Quantum spin transport is studied in an interacting quantum dot. It is found that a conductance "plateau" emerges in the non-linear charge conductance by a spin bias in the Kondo regime. The conductance plateau, as a complementary to the Kondo peak, originates from the strong electron correlation and exchange processes in the quantum dot, and can be regarded as one of the characteristics in quantum spin transport.
The integer quantized conductance of one-dimensional electron systems is a well understood effect of quantum confinement. A number of fractionally quantized plateaus are also commonly observed. They are attributed to many-body effects, but their precise origin is still a matter of debate, having attracted considerable interest over the past 15 years. This review reports on experimental studies of fractionally quantized plateaus in semiconductor quantum point contacts and quantum wires, focusing on the 0.7 x 2e^2/h conductance anomaly, its analogs at higher conductances, and the zero bias peak …
Quantum point contact devices are indispensable tools for probing the edge structure of the fractional quantum Hall (FQH) states. Recent observations of quantized conductance plateaus accompanied by shot noise in such devices, as well as suppression of Mach-Zehnder interference, call for theoretical explanations. In this paper, we develop a theory of FQH edge state transport through quantum point contacts, which allows for a generic Abelian edge structure and assumes strong equilibration between edge modes (incoherent transport regime). We find that conductance plateaus are found whenever the …
The conductance of breaking metallic nanojunctions shows plateaus alternated with sudden jumps, corresponding to the stretching of stable atomic configurations and atomic rearrangements, respectively. We investigate the structure of the conductance plateaus both by measuring the voltage dependence of the plateaus' slope on individual junctions and by a detailed statistical analysis on a large amount of contacts. Though the atomic discreteness of the junction plays a fundamental role in the evolution of the conductance, we find that the fine structure of the conductance plateaus is determined b…
We address the question about the origin of the $\frac12 \frac{e^2}{h}$ conductance plateau observed in a recent experiment on an integer quantum Hall (IQH) film covered by a superconducting (SC) film. Since 1-dimensional (1D) chiral Majorana fermions on the edge of the above device can give rise to the half quantized plateau, such a plateau was regarded as a smoking-gun evidence for the chiral Majorana fermions. However, in this paper we give another mechanism for the $\frac12 \frac{e^2}{h}$ conductance plateau. We find the $\frac12 \frac{e^2}{h}$ conductance plateau to be a general feature o…
We propose to realize a two-dimensional chiral topological superconducting (TSC) state from the quantum anomalous Hall plateau transition in a magnetic topological insulator thin film through the proximity effect to a conventional $s$-wave superconductor. This state has a full pairing gap in the bulk and a single chiral Majorana mode at the edge. The optimal condition for realizing such chiral TSC is to have inequivalent superconducting pairing amplitudes on top and bottom surfaces of the doped magnetic topological insulator. We further propose several transport experiments to detect the chira…
The anomalous 0.5 and 0.7 conductance plateaus in quantum point contacts in zero magnetic field are analyzed within a phenomenological model. The model utilizes the Landauer-Buttiker formalism and involves enhanced spin correlations and thermal depopulation of spin subbands. In particular we can account for the plateau values 0.5 and 0.7, as well as the unusual temperature and magnetic field dependences of the 0.7 plateau. Finally, the model predicts the possibility of coexisting 0.5 and 0.7 plateaus.
We present a simple phenomenological model which offers a unifying interpretation of the experimental observations on the 0.7 conductance anomaly of quantum point contacts. The model utilizes the Landauer-Buttiker formalism and involves enhanced spin correlations and thermal depopulation of spin subbands. In particular our model can account for the plateau value 0.7 and the unusual temperature and magnetic field dependence. Furthermore it predicts an anomalous suppression of shot noise at the 0.7 plateau.
Ground-state properties of a double-large-dot sample connected to a reservoir via a single-mode point contact are investigated. When the interdot transmission is perfect and the dots controlled by the same dimensionless gate voltage, we find that for any finite backscattering from the barrier between the lead and the left dot, the average dot charge exhibits a Coulomb-staircase behavior with steps of size e/2 and the capacitance peak period is halved. The interdot electrostatic coupling here is weak. For strong tunneling between the left dot and the lead, we report a conspicuous intermediate p…
We show that partially separated Andreev bound states (ps-ABSs), comprised of pairs of overlapping Majorana bound states (MBSs) emerging in quantum dot-semiconductor-superconductor heterostructures, produce robust zero bias conductance plateaus in end-of-wire charge tunneling experiments. These plateaus remain quantized at $2e^2/h$ over large ranges of experimental control parameters. In light of recent experiments reporting the observation of robust $2e^2/h$-quantized conductance plateaus in local charge tunneling experiments, we perform extensive numerical calculations to explicitly show tha…
We study non-equilibrium differential conductance and current fluctuations in a single quantum point contact. The two-terminal electrical transport properties -- differential conductance and shot noise -- are measured at 1.5 K as a function of the drain-source voltage and the Schottky split-gate voltage. In differential conductance measurements, conductance plateaus appear at integer multiples of $2e^2/h$ when the drain-source voltage is small, and the plateaus evolve to a fractional of $2e^2/h$ as the drain-source voltage increases. Our shot noise measurements correspondingly show that the sh…
A weak superconducting proximity effect in the vicinity of the topological transition of a quantum anomalous Hall system has been proposed as a venue to realize a topological superconductor (TSC) with chiral Majorana edge modes (CMEMs). A recent experiment [Science 357, 294 (2017)] claimed to have observed such CMEMs in the form of a half-integer quantized conductance plateau in the two-terminal transport measurement of a quantum anomalous Hall-superconductor junction. Although the presence of a superconducting proximity effect generically splits the quantum Hall transition into two phase tran…
We analyze the conductance fluctuations observed in the quantum Hall regime for a bulk two-dimensional electron system in a Corbino geometry. We find that characteristics like the power spectral density and the temperature dependence agree well with simple expectations for universal conductance fluctuations in metals, while the observed amplitude is reduced. In addition, the dephasing length $L_Φ\propto T^{-1/2}$, which governs the temperature dependence of the fluctuations, is surprisingly different from the scaling length $L_{sc}\propto T^{-1}$ governing the width of the quantum Hall plateau…
We analyze the conductance of the quantum point contact containing large localized spin J. The additional plateau is formed on a ballistic conductance staircase if only one propagating channel is rendered conducting. The conductance value at this plateau is shown to depend strongly on J and decrease from 3e^2/2h to e^2/h when J increases from 1/2 to infinity, which is in a good agreement with the experimental observations [D.J. Reilly, et al, Phys. Rev. B 63, 121311 (2001)].
We study numerically conductance fluctuations near the integer quantum Hall effect plateau transition. The system is presumed to be in a mesoscopic regime, with phase coherence length comparable to the system size. We focus on a two-terminal conductance G for square samples, considering both periodic and open boundary conditions transverse to the current. At the plateau transition, G is broadly distributed, with a distribution function close to uniform on the interval between zero and one in units of e^2/h. Our results are consistent with a recent experiment by Cobden and Kogan on a mesoscopic…
We present detailed experimental studies of the temperature dependence of the plateau conductance of GaAs quantum point contacts in the temperature range from 0.3 K to 10 K. Due to a strong lateral confinement produced by a shallow-etching technique we are able to observe the following unexpected feature: a linear temperature dependence of the measured mid-plateau conductance. We discuss an interpretation in terms of a temperature dependent, intrinsic series resistance, due to non-ballistic effects in the 2D-1D transition region. These results have been reproduced in several samples from diffe…
We calculate the linear and nonlinear conductance of spinless fermions in clean, long quantum wires where short-ranged interactions lead locally to equilibration. Close to the quantum phase transition where the conductance jumps from zero to one conductance quantum, the conductance obtains an universal form governed by the ratios of temperature, bias voltage and gate voltage. Asymptotic analytic results are compared to solutions of a Boltzmann equation which includes the effects of three-particle scattering. Surprisingly, we find that for long wires the voltage predominantly drops close to one…
We present numerical renormalization group calculations for the zero-bias conductance of quantum dots made from semiconducting carbon nanotubes. These explain and reproduce the thermal evolution of the conductance for different groups of orbitals, as the dot-lead tunnel coupling is varied and the system evolves from correlated Kondo behavior to more weakly correlated regimes. For integer fillings $N=1,2,3$ of an SU(4) model, we find universal scaling behavior of the conductance that is distinct from the standard SU(2) universal conductance, and concurs quantitatively with experiment. Our resul…
In a recent paper [Physica E 39 (2007) 214, arXiv:0708.1842v1] Crisan, Grosu and Tifrea revisited the problem of the conductance through a double quantum dot molecule connected to electrodes in a T-shape configuration. The authors obtained an expression for the conductance that disagrees with previous results in the literature. We point out an error in their derivation of the conductance formula and show that it gives unphysical results even for non-interacting quantum dots.
Clean topological insulators exposed to a magnetic field develop Landau levels accompanied by a nonzero Hall conductivity for the infinite slab geometry. In this work we consider the case of disordered topological insulator nanowires and find, in contrast, that a zero Hall plateau emerges within a broad energy window close to the Dirac point. We numerically calculate the conductance and its distribution for a statistical ensemble of disordered nanowires, and use the conductance fluctuations to study the dependence of the insulating phase on system parameters, such as the nanowire length, disor…
We study the linear conductance through a double-quantum-dot system consisting of an interacting dot in its Kondo regime and an effectively noninteracting dot, connected in parallel to metallic leads. Signatures in the zero-bias conductance at temperatures $T>0$ mark a pair of quantum (T=0) phase transitions between a Kondo-screened many-body ground state and non-Kondo ground states. Notably, the conductance features become more prominent with increasing $T$, which enhances the experimental prospects for accessing the quantum-critical region through tuning of gate voltages in a single device.
A consistent approach in forming the 0.7 structure by using a quantum dot rather than a quantum point contact is demonstrated. With this scheme, it was possible to tune on and off the 0.7 structure. The 0.7 structure continuously evolved into a normal integer conductance plateau by varying the tuning condition. Unlike the conventional 0.7 plateau, the new 0.7 structure was observed even at low electron temperatures down to 100 mK, with unprecedented flatness. From our results, it is concluded that electron interference should be taken into consideration to explain the 0.7 structure.
The interplay between groups of water molecules and single-atom contacts, as reflected in the electrical conductances and mechanical forces of copper atomic junctions, is explored by means of first-principles theory and semi-empirical calculations. We study the influence of the atomic geometries of copper electrodes with pyramidal and non-crystalline structures in the presence and absence of water on the conductance profiles as the electrodes approach each other. It is shown that the atomic arrangements of nano-contacts have crucial effects on the formation of plateaus and the conductance valu…
We propose that two-channel orbital Kondo ``spin'' 1/2 conductance can be measured in a quantum dot at Coulomb Blockade with an odd number of electrons with contacts in a pillar configuration, if an orthogonal magnetic field induces an appropriate level crossing. At the zero-temperature strong coupling fixed point the conductance reaches the unitarity limit with a non-Fermi liquid sqrt(T)-law.
A numerical renormalization-group study of the conductance through a quantum wire side-coupled to a quantum dot is reported. The temperature and the dot-energy dependence of the conductance are examined in the light of a recently derived linear mapping between the Kondo-regime temperature-dependent conductance and the universal function describing the conductance for the symmetric Anderson model of a quantum wire with an embedded quantum dot. Two conduction paths, one traversing the wire, the other a bypass through the quantum dot, are identified. A gate potential applied to the quantum wire i…
The zero bias conductance of quantum dots coupled to ferromagnetic leads is investigated. In the strong coupling regime, it is found that the conductance is a non-monotonic function of the angle between the magnetisation directions in the two contacts. This behaviour is an effect of the presence of the leads which induces an angle dependent spin split of the quantum dot states, and spin flip transitions between the quantum dot states whenever the magnetisation directions of the leads are non-collinear which enhances the current density at the chemical potential. In the weak coupling regime, th…
The electrostatic interaction between two capacitively-coupled metal double-dots is studied at low temperatures. Experiments show that when the Coulomb blockade is lifted by applying appropriate gate biases to both double-dots, the conductance through each double-dot becomes significantly lower than when only one double-dot is conducting. A master equation is derived for the system and the results obtained agree well with the experimental data. The model suggests that the conductance lowering in each double-dot is caused by a single-electron tunneling in the other double-dot. Here, each double…
We report on magnetic field dependence of half-integer quantized conductance plateaus (HQPs) in InAs quantum wires. We observed HQPs at zero applied magnetic field in InAs quantum wires fabricated from a high-quality InAs quantum well. The application of in-plane magnetic field causes Zeeman splitting of the HQP features, indicating that the origin of the observed HQP is not spontaneous spin polarization. Additionally we observe that conductance of the split HQPs decreases gradually as the in-plane magnetic field increases. We finally assume electron-electron interaction as a possible mechanis…
The conductance through two quantum dots in series is studied using general qualitative arguments and quantitative slave-boson mean-field theory. It is demonstrated that measurements of the conductance can explore the phase diagram of the two-impurity Anderson model. Competition between the Kondo effect and the inter-dot magnetic exchange leads to a two-plateau structure in the conductance as a function of gate voltage and a two or three peak structure in the conductance vs. inter-dot tunneling.
The conductance of a two-dimensional electron gas at the transition from one quantum Hall plateau to the next has sample-specific fluctuations as a function of magnetic field and Fermi energy. Here we identify a universal feature of these mesoscopic fluctuations in a Corbino geometry: The amplitude of the magnetoconductance oscillations has an e^2/h resonance in the transition region, signaling a change in the topological quantum number of the insulating bulk. This resonance provides a signed scaling variable for the critical exponent of the phase transition (distinct from existing positive de…
A numerically exact calculation of the T=0 transport properties of a quantum wire interacting with a lateral two-level quantum dot is presented. The wire conductance is calculated for all different states of charge and spin of the quantum dot. For a dot with two electrons we obtain an enhancement of the Kondo temperature at the singlet-triplet transition and a non-universal scaling law for its dependence upon the dot energy spacing. We find that the Kondo correlation is stronger for a dot spin $S_D \sim 1$ than for $S_D \sim 1/2$. In both cases the wire current is totally quenched by the Kondo…
We study the electronic transport through a quantum wire (QW) with a strong side coupled quantum dot (QD). We obtain a linear conductance with lateral peaks when the gate voltage, is located near the edge of the conduction band. The calculated density of states shows, that these peaks are associated with renormalized localized levels out side of the conduction band. These results are compatibles with recent experimental results.
Under periodic boundary condition in the transverse direction, we calculate the averaged zero-temperature two-terminal conductance ($<G>$) and its statistical fluctuations ($<(\dg)^{2n}>$ for $n\le 4$) at the critical point of integer quantum Hall plateau transitions. We find {\it universal} values for $<G>=(0.58\pm0.03){e^2\over h}$, and $<(\dg)^{2n}>=({e^2\over h})^{2n}A_{2n}$, where $A_{2,4,6,8}=0.081\pm0.005$; $0.013\pm0.003$; $0.0026\pm0.005$; and $(8\pm2)\times10^{-4}$ respectively. We also determine the leading finite size scaling corrections to these observables. Comparisons with exper…
We study the Kondo effect in a single quantum dot system with two or three orbitals by using the Bethe-ansatz exact solution at zero temperature and the non-crossing approximation at finite temperatures. For the two-orbital Kondo effect, the conductance is shown to be constant at absolute zero in any magnetic fields, but decrease monotonically with increasing fields at finite temperatures. In the case with more orbitals, the conductance increases at absolute zero, while it features a maximum structure as a function of the magnetic field at finite temperatures. We discuss how these characterist…
The observation of a 0.5 conductance plateau in asymmetrically biased quantum point contacts with in-plane side gates has been attributed to the onset of spin-polarized current through these structures. For InAs quantum point contacts with the same width but longer channel length, there is roughly a fourfold increase in the range of common sweep voltage applied to the side gates over which the 0.5 conductance plateau is observed when the QPC aspect ratio (ratio of length over width of the narrow portion of the structure) is increased by a factor 3. Non-equilibrium Green s function simulations …
The appearance and evolution of an anomalous conductance plateau at 0.4 (in units of 2e2/h) in an In0.52Al0.48As/InAs quantum point contact (QPC), in the presence of lateral spin-orbit coupling, has been studied at T=4.2K as a function of the potential asymmetry between the in-plane gates of the QPC. The anomalous plateau, a signature of spin polarization in the channel, appears only over an intermediate range (around 3 V) of bias asymmetry. It is quite robust, being observed over a maximum range of nearly 1V of the sweep voltage common to the two in-plane gates. Our conductance measurements s…
Using exact-diagonalization of small clusters and Dyson equation embedding techniques, the conductance $G$ of linear arrays of quantum dots is investigated. The Hubbard interaction induces Kondo peaks at low temperatures for an odd number of dots. Remarkably, the Kondo peak is split in half by a deep minimum, and the conductance vanishes at one value of the gate voltage. Tentative explanations for this unusual effect are proposed, including an interference process between two channels contributing to $G$, with one more and one less particle than the exactly-solved cluster ground-state. The Hub…
Majorana zero-modes (MZMs) are spatially-localized zero-energy fractional quasiparticles with non-Abelian braiding statistics that hold a great promise for topological quantum computing. Due to its particle-antiparticle equivalence, an MZM exhibits robust resonant Andreev reflection and 2e2/h quantized conductance at low temperature. By utilizing variable-tunnel-coupled scanning tunneling spectroscopy, we study tunneling conductance of vortex bound states on FeTe0.55Se0.45 superconductors. We report observations of conductance plateaus as a function of tunnel coupling for zero-energy vortex bo…
The transport properties of an interacting one-dimensional quantum dot capacitively coupled to an atomic force microscope probe are investigated. The dot is described within a Luttinger liquid framework which captures both Friedel and Wigner oscillations. In the linear regime, we demonstrate that both the conductance peak position and height oscillate as the tip is scanned along the dot. A pronounced beating pattern in the conductance maximum is observed, connected to the oscillations of the electron density. Signatures of the effects induced by a Wigner molecule are clearly identified and the…
Quantum transport properties in quantum Hall wires in the presence of spatially correlated random potential are investigated numerically. It is found that the potential correlation reduces the localization length associated with the edge state, in contrast to the naive expectation that the potential correlation increases it. The effect appears as the sizable shift of quantized conductance plateaus in long wires, where the plateau transitions occur at energies much higher than the Landau band centers. The scale of the shift is of the order of the strength of the random potential and is insensit…
We interpret the recent observation of a zero-bias anomaly in spin-1 quantum dots in terms of an underscreened Kondo effect. Although a spin-1 quantum dots are expected to undergo a two-stage quenching effect, in practice the log normal distribution of Kondo temperatures leads to a broad temperature region dominated by underscreened Kondo physics. General arguments, based on the asymptotic decoupling between the partially screened moment and the leads, predict a singular temperature and voltage dependence of the conductance $G$ and differential conductance $g$, resulting in $dg/dT\sim 1/T$ and…
Electron tunneling through a two stage Kondo system constituted by a double quantum-dot molecule side coupled to a quantum wire, under the effect of a finite external potential is studied. We found that $I$-$V$ characteristic shows a negative differential conductance region induced by the electronic correlation. This phenomenon is a consequence of the properties of the two stage Kondo regime under the effect of an external applied potential that takes the system out of equilibrium. The problem is solved using the mean-field finite-$U$ slave-boson formalism.
We discuss the calculation of the linear conductance through a Coulomb-blockade quantum dot in the presence of interactions beyond the charging energy. In the limit where the temperature is large compared with a typical tunneling width, we use a rate-equations approach to describe the transitions between the corresponding many-body states. We discuss both the elastic and rapid-thermalization limits, where the rate of inelastic scattering in the dot is either small or large compared with the elastic transition rate, respectively. In the elastic limit, we find several cases where a closed soluti…
We propose a simple approach to study the conductance through an array of $N$ interacting quantum dots, weakly coupled to metallic leads. Using a mapping to an effective site which describes the low-lying excitations and a slave-boson representation in the saddle-point approximation, we calculated the conductance through the system. Explicit results are presented for N=1 and N=3: a linear array and an isosceles triangle. For N=1 in the Kondo limit, the results are in very good agreement with previous results obtained with numerical renormalization group (NRG). In the case of the linear trimer …
We demonstrate experimentally that the transitions between adjacent integer quantum Hall (QH) states are equivalent to a QH-to-insulator transition occurring in the top Landau level, in the presence of an inert background of the other completely filled Landau levels, each contributing a single unit of quantum conductance, $e^{2}/h$, to the total Hall conductance of the system.
Motivated by recent experiments, we consider a single-electron transistor in the Kondo regime which is coupled to three leads in the presence of large bias voltages. Such a steady-state non-equilibrium system is to a large extent governed by a decoherence rate induced by the current through the dot. As the two-terminal conductance turns out to be rather insensitive to the decoherence rate, we study the conductance in a three-terminal device using perturbative renormalization group and calculate the characteristic splitting of the Kondo resonance. The interplay between potential biases and anis…
We investigate a lateral semiconductor quantum dot with a large number of electrons in the limit of strong coupling to the leads. A Kondo effect is observed and can be tuned in a perpendicular magnetic field. This Kondo effect does not exhibit Zeeman splitting. It shows a modulation with the periodicity of one flux quantum per dot area at low temperatures. The modulation leads to a novel, strikingly regular stripe pattern for a wide range in magnetic field and number of electrons.
The transport properties of a double quantum-dot device with one of the dots coupled to perfect conductors are analyzed using the numerical renormalization group technique and slave-boson mean-field theory. The coupling between the dots strongly influences the transport through the system leading to a non-monotonic dependence of the conductance as a function of the temperature and the magnetic field. For small inter-dot coupling and parameters such that both dots are in the Kondo regime, there is a two-stage screening of the dot's magnetic moments that is reflected in the conductance. In an in…
We explore the effects of asymmetry of hopping parameters between double parallel quantum dots and the leads on the conductance and a possibility of local magnetic moment formation in this system using functional renormalization group approach with the counterterm. We demonstrate a possibility of a quantum phase transition to a local moment regime (so called singular Fermi liquid (SFL) state) for various types of hopping asymmetries and discuss respective gate voltage dependences of the conductance. It is shown, that depending on the type of the asymmetry, the system can demonstrate either a f…
Using exact-diagonalization techniques supplemented by a Dyson equation embedding procedure, the transport properties of multilevel quantum dots are investigated in the Kondo regime. The conductance can be decomposed into the contributions of each level. It is shown that these channels can carry a different phase, and destructive interference processes are observed when the phase difference between them is $\pmπ$. This effect is very different from those observed in bulk metals with magnetic impurities, where the phase differences play no significant role. The effect is also different from oth…
Probing an isolated Majorana zero mode is predicted to reveal a tunneling conductance quantized at $2e^2/h$ at zero temperature. Experimentally, a zero-bias peak (ZBP) is expected and its height should remain robust against relevant parameter tuning, forming a quantized plateau. Here, we report the observation of large ZBPs in a thin InAs-Al hybrid nanowire device. The ZBP height can stick close to $2e^2/h$, mostly within $5\%$ tolerance, by sweeping gate voltages and magnetic field. We further map out the phase diagram and identify two plateau regions in the phase space. Despite the presence …
We present analytic results for the finite-frequency current noise and the nonequilibrium ac conductance for a Kondo quantum dot in presence of a magnetic field. Using the real-time renormalization group method, we determine the line shape close to resonances and show that while all resonances in the ac conductance are broadened by the transverse spin relaxation rate, the noise at finite field additionally involves the longitudinal rate as well as sharp kinks resulting in singular derivatives. Our results provide a consistent theoretical description of recent experimental data for the emission…
The $ν= 2/3$ fractional quantum Hall state is the hole-conjugate state to the primary Laughlin $ν= 1/3$ state. We investigate transmission of edge states through quantum point contacts fabricated on a GaAs/AlGaAs heterostructure designed to have a sharp confining potential. When a small but finite bias is applied, we observe an intermediate conductance plateau with $G = 0.5 \frac{e^2}{h}$. This plateau is observed in multiple QPCs, and persists over a significant range of magnetic field, gate voltage, and source-drain bias, making it a robust feature. Using a simple model which considers scatt…
We derive a closed expression for the finite-temperature conductance of a Coulomb-blockade quantum dot in the presence of an exchange interaction and a parallel magnetic field. Parallel-field dependence of Coulomb-blockade peak position has been used to determine experimentally the ground-state spin of quantum dots. We find that for a realistic value of the exchange interaction, the peak motion can be significantly affected at temperatures as low as kT ~ 0.1 Delta, with Delta being the mean level spacing in the dot. This temperature effect can lead to misidentification of the ground-state spin…
We obtain a ``mean field'' scaling flow of the longitudinal and the Hall conductivities in the fractional quantum Hall regime. Using the composite fermion picture and assuming that the composite fermions follow the Khmelnitskii-Pruisken scaling flow for the integer quantum Hall effect, the unstable fixed points which govern the transitions between different fractional quantum Hall states are identified. Distributions of the critical longitudinal and Hall conductivities at the unstable fixed points are obtained and implications of the results for the experiments on mesoscopic quantum Hall syste…
Conductance, on-site and inter-site charge fluctuations and spin correlations in the system of two side-coupled quantum dots are calculated using the Wilson's numerical renormalization group (NRG) technique. We also show spectral density calculated using the density-matrix NRG, which for some parameter ranges remedies inconsistencies of the conventional approach. By changing the gate voltage and the inter-dot tunneling rate, the system can be tuned to a non-conducting spin-singlet state, the usual Kondo regime with odd number of electrons occupying the dots, the two-stage Kondo regime with two…
In a recent work [L. Merker, S. Kirchner, E. Munoz, and T. A. Costi, Phys. Rev. B 87, 165132 (2013)], the authors compared results of numerical renormalization group and a perturbative approach for the dependence on temperature T and magnetic field B of the conductance through a quantum dot described by the impurity Anderson model, for small T and B. We show that the equation used to extract the dependence on B from NRG results is incorrect out of the particle-hole symmetric case. As a consequence, in the Kondo regime, the correct NRG results have a weaker dependence on B and the disagreement …
We study finite-frequency transport properties of the double-dot system recently constructed to observe the two-channel Kondo effect [R. M. Potok et al., Nature 446, 167 (2007)]. We derive an analytical expression for the frequency-dependent linear conductance of this device in the Kondo regime. We show how the features characteristic of the 2-channel Kondo quantum critical point emerge in this quantity, which we compute using the results of conformal field theory as well as numerical renormalization group methods. We determine the universal cross-over functions describing non-Fermi liquid vs.…
A non-equilibrium Green function formalism (NEGF) is used to study the conductance of a side-gated quantum point contact (QPC) in the presence of lateral spin-orbit coupling (LSOC). A small difference of bias voltage between the two side gates (SGs) leads to an inversion asymmetry in the LSOC between the opposite edges of the channel. In single electron modeling of transport, this triggers a spontaneous but insignificant spin polarization in the QPC. However, the spin polarization of the QPC is enhanced substantially when the effect of electron-electron interaction is included. The spin polari…
Electron transport through parallel double quantum dot system with interdot tunneling and strong on-site Coulomb interaction is studied in the Kondo regime by using the finite-$U$ slave boson technique. For a system of quantum dots with degenerate energy levels, the linear conductance reaches the unitary limit ($2e^2/h$) due to the Kondo effect at low temperature when the interdot tunneling is absent. As the interdot tunneling amplitude increases, the conductance decreases in the singly occupied regime and a conductance plateau structure appears. In the crossover to the doubly occupied regime,…
