arXiv subjects

Yigal Meir

Publications and source records attributed to Yigal Meir.

At least 73 records · Page 4Linked to original sources

arXiv2010

Theory of the Quantum Hall Insulator

The quantum Hall transition is one of the simplest and most studied quantum phase transitions. Nevertheless, the experimental observation of a new phase in this regime, the quantum Hall insulator, still remains a puzzle since the first report more than a decade ago, as it is in contradiction with all theoretical studies based on microscopically coherent quantum calculations. In this work we introduce into the coherent quantum theory a new ingredient - rare incoherent events, in a controlled manner. Using both direct numerical solutions and real-space renormalization, we demonstrate that these decoherence events stabilize the elusive quantum Hall insulator phase, which becomes even more stable with increasing temperature and voltage bias, in agreement with experiments.

cond-mat.dis-nn↗

arXiv2010

How to Measure the Transmission Phase via a Quantum Dot in a Two-Terminal Interferometer

Measurement of the transmission phase through a quantum dot (QD) embedded in an arm of a two-terminal Aharonov-Bohm (AB) interferometer is inhibited by phase symmetry, i.e. the property that the linear response conductance of a two-terminal device is an even function of magnetic field. It is demonstrated that in a setup consisting of an interferometer with a QD in each of its arms, with one of the QDs capacitively coupled to a nearby quantum point contact (QPC), phase symmetry is broken when a finite voltage bias is applied to the QPC. The transmission phase via the uncoupled QD can then be deduced from the amplitude of the odd component of the AB oscillations.

cond-mat.mes-hall↗

arXiv2010

Thermal Phase Transition in Two-Dimensional Disordered Superconductors: Kosterlitz-Thouless vs Percolation

Weakly disordered two-dimensional superconductors undergo a Kosterlitz-Thouless (KT) transition, where at a critical temperature vortices proliferate through the system and destroy the superconducting (SC) order. On the other hand, it was suggested that for large disorder the systems separates into regions of high SC order, and it is the percolation of coherence between these regions that is lost at the critical temperature. Here we demonstrate that these two descriptions are just the dual of each other. A vortex causes loss of local correlations, and thus the loss of percolation of correlations is concomitant with percolation of vortices on the dual lattice, in the perpendicular direction, i.e. the KT transition.

cond-mat.supr-con↗

arXiv2009

Breaking of Phase Symmetry in Non-Equilibrium Aharonov-Bohm Oscillations through a Quantum Dot

Linear response conductance of a two terminal Aharonov-Bohm (AB) interferometer is an even function of magnetic field. This "phase symmetry" is no expected to hold beyond the linear response regime. In simple AB rings the phase of the oscillations changes smoothly (almost linearly) with voltage bias. However, in an interferometer with a quantum dot in its arm, tuned to the Coulomb blockade regime, experiments indicate that phase symmetry seems to persist even in the nonlinear regime. In this letter we discuss the processes that break AB phase symmetry. In particular we show that breaking of phase symmetry in such an interferometer is possible only after the onset of inelastic cotunneling, i.e. when the voltage bias is larger than the excitation energy in the dot. The asymmetric component of AB oscillations is significant only when the contributions of different levels to the symmetric component nearly cancel out (e.g., due to different parity of these levels), which explains the sharp changes of the AB phase. We show that our theoretical results are consistent with experimental findings.

cond-mat.mes-hall↗

arXiv2007

Phase switching in a voltage-biased Aharonov-Bohm interferometer

Recent experiment [Sigrist et al., Phys. Rev. Lett. {\bf 98}, 036805 (2007)] reported switches between 0 and $π$ in the phase of Aharonov-Bohm oscillations of the two-terminal differential conductance through a two-dot ring with increasing voltage bias. Using a simple model, where one of the dots contains multiple interacting levels, these findings are explained as a result of transport through the interferometer being dominated at different biases by quantum dot levels of different "parity" (i.e. the sign of the overlap integral between the dot state and the states in the leads). The redistribution of electron population between different levels with bias leads to the fact that the number of switching events is not necessarily equal to the number of dot levels, in agreement with experiment. For the same reason switching does not always imply that the parity of levels is strictly alternating. Lastly, it is demonstrated that the correlation between the first switching of the phase and the onset of the inelastic cotunneling, as well as the sharp (rather than gradual) change of phase when switching occurs, give reason to think that the present interpretation of the experiment is preferable to the one based on electrostatic AB effect.

cond-mat.mes-hall↗

arXiv2007

Nature of the superconductor-insulator transition in disordered superconductors

As a superconducting thin film becomes disordered and subject to an increasing magnetic field, a point is reached when it undergoes a transition from a superconducting to an insulating state. We use the Bogoliubov-De-Gennes equations and a novel Monte-Carlo approach to study this transition numerically, starting from a microscopic hamiltonian. The key effect of disorder is to create 'islands' of strong superconductivity, coupled by regions that are only weakly superconducting. In the case of weak disorder, an increasing magnetic field eventually destroys the superconducting state throughout the material, leading to an insulator. On the other hand, when disorder is strong, superconductivity persists in the islands, and the effect of a magnetic field is to suppress the coupling between them, resulting in strong superconducting phase fluctuations, again leading to an insulating state. These findings may be relevant to the high-temperature superconductors, where intrinsic disorder may play a role.

cond-mat.supr-con↗

arXiv2007

The theory of the "0.7 anomaly" in quantum point contacts

The phenomenology of the "0.7 anomaly" in quantum point contacts is fully explained in terms of a quasi-localized state, which forms as the point contact opens up. Detailed numerical calculations within spin-density functional theory indeed con rm the emergence of such a state. Quantitative calculations of the conductance and the noise are obtained using a model based on these observations, and are in excellent agreement with existing experimental observations.

cond-mat.mes-hall↗

arXiv2007

Charge rearrangement and screening in a quantum point contact

Compressibility measurements, sensitive to charge rearrangements, are performed on a quantum point contact (QPC). Screening due to mobile charges in the QPC is quantitatively measured, using a second point contact to detect the screened electrical potential. These measurements are performed from pinch-off through the opening of the first few modes in the QPC. While the measured signal closely matches a Thomas-Fermi-Poisson prediction, deviations from the classical behavior, in the form of additional dips, are apparent near the openings of the different modes, with the largest dip at the opening of the first mode. Density functional calculations attribute the first dip to exchange interactions. The other dips reflect the diverging density of states at the opening of each one-dimensional mode, which affects both kinetic and exchange contributions to the energy.

cond-mat.mes-hall↗

arXiv2006

Magnetic impurity formation in quantum point contacts

A quantum point contact (QPC), a narrow region separating two wider electron reservoirs, is the standard building block of sub-micron devices, such as quantum dots - small boxes of electrons, and qubits - the proposed basic elements of quantum computers. As a function of its width, the conductance through a QPC changes in integer steps of G0 = $2e^2/h$, signalling the quantization of its tranverse modes.1,2 Such measurements also reveal an additional shoulder at a value around 0.7, an observation which remains a puzzle even after more than a decade. Recently it has been suggested5,6 that this phenomenon can be explained if one invokes the existence of a magnetic impurity in the QPC at low densities. Here we present extensive numerical density-functional calculations that reveal the formation of a magnetic moment in the channel as the density increases above pinch-off, under very general conditions. In addition we show that such an impurity will also form at large magnetic fields, for a specific value of the field (corresponding to a degeneracy point between the upper spin state in the first mode and the lower spin state in the second mode), and sometimes even at the opening of the second mode in the QPC. Beyond explaining the source of the "0.7 anomaly", these results may have far reaching implications on spin filling of electronic states in quantum dots and on dephasing of quantum information stored in semiconductor qubits.

cond-mat.mes-hall↗

arXiv2006

Suppression of Shot Noise in Quantum Point Contacts in the "0.7" Regime

Experimental investigations of current shot noise in quantum point contacts show a reduction of the noise near the 0.7 anomaly. It is demonstrated that such a reduction naturally arises in a model proposed recently to explain the characteristics of the 0.7 anomaly in quantum point contacts in terms of a quasi-bound state, due to the emergence of two conducting channels. We calculate the shot noise as a function of temperature, applied voltage and magnetic field, and demonstrate an excellent agreement with experiments. It is predicted that with decreasing temperature, voltage and magnetic field, the dip in the shot noise is suppressed due to the Kondo effect.

cond-mat.mes-hall↗

arXiv2005

Theory of Magneto-resistance of Disordered Superconducting Films

Recent experimental studies of magneto-resistance in disordered superconducting thin films reveal a huge peak (about 5 orders of magnitude). While it may be expected that magnetic field destroys superconductivity, leading to an enhanced resistance, attenuation of the resistance at higher magnetic fields is surprising. We propose a model which accounts for the experimental results in the entire range of magnetic fields, based on the formation of superconducting islands due to fluctuations in the superconducting order parameter amplitude. At strong magnetic fields Coulomb blockade in these islands gives rise to negative magneto-resistance. As the magnetic field is reduced the effect of Coulomb blockade diminishes and eventually the magneto-resistance changes sign. Numerical calculations show good qualitative agreement with experimental data.

cond-mat.mes-hall↗

arXiv2005

Quasi-particle Tunneling Through a Barrier in the Fractional Quantum Hall Regime

Tunneling of fractionally charged quasi-particles (QPs) through a barrier is considered in the context of a multiply connected geometry. In this geometry global constraints do not prohibit such a tunneling process. The tunneling amplitude is evaluated and the crossover from mesoscopic QP-dominated to electron-dominated tunneling as the system's size is increased is found. The presence of disorder enhances both electron and QP tunneling rates.

cond-mat.mes-hall↗

arXiv2004

A unifying model for several two-dimensional phase transitions

A relatively simple and physically transparent model based on quantum percolation and dephasing is employed to construct a global phase diagram which encodes and unifies the critical physics of the quantum Hall, "two-dimensional metal-insulator", classical percolation and, to some extent, superconductor-insulator transitions. Using real space renormalization group techniques, crossover functions between critical points are calculated. The critical behavior around each fixed point is analyzed and some experimentally relevant puzzles are addressed.

cond-mat.mes-hall↗

arXiv2004

Can Quasi-Particles Tunnel Through A Barrier?

This is a qualitative description of a systematic analysis carried out by us. We address the question of whether fractionally charged particles, in the context of the fractional quantum Hall effect (FQHE) can tunnel through a potential barrier (around which the density of the quantum Hall liquid is practically zero). Setting the barrier in a multiply-connected FQHE geometry removes the "global constraint" which prohibits such tunnelling. We have performed a microscopic analysis of adiabatic charge pumping and tunnelling in a torus geometry. Below we summarize our analysis in semi-qualitative terms. We also propose a setup-- different from the torus geometry-- amenable to experimental verification.

cond-mat.mes-hall↗

arXiv2004

Quantum Hall Criticality, Superconductor-Insulator Transition and Quantum Percolation

A model consisting of a mixture of superconducting and quantum links is proposed to describe the integer quantum Hall transition. The quantum links correspond to tunneling of electrons between trajectories trapped in adjacent potential valleys, while the superconducting links correspond to merging of these trajectories once the Fermi energy crosses the saddle point energy separating the two valleys. The quantum Hall transition in this model corresponds to percolation of the superconducting links. Numerical calculations and scaling analysis using two different approaches yield the critical exponent $ν\approx 2.4$ and a two-peak conductance distribution at the critical point. The role of quantum coherence is discussed, and an explanation of experimental observations claiming different universality class for the quantum Hall transition is suggested. The model suggests that the critical behavior of the superconductor-insulator transition from the insulating side is in the same universality class as the quantum Hall transition.

cond-mat.mes-hall↗

arXiv2002

Local moment formation in quantum point contacts

Spin-density-functional theory of quantum point contacts (QPCs) reveals the formation of a local moment with a net of one electron spin in the vicinity of the point contact - supporting the recent report of a Kondo effect in a QPC. The hybridization of the local moment to the leads decreases as the QPC becomes longer, while the onsite Coulomb-interaction energy remains almost constant.

cond-mat.mes-hall↗

arXiv2002

Luttinger liquid behavior in tunneling through a quantum dot at zero magnetic field

Thermodynamic and transport properties of a two dimensional circular quantum dot are studied theoretically at zero magnetic field. In the limit of a large confining potential, where the dot spectrum exhibits a shell structure, it is argued that both spectral and transport properties should exhibit Luttinger liquid behavior. These predictions are verified by direct numerical diagonalization. The experimental implications of such Luttinger liquid characteristics are discussed.

cond-mat.mes-hall↗

arXiv2002

Kondo model for the "0.7 anomaly" in transport through a quantum point contact

Experiments on quantum point contacts have highlighted an anomalous conductance plateau at $0.7 (2e^2/h)$, with features suggestive of the Kondo effect. Here we present an Anderson model for transport through a point contact which we analyze in the Kondo limit. Hybridization to the band increases abruptly with energy but decreases with valence, so that the background conductance and the Kondo temperature $T_K$ are dominated by different valence transitions. This accounts for the high residual conductance above $T_K$. A spin-polarized current is predicted for Zeeman splitting $g^* μ_B B > k_B T_K,k_BT$.

cond-mat.mes-hall↗