées dans une image SGM et le potentiel de désordre est réalisée grâce à l'utilisation d'un algorithme de deep learning. Nous avons démontré que ce problème inverse peut être résolu, et avons déterminé le potentiel de désordre au sein d'une véritable hétérostructure expérimentale. Dans ce manuscrit, nous présentons en détail la méthodologie employée, la rendant ainsi reproductible pour d'autres dispositifs
Heterostructures represent one of the most widely employed devices for generating a two-dimensional electron gas (2DEG). However, the precise transport properties of electrons within such devices remain neither completely controllable nor predictable. This is mainly due to the presence of randomly located ionized dopants, that are necessary for creating the electron gas. However, due to the Coulombic interactions, the electron motion is impacted, which results in a disorder potential. In this study, we introduce a method to determine this disorder potential using local transport information obtained from scanning gate microscopy (SGM) experiments. The mapping between the data captured in an SGM image and the disorder potential is achieved through the utilization of a deep learning algorithm. We have demonstrated that this inverse problem can be solved, and we have been able to determine the disorder potential within a real experimental heterostructure. In this manuscript, we detail the employed methodology, making it replicable for other devices

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Ly, Ousmane. "Microscopie à grille locale comme outil d’extraction des propriétés électroniques locales en transport quantique." Thesis, Strasbourg, 2017. http://www.theses.fr/2017STRAE022/document.

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La technique de la microscopie à grille de balayage (SGM) consiste à mesurer la conductance d'un gaz bidimensionnel d'électrons (2DEG) sous l'influence d'une pointe balayant la surface de l'échantillon. Dans ce travail, une approche analytique complétée par des simulations numériques est développée pour étudier la relation entre les mesures SGM et les propriétés électroniques locales dans des systèmes mésoscopiques. La correspondance entre la réponse SGM et la densité locale partielle (PLDOS) est étudiée pour un contact quantique entouré d’un 2DEG en présence ou en absence de désordre, pour une pointe perturbative ou non perturbative. Une correspondance SGM-PLDOS parfaite est trouvée pour des transmissions entières et des pointes locales. La dégradation de la correspondance en dehors de cette situation est étudiée. D’autre part, la liaison entre la réponse SGM et la transformée de Hilbert de la densité locale est discutée. Pour étudier le rôle de la force de la pointe sur la conductance SGM, une formule analytique donnant la conductance totale est obtenue. Dans le cas d'une pointe à taille finie nous proposons une méthode basée sur les fonctions de Green permettant de calculer la conductance en connaissant les propriétés non-perturbées. En plus, nous avons étudié la dépendance des branches de la PLDOS en fonction de l’énergie de Fermi
The scanning gate microscopy (SGM) technique consists in measuring the conductance of a two dimensional electron gas (2DEG) under the influence of a scanning tip. In this work, an analytical approach complemented by numerical simulations is developed to study the connection between SGM measurements and local electronic properties in mesoscopic devices. The connection between the SGM response and the partial local density of states (PLDOS) is studied for the case of a quantum point contact surrounded by clean or disordered 2DEG for perturbative or non-perturbative, local or extended tips. An SGM-PLDOS correspondence is found for integer transmissions and local tips. The degradation of this correspondence out of these conditions is studied. Moreover, a presumed link between the SGM response and the Hilbert transform of the LDOS is discussed. To study the role of the tip strength, an analytical formula giving the full conductance in the case of local tips is obtained. Furthermore, a Green function method enabling to calculate the quantum conductance in the presence of a finite size tip in terms of the unperturbed properties is proposed. Finally the dependence of the PLDOS branches on the Fermi energy is studied

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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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Gustafsson, Alexander. "Quantum point contact : A theoretical study." Thesis, Växjö University, School of Mathematics and Systems Engineering, 2010. http://urn.kb.se/resolve?urn=urn:nbn:se:lnu:diva-6571.

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Experiments shows that the conductance of a quantum point contact is quantized in steps of 2e²/h, where e is the charge of the electron and h is Planck’s constant, and thereby Ohm’s law is not valid for nanostructures. By using the approximation method finite difference, the transmission for one-dimensional contacts and one- and two-dimensional potentials are investigated. In the case of two-dimensional contacts and a two-dimensional potential the Green’s function method is used. It turns out that if electrons are treated as waves, the transmission and the conductance just differ by the constant 2e²/h, which in this thesis is interpreted numerically in Matlab by using the Green’s function method.

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Johansson, Erik. "Quantized Transmission in an Asymmetrically Biased Quantum Point Contact." Thesis, Linköpings universitet, Teoretisk Fysik, 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-133185.

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In this project work we have studied how a two-dimensional electron gas (2DEG) in a GaAs/AlGaAs semiconductor heterostructure can be locally confined down to a narrow bottleneck constriction called a quantum point contact (QPC) and form an artificial quantum wire using a split-gate technique by application of negative bias voltages. The electron transport through the QPC and how asymmetric loading of bias voltages affects the nature of quantized conductance were studied. The basis is Thomas-Fermi simulations that within the Büttiker model give results somewhat similar to experimental work in aspects regarding electron density effects. An extension of the model to include exchange and correlation interaction was investigated, as well as compared to density functional theory.

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29

Das, Partha Pratim. "Generation of Spin Polarization in Side-Gated InAs Quantum Point Contact." University of Cincinnati / OhioLINK, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1344874808.

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Vaidya, Nikhilesh Avanish. "NOISE SPECTRUM OF A QUANTUM POINT CONTACT COUPLED TO A NANO-MECHANICAL OSCILLATOR." Diss., Temple University Libraries, 2017. http://cdm16002.contentdm.oclc.org/cdm/ref/collection/p245801coll10/id/447885.

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Physics
Ph.D.
With the advance in nanotechnology, we are more interested in the "smaller worlds". One of the practical applications of this is to measure a very small displacement or the mass of a nano-mechanical object. To measure such properties, one needs a very sensitive detector. A quantum point contact (QPC) is one of the most sensitive detectors. In a QPC, electrons tunnel one by one through a tunnel junction (a "hole"). The tunnel junction in a QPC consists of a narrow constriction (nm-wide) between two conductors. To measure the properties of a nano-mechanical object (which acts as a harmonic oscillator), we couple it to a QPC. This coupling effects the electrons tunneling through the QPC junction. By measuring the transport properties of the tunneling electrons, we can infer the properties of the oscillator (i.e. the nano-mechanical object). However, this coupling introduces noise, which reduces the measurement precision. Thus, it is very important to understand this source of noise and to study how it effects the measurement process. We theoretically study the transport properties of electrons through a QPC junction, weakly coupled to a vibration mode of a nano-mechanical oscillator via both the position and the momentum of the oscillator. %We study both the position and momentum based coupling. The transport properties that we study consist of the average flow of current through the junction, given by the one-time correlation of the electron tunneling event, and the current noise given by the two-time correlation of the average current, i.e, the variance. The first comprehensive experimental study of the noise spectrum of a detector coupled to a QPC was performed by the group of Stettenheim et al. Their observed spectral features had two pronounced peaks which depict the noise produced due to the coupling of the QPC with the oscillator and in turn provide evidence of the induced feedback loop (back-action). Benatov and Blencowe theoretically studied these spectral features using the Born approximation and the Markovian approximation. In this case the Born approximation refers to second order perturbation of the interaction Hamiltonian. In this approximation, the electrons tunnel independently, i.e., one by one only, and co-tunneling is disregarded. The Markovian approximation does not take into account the past behavior of the system under time evolution. These two approximations also enable one to study the system analytically, and the noise is calculated using the MacDonald formula. Our main aim for this thesis is to find a suitable theoretical model that would replicate the experimental plots from the work of Stettenheim et al. Our work does not use the Markovian approximation. However, we do use the Born approximation. This is justified as long as the coupling between the oscillator and QPC is weak. We first obtain the non-Markovian unconditional master equation for the reduced density matrix of the system. Non-Markovian dynamics enables us to study, in principle, the full memory effects of the system. From the master equation, we then derive analytical results for the current and the current noise. Due to the non-Markovian nature of our system, the electron tunneling parameters are time-dependent. Therefore, we cannot study the system analytically. We thus numerically solve the current noise expression to obtain the noise spectrum. We then compare our noise spectrum with the experimental noise spectrum. We show that our spectral noise results agree better with the experimental evidence compared to the results obtained using the Markovian approximation. We thus conclude that one needs non-Markovian dynamics to understand the experimental noise spectrum of a QPC coupled to a nano-mechanical oscillator.
Temple University--Theses

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31

Schubert, Enrico [Verfasser], and Alexander [Akademischer Betreuer] Högele. "Transport and optical spectroscopy of a quantum point contact / Enrico Schubert ; Betreuer: Alexander Högele." München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2016. http://d-nb.info/1125883960/34.

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32

Choi, Deung jang. "Kondo effect and detection of a spin-polarized current in a quantum point contact." Thesis, Strasbourg, 2012. http://www.theses.fr/2012STRAE029/document.

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L'effet Kondo observé dans des objets individuels constitue un système modèle pour l’étude de corrélations électroniques. Ces dernières jouent un rôle moteur dans le domaine émergent de l'électronique de spin (ou spintronique) où l’utilisation d’atomes issus des terres rares et des métaux de transition est incontournable. Dans ce contexte, l’étude de l'interaction d’une impureté Kondo avec des électrodes ferromagnétiques ou avec d’autres impuretés magnétiques peut donc s’avérer fondamental pour la spintronique. L’effet Kondo est sensible à son environnement magnétique car en présence d’interactions magnétiques la résonance ASK se dédouble. Dans une certaine mesure, la résonance ASK agit comme un niveau atomique discret doublement dégénérée qui subit un dédoublement Zeeman en présence d'un champ magnétique ou plus généralement d’un champ magnétique effectif. Inversement, la détection d'un dédoublement Zeeman indique l'existence d'un champ magnétique. Dans une boîte quantique, le couplage de la boîte avec les deux électrodes est faible en général et la largeur de la résonance ASK est donc de l'ordre de quelques meV. Beaucoup d’études de l’effet Kondo en présence d’interactions magnétiques ont été menées sur les boîtes quantiques, grâce notamment au contrôle qui peut être exercé sur la résonance ASK, mais aussi grâce au faible élargissement de la résonance qui peut alors être dédoublée avec un champ magnétique de l’ordre de 10 Tesla ou moins. A ces études, s’ajoutent de nombreux travaux similaires menés avec des dispositifs tels des jonctions cassées comprenant une molécule individuelle jouant le rôle de l’impureté magnétique. En revanche, peu d’études de ce type ont été consacrées aux atomes individuels. Cela est dû à l’hybridation plus marquée entre l'impureté atomique et la surface comparée aux boîtes quantiques, qui entraine une largeur typique de 10 meV ou plus pour la résonance ASK. Un champ magnétique d'environ 100 T ou plus est alors nécessaire afin de dédoubler la résonance et donc en pratique difficile à mettre en oeuvre. Cette thèse est consacrée précisément à l’étude de l'interaction entre une impureté Kondo individuel et son environnement magnétique à l’aide d’un STM. Une nouvelle stratégie est adoptée ici par rapport aux études antérieures de ce genre. Tout d'abord, nous éliminons la barrière tunnel en établissons un contact pointe-atome. Nous formons ainsi un point de contact quantique comprenant une seule impureté Kondo. Deuxièmement, nous utilisons des pointes ferromagnétiques. Le contact pointe-atome permet de sonder l'influence du ferromagnétisme sur l'impureté Kondo vial’observation de la résonance ASK. La géométrie de contact permet tout particulièrement de produire une densité de courant polarisé en spin suffisamment élevée pour qu’elle entraîne un dédoublement de la résonance ASK. Ce dédoublement constitue la première observation à l’échelle atomique d’un phénomène connu sous le nom d’accumulation de spin, laquelle se trouve être une propriété fondamentale de la spintronique
The Kondo effect of these single objects represents a model system to study electron correlations, which are nowadays of importance in relation to the emerging field of spin electronics, also known as spintronics, where chemical elements with partially filled d or f shells play a central role. Also of particular interest to spintronics is the interaction of single Kondo impurities with ferromagnetic leads or with other magnetic impurities. A Kondo impurity is in fact sensitive to its magnetic environment as the ASK resonance is usually split into two resonances in the presence of magnetic interactions. To some extent, the ASK resonance acts as a two-fold degenerate energy level of an atom which undergoes a Zeeman splitting in the presence of an effective magnetic field. Conversely, the detection of a Zeeman splitting indicates the existence of a magnetic field. In a QD, the coupling of the QD to the two leads is very weak in general, and the Kondo resonance is in the range of a few meV. Many studies focusing on magnetic interaction have been carried out on QDs, due to the high control that can be extended to the ASK resonance and its low energy range, allowing to split the resonance with a magnetic field of 10 T. Similar work has also been carried out in single-molecule or lithographically-defined devices. Although STM is an ideal tool to study the Kondo effect of single atoms, there is still a strong lack of experimental studies concerning atoms in the presence of magnetic interactions. This is partly due to the stronger impurity-metal hybridization compared to QDs, which places the ASK width in the range of 10 meV. An effective magnetic field of 100 T would be needed to split the resonance. The present Thesis is devoted precisely at studying the interaction between a single Kondo impurity with its magnetic environment through STM. A new strategy is adopted herecompared to former studies of this kind. Firstly, we contact a single-magnetic atom on a surface with a STM tip thereby eliminating the vacuum barrier. Secondly, we use ferromagnetic tips. The contact with a single atom allows probing the influence of ferromagnetism on the Kondo impurity i. e. its ASK resonance. But most importantly, the contact geometry produces sufficiently high current densities compared to the tunneling regime, so that the ASK resonance becomes sensitive to the presence of a spin-polarized current. This constitutes the first atomic scale detection of a spin-polarized current with a single Kondo impurity

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33

Brun, Boris. "Electron interactions in mesoscopic physics : Scanning Gate Microscopy and interferometry at a quantum point contact." Thesis, Grenoble, 2014. http://www.theses.fr/2014GRENY049/document.

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Au cours de cette thèse nous avons étudié les effets des interactions entre électrons dansles contacts ponctuels quantiques (QPCs). Les contacts ponctuels quantiques sont des petitscanaux quasi-unidimensionnels, définis à partir de gaz électroniques bidimensionnelsde haute mobilité (2DEG). Une tension négative appliquée sur des grilles métalliques audessus de la surface permet d’ouvrir ou fermer le QPC. Lorsqu’un QPC s’ouvre, de plusen plus de modes électroniques peuvent traverser le QPC, et sa conductance augmente parpas discrets, séparés par un quantum de conductance 2e2/h. On peut le comprendre parle transport unidimensionnel d’une seule particule, car chaque mode transverse contribuepour un quantum de conductance.Mais depuis leurs premières réalisations, les QPCs ont montré des déviations par rapportà ce modèle à une particule. Les plus connues sont un épaulement sous le premier plateau,autour de 0.7×2e2/h, appelé "l’anomalie 0.7", et un pic dans la conductance différentiellequi apparaît à basse température: l’anomalie à zéro polarisation (ZBA).L’instrument que nous avons utilisé pour étudier ces effets d’interactions est un microscopeà effet de grille local (SGM). Cette technique consiste à modifier localement le potentield’un dispositif à l’aide d’une pointe de microscope à force atomique (AFM) chargée négativement,et enregistrer les modifications de la conductance en fonction de la position dela pointe. En utilisant cette technique à très basse température, nous avons montré quenous pouvons moduler les anomalies de conductance du QPC. Nous avons interprété nosrésultats comme la signature d’un cristal d’électrons se formant spontanément à bassedensité dans le QPC à cause de la répulsion Coulombienne: un cristal de Wigner. Onpeut modifier le nombre d’électrons cristallisés en approchant la pointe, et obtenir dessignatures de la parité du nombre d’électrons localisés dans le transport électronique.En fonction de cette parité, le cristal de Wigner présente un état de spin différent, etl’écrantage de ce spin par les électrons de conduction au travers d’un mécanisme appeléeffet Kondo donne une anomalie à zéro polarisation formant alternativement un simplepic ou un double pic. Cette découverte apporte une avancée significative à ce domaine,qui a concentré les efforts de plusieurs groupes importants ces 15 dernières années.Nous avons ensuite réalisé des mesures interférométriques à l’aide du microscope SGM,en créant in situ des interféromètres dans le gaz 2D. Nous avons obtenu les signaturesd’un déphasage supplémentaire dans le régime de la ZBA. Nous attribuons cet effet audéphasage universel accumulé par les électrons à la traversée d’un singulet Kondo, ce quirenforce le fait que la ZBA trouve son origine dans les phénomènes Kondo.Enfin, nous avons adapté la technique SGM au transport thermoélectrique dans les QPCs,et avons imagé pour la première fois les interférences d’électrons se déplaçant sous l’effetd’une différence de température
In this thesis, we studied the effect of electron electron interactions in quantum pointcontacts (QPCs). Quantum point contacts are small quasi-one dimensional channels,designed on a high mobility two-dimensional electron gas (2DEG). A negative voltageapplied on a pair of metallic split gates above the sample surface allows to open or closethe QPC. As a QPC opens, more and more electronic modes are allowed to cross theQPC, and its conductance increases by discrete steps, separated by a conductance quantum2e2/h. This can be understood from a single-particle picture in one-dimensionaltransport, as each transverse mode carries a conductance quantum.But from their first realization 25 years ago, quantum point contacts have shown deviationsfrom this picture, attributed to electron electron interactions. The most well knownare a shoulder below the first plateau, around 0.7×2e2/h, called the "0.7 anomaly", and apeak in the differential conductance that arises at low temperature: the zero bias anomaly(ZBA).The tool we used to study these interaction effects is a scanning gate microscope (SGM).It consists by changing locally the device’s potential with the polarized tip of an atomicforce microscope (AFM), and record the changes in conductance as a function of the tipposition. By performing this technique at very low temperature, we showed that we canmodulate the conductance anomalies of QPCs. We interpret our result as the signatureof a small electrons crystal forming spontaneously at low density in the QPC due to theCoulomb repulsion: a Wigner crystal. We can modify the number of crystallized electronsby approaching the tip, and obtain signatures of the parity of the localized electrons numberin transport features. Depending on this parity, the Wigner crystal has a differentspin state, and screening of this spin by the surrounding electrons through the so-calledKondo effect leads alternatively to a single peak or a split ZBA. This discovery bringsa significant advance in this field, that has attracted research efforts of many importantgroups in the world over the past 15 years.We then performed interferometric measurements thanks to the scanning gate microscopeby creating in-situ interferometers in the 2DEG. We obtained signatures of an additionalphase shift accumulated by the electrons in the ZBA regime. We attribute this effect tothe universal phase shift that electrons accumulate when crossing a Kondo singlet, reinforcingthat the debated origin of the ZBA lies in Kondo physics.Finally, we adapted the SGM technique to the study of thermoelectric transport in QPCs,and for the first time imaged interferences of electrons driven by a temperature difference

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Oxtoby, Neil Paul. "Keeping it real': A Quantum Trajectory Approach to Realistic Measurement of Solid-State Quantum Systems." Thesis, Griffith University, 2007. http://hdl.handle.net/10072/365770.

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To obtain information about a system of interest a measurement has to be made. In experiments that probe the quantum nature of our world, the system itself is, in general, necessarily affected by the act of measurement. If the system is weakly coupled to its bath and the dynamics are such that information concerning the system is spread throughout the many degrees of freedom of the bath, and the bath is being measured then a stochastic master equation for the conditioned state of the system can be found. This is termed a quantum trajectory equation. Realistic detectors are not perfect. Information is lost in the conversion to a signal that the observer can use. This loss may occur in the detector itself, in the circuit containing the detector (described by a response time and electronic noise) or at the circuit output (electronic output noise). In order to obtain a true quantum trajectory for the experiment, the observer must condition the state of the quantum system on results that are available in the laboratory rather than on the microscopic events considered previously in quantum trajectories. A method for treating this was first proposed by Warszawski, Wiseman and Mabuchi [Phys. Rev. A 65, 023802 (2002)], in which the quantum system is embedded within a supersystem that also contains the state of the detector. They applied their theory to photodetectors of various sorts. Warszawski has also done the preliminary work on applying this theory to detecting the state of a pair of quantum dots using a SET (single-electron transistor) [MSc. Thesis, Griffith University (2001)]. The resulting theory is termed 'realistic' quantum trajectory theory. In this thesis, the approach of Warszawski, et al.is applied to various solidstate readout devices. These include the SET, the QPC (quantum point contact), and the RF-QPC (radio-frequency QPC). Numerically obtained realistic quantum trajectories for the QPC agree with heuristic results. In particular, in certain limits, the realistic quantum trajectories can take on the appearance of ideal quantum trajectories. This thesis also resolves a problem in solid-state continuous quantum measurement theory by deriving a quantum trajectory model for a SET-monitored charge qubit, that guarantees physically meaningful qubit states. The particular limit necessary to achieve this is discussed, and the SET measurement quality is analysed using techniques borrowed from quantum optics. Conditions for which the SET can approach operation at the limit allowed by quantum mechanics are given. This is also done for the QPC, for which the results agree with previous work.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Science
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35

Dillard, Colin. "Quasiparticle Tunneling and High Bias Breakdown in the Fractional Quantum Hall Effect." Thesis, Harvard University, 2012. http://dissertations.umi.com/gsas.harvard:10334.

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The integer and fractional quantum Hall eﬀects arise in two-dimensional electron systems subject to low temperature and high perpendicular magnetic ﬁeld. The phenomenology of these two eﬀects is rich and provides interesting insight into quantum physics. We present two experimental studies of phenomena in the fractional quantum Hall regime. The ﬁrst examines the tunneling conductance of quasiparticles at ﬁlling factor 5/2. This state is of signiﬁcant interest because it lies outside the traditional Jain hierarchy of fractional quantum Hall states and because it may be the ﬁrst physical system found to exhibit non-abelian particle statistics. A quantum point contact is used to bring edge states on opposite sides of the system in proximity to each other, allowing quasiparticles to tunnel between the edge states. By annealing the gates forming the quantum point contact at diﬀerent voltages we control the tunneling strength for ﬁxed temperature and bias. We demonstrate a transition from strong to weak tunneling controlled in this manner. In the weak tunneling regime, the DC bias and temperature dependence of the tunneling conductance is ﬁt to a theoretical form, resulting in values for the quasiparticle charge $e*$ and the interaction parameter $g$. The values of these parameters are used to help distinguish between proposed candidate states for the 5/2 wave function. Quantitative and qualitative results are most consistent with the abelian 331 state. Our second main focus is the breakdown of the fractional quantum Hall states at ﬁlling factors 4/3 and 5/3. Breakdown of integer and fractional quantum Hall states is known to occur when the Hall and longitudinal resistances deviate from their ideal values at nonzero critical currents. Although multiple studies of breakdown in the integer quantum Hall regime have been reported, corresponding results for the fractional regime are scarce. We observe breakdown over a range of integer states that is consistent with previous results. However, breakdown in the fractional regime is found to exhibit markedly diﬀerent behavior. In particular, the magnitude of the critical current decreases with increased sample width. This behavior is opposite that observed for integer ﬁlling factors and does not seem to be explicable based on current theories of breakdown.
Physics

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Ma, Pengcheng. "Using multiplexers to study the statistics of quantum phenomenon in one-dimensional wires." Thesis, University of Cambridge, 2017. https://www.repository.cam.ac.uk/handle/1810/270301.

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The quantum point contact (QPC) is a one-dimensional constriction with the differential conductance quantised in units of $G_Q=2e^2/h$. However, the transport behaviour below the first plateau is still not fully understood, including the 0.7 anomaly and the 0.25 anomaly in the linear and non-linear transport regimes respectively. In this work, we utilise a multiplexing technique and statistically investigate the 0.7 anomaly observed on the first three plateaus respectively in 571 QPCs, fitting well the van-Hove model. The 0.7 anomaly shows the transconductance suppression due to the effective electron interactions which are modified by the local density of states (LDOS). At the maximum of LDOS, the interaction strength becomes strongest, resulting in the strongest transconductance suppression. The strongest interaction strength is determined by the ratio of transverse confinement curvature and longitudinal barrier curvature. Moreover, we realise measurements of the effective g factor ($g^*$) and high-field offset ($\Delta E^{hfo}$) in numerous devices in a single cooldown at T=40 mK. The statistical results show both the $g^*$ and $\Delta E^{hfo}$ increase with the potential confinement, which supports the predictions about the role of interaction strength on $g^*$ and $\Delta E^{hfo}$ in a 1D tight-binding model. We explore the origin of $\Delta E^{hfo}$ and find that it is only considerable for the first plateau. Using a short and narrow QPC could result in a stronger potential confinement and thus a higher $g^*$, which could be beneficial for its use in spintronic applications. Last, we investigate the formation and development of the DC-bias-induced 0.75 and 0.25 anomalies for 402 QPCs. We find the anomalies evolve similarly in a magnetic field. To explain the anomaly behaviours, we propose a phenomenological DC-bias-induced spin-splitting model. In the model, with the increasing DC bias (V_DC), the 0.75 anomaly occurs first at a differential conductance of 0.75 $G_Q$, while the 0.25 anomaly is formed at a differential conductance of 0.5 $G_Q$ and moves to 0.375 $G_Q$. The spin gap of the first subband opens to be e|V_DC|, which enables an all-electric manipulation of spin polarisation simply by applying a DC bias.

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Grémion, Emile. "Transistor balistique quantique et HEMT bas-bruit pour la cryoélectronique inférieure à 4. 2 K." Paris 11, 2008. http://www.theses.fr/2008PA112017.

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Pour augmenter la résolution globale des détecteurs à très basse température, aujourd'hui couramment utilisés dans de nombreux champs de la physique des particules et de l'univers, les expériences à venir ne pourront faire l'économie du développement d'une cryo-électronique performante, à la fois moins bruyante et plus proche du détecteur. Dans ce contexte, ce travail s'intéresse aux possibilités offertes par les gaz d'électrons bidimensionnels (2DEG) GaAlAs/GaAs à travers l'étude expérimentale de deux composants distincts : les QPC (Quantum Point Contact) et les HEMT (High Electron Mobility Transistor). En s'appuyant sur la quantification de la conductance dans les QPC, phénomène issu de la physique mésoscopique, un transistor balistique quantique fonctionnant à 4. 2 K a été réalisé. Le transport électronique à travers les bandes 1D permet d'obtenir un gain en tension supérieur à 1 avec une puissance dissipée d'environ 1 nW. En raison de leur très faible capacité d'entrée, ces dispositifs constituent également des candidats idéaux pour multiplexer des matrices de bolomètres haute impédance (collaboration DCMB). Les HEMT présentent des performances compatibles avec une utilisation à basse température, ayant une puissance dissipée de ~ 100 μW et un gain supérieur à 20. Le faible bruit en tension équivalent en entrée (1. 2 nV/Hz^(1/2) à 1 kHz et 0. 13 nV/Hz^(1/2) à 100 kHz) ouvre la voie à leur utilisation dans la lecture de détecteur de forte impédance. Conformément à la loi de Hooge, ces performances sont obtenues au détriment d'une capacité d'entrée élevée estimée à environ 60 pF
Next generations of cryodetectors, widely used in physics of particles and physics of universe, will need in the future high-performance cryoelectronics less noisy and closer to the detector. Within this context, this work investigates properties of two dimensional electron gas GaAlAs/GaAs by studying two components, quantum point contact (QPC) and high electron mobility transistor (HEMT). Thanks to quantized conductance steps in QPC, we have realized a quantum ballistic transistor (voltage gain higher than 1), a new component useful for cryoelectronics thanks to its operating temperature and weak power consumption (about 1 nW). Moreover, the very low capacity of this component leads to promising performances for multiplexing low temperature bolometer dedicated to millimetric astronomy. The second study focused on HEMT with very high quality 2DEG. At 4. 2 K, a voltage gain higher than 20 can be obtained with a very low power dissipation of less than 100 μW. Under the above experimental conditions, an equivalent input voltage noise of 1. 2 nV/Hz^(1/2) at 1 kHz and 0. 12 nV/Hz^(1/2) at 100 kHz has been reached. According to the Hooge formula, these noise performances are get by increasing gate capacity estimated to 60 pF

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38

Hakanen, Jani. "Modeling of nanostructures with complex source and drain." Thesis, Linköping University, The Department of Physics, Chemistry and Biology, 2004. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-4285.

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In this thesis we report on calculations for open quantum mechanical and certain microwave systems. The models refer to a quantum point contact and an electron cavity. We model this open system with an imaginary potential as source and drain, and use the finite difference method to make our calculations. We report on general features of the model we have found, and compare our calculations with measurements made on microwave cavities.

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39

Seoudi, Tarek. "Non-intrusive CdSe-based quantum dots for sensing pressure and temperature in lubricated contacts." Thesis, Lyon, 2020. http://www.theses.fr/2020LYSEI009.

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Cette thèse est dédiée à la mesure des pressions et des températures locales et à la comparaison de la génération de chaleur dans les contacts élastohydrodynamiques (EHD) de type tout acier et hybride (nitrure de silicium-acier). Le but ultime de ce travail est de développer une nouvelle technique in situ non-intrusive, exploitant la sensibilité de la photoluminescence (PL) des boîtes quantiques (QDs) de CdSe/CdS/ZnS aux variations de pression et température, afin de cartographier ces deux paramètres dans les contacts EHD. Dispersible à faible concentration dans les lubrifiants, il est montré que les QDs ne modifient pas le comportement rhéologique du fluide porteur et que le cisaillement n’est pas perturbateur à la réponse en PL. La calibration des QDs en suspension confirme la dépendance de la réponse en PL des QDs à la pression et à la température. Les mesures in situ sont effectuées en utilisant un banc d’essai bille-disque. La comparaison entre les mesures in situ de pression et de température et celles prédites à l'aide d'un modèle éléments finis TEHD interne montre une bonne concordance, ce qui démontre la faisabilité de la méthodologie proposée. Les effets du glissement et du chargement normal sur la pression, la température et la chaleur générée sont reportés. L’effet des propriétés thermiques des solides est souligné et la répartition de la chaleur générée entre les solides en contact est étudiée. L'équilibre énergétique entre l'énergie mécanique et l'énergie thermique interne générée par compression et cisaillement est démontré en comparant les pertes de puissance expérimentales et la chaleur générée issue du modèle numérique, pour des contacts acier-acier et hybrides
This thesis is dedicated to the measurement of local pressure and temperature and to compare the heat generation in all-steel and silicon nitride-steel (hybrid) elastohydrodynamic (EHD) contacts. The ultimate goal of this work is to develop a new non-intrusive in situ technique, exploiting the sensitivity of the photoluminescence (PL) of CdSe/CdS/ZnS quantum dots (QDs) to pressure and temperature. Dispersible in small concentration in lubricants, it is shown that the QDs doesn’t modify the rheological behavior of the carrier fluid and that shearing is not perturbative to the QDs PL response. The calibration of QDs in the suspension confirms the QDs PL dependence on temperature and pressure. The in situ measurements were conducted in EHD contacts using a ball-on-disc test rig. Comparisons between pressure and temperature measurements and predictions, using an in–house finite element thermal EHD model, showed a good agreement which demonstrates the feasibility of the proposed methodology. The effects of sliding and normal loading on pressure, temperature and heat generation are indicated. The effect of the thermal properties of the solid materials is underlined and the partition of the generated heat between the contacting solids is investigated. The energy equilibrium between the mechanical energy and the internal thermal energy generated by compression and shearing is demonstrated by comparing experimental power losses and numerical heat generation, in steel-steel and hybrid contacts

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40

Wan, Junjun. "Towards the realization of an all electrically controlled Spin Field Effect Transistor." University of Cincinnati / OhioLINK, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1292519781.

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Bhandari, Nikhil K. "Tunable All Electric Spin Polarizer." University of Cincinnati / OhioLINK, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1393237571.

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42

Ruiz-Tijerina, David A. "Kondo Physics and Many-Body Effects in Quantum Dots and Molecular Junctions." Ohio University / OhioLINK, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1385982088.

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Chen, Peng-Jen. "Density functional study on quantum point contacts." 2008. http://proquest.umi.com/pqdweb?did=1588785451&sid=4&Fmt=2&clientId=39334&RQT=309&VName=PQD.

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Thesis (M.S.)--State University of New York at Buffalo, 2008.
Title from PDF title page (viewed on Jan. 15, 2009) Available through UMI ProQuest Digital Dissertations. Thesis adviser: Han, Jong E. Includes bibliographical references.

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Chen, Wei-Ren, and 陳偉仁. "Electrical transport in multiple gated quantum point contacts." Thesis, 2005. http://ndltd.ncl.edu.tw/handle/zm3nk7.

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Yoon, Youngsoo. "Readout of electron spins in quantum point contacts." 2008. http://proquest.umi.com/pqdweb?did=1542160001&sid=8&Fmt=2&clientId=39334&RQT=309&VName=PQD.

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Thesis (Ph.D.)--State University of New York at Buffalo, 2008.
Title from PDF title page (viewed on Dec. 2, 2008) Available through UMI ProQuest Digital Dissertations. Thesis adviser: Bird, Jonathan P. Includes bibliographical references.

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Hsieh, Sung-Hsien, and 謝松憲. "Nonequilibrium Transport phenomena in GaAs Quantum Point Contacts." Thesis, 2003. http://ndltd.ncl.edu.tw/handle/37453322413045813462.

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唐志雄. "= Time-modulated effects on the quantum transport in quantum point contacts." Thesis, 1998. http://ndltd.ncl.edu.tw/handle/85670044905416118853.

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Zhang, Hao. "Electronic and Spin Correlations in Asymmetric Quantum Point Contacts." Diss., 2014. http://hdl.handle.net/10161/9036.

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A quantum point contact (QPC) is a quasi-one dimensional electron system, for which the conductance is quantized in unit of $2e^2/h$. This conductance quantization can be explained in a simple single particle picture, where the electron density of states cancels the electron velocity to a constant. However, two significant features in QPCs were discovered in the past two decades, which have drawn much attention: the 0.7 effect in the linear conductance and zero-bias-anomaly (ZBA) in the differential conductance. Neither of them can be explained by single particle pictures.

In this thesis, I will present several electron correlation effects discovered in asymmetric QPCs, as shown below:

The linear conductance of our asymmetric QPCs shows conductance resonances. The number of these resonances increases as the QPC channel length increases. The quantized conductance plateau is also modulated by tuning the gate voltage of the QPCs. These two features, observed in the linear conductance, are ascribed to the formation of quasi-bound states in the QPCs, which is further ascribed to the electron-correlation-induced barriers.

The differential conductance for long channel QPCs shows the zero-bias-anomaly for every other linear conductance resonance valley, suggesting a near even-odd behavior. This even-odd law can be interpreted within the electron-correlation-induced barrier picture, where the quasi-localized non-zero spin in the quasi-bound state (Kondo-like) couples to the Fermi sea in the lead. For a specific case, triple-peak structure is observed in the differential conductance curves, while the electron filling number is still even, suggesting a spin triplet formation at zero magnetic field.

Small differential conductance oscillations as a function of bias voltage were discovered and systematically studied in an asymmetric QPC sample. These oscillations are significantly suppressed in a low in-plane magnetic field, which is completely unexpected. The oscillations are washed out when the temperature is increased to 0.8K. Numerical simulation, based on the thermal smearing of the Fermi distribution, was performed to simulate the oscillation behavior at high temperatures, using the low temperature data as an input. This simulation agrees with the oscillations off zero-bias region, but does not agree with the temperature evolution of the structure near zero-bias. Based on the above oscillation characteristics, all simple single particle pictures were carefully considered, and then ruled out. After exhausting all these pictures, we think these small oscillations are related to novel electronic and spin correlations.

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羅東昇. "Quantum decoherence and spin-polarized currents in quantum wires — studied by parallel coupled double quantum point contacts." Thesis, 2013. http://ndltd.ncl.edu.tw/handle/852t6r.

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博士
國立清華大學
物理系
101
We employ coupled parallel double quantum point contacts to study the quantum decoherence and the transport of spin-polarized currents in quantum wires. The devices are fabricated on the GaAs/Al0.3Ga0.7As heterostructures with two-dimensional electron gas. In the experiment, the structure of quantum wires are formed on the layer of two-dimensional electron gas by means of quantum point contacts, and the electrical transport measurements are done at ultra-low temperatures. We study the phase coherence in a quantum-wire system by quantum interference phenomena, and besides we utilize the conductance additivity in a pair of parallel quantum wires to study the interaction between the quantum wires. Firstly, we use the Aharonove-Bohm oscillation to see how quantum interference depends on the mode number in parallel double quantum wires. We find that the magnitude of the oscillation decreases with the decrease of the mode number, accompanying fluctuation. The decrease of the oscillation magnitude can be understood as the result of the decrease of transmission probability, and the fluctuation possibly relates to the phase coherence. Then, we investigate the temperature dependence of the Ramsauer-type resonance to study the quantum decoherence in a quantum-wire system. We find that the temperature dependence of the Ramsauer-type resonance can be explained and analyzed by thermal averaging effect, and the tendency of the temperature dependence does not vary with the mode number in the system. Finally, we use conductance additivity as a tool to study the interaction between the spin-polarized currents through the coupled parallel double quantum wires. The conductance additivity is valid at zero magnetic field. However, when the electrons are spin-polarized at a high magnetic field parallel to the two-dimensional electron gas, the additivity is failed, and extra quantum states are observed. The breakdown of the additivity and the emergence of the extra states possibly result from the interaction between the spin-polarized currents.

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Keyser, Ulrich Felix [Verfasser]. "Nanolithography with an atomic force microscope : quantum point contacts, quantum dots, and quantum rings / von Ulrich Felix Keyser." 2002. http://d-nb.info/966282337/34.

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Dissertations / Theses on the topic 'Quantum point contacts'

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