arXiv subjects

Yuval Gefen

Publications and source records attributed to Yuval Gefen.

At least 163 records · Page 9Linked to original sources

arXiv2006

Transmission through Quantum Dots: Focus on Phase Lapses

Measurements of the transmission phase in transport through a quantum dot embedded in an Aharonov-Bohm interferometer show systematic sequences of phase lapses separated by Coulomb peaks. Using a two-level quantum dot as an example we show that this phenomenon can be accounted for by the combined effect of asymmetric dot-lead coupling and interaction-induced "population switching" of the levels, rendering this behavior generic. In addition, we use the notion of spectral shift function to analyze the relationship between transmission phase lapses and the Friedel sum rule.

cond-mat.str-el↗

arXiv2006

Electronic Mach-Zehnder interferometer as a tool to probe fractional statistics

We study transport through an electronic Mach-Zehnder interferometer recently devised at the Weizmann Institute. We show that this device can be used to probe statistics of quasiparticles in the fractional quantum Hall regime. We calculate the tunneling current through the interferometer as the function of the Aharonov-Bohm flux, temperature and voltage bias, and demonstrate that its flux-dependent component is strongly sensitive to the statistics of tunneling quasiparticles. More specifically, the flux-dependent and flux-independent contributions to the current are related by a power law, the exponent being a function of the quasiparticle statistics.

cond-mat.mes-hall↗

arXiv2005

Comment on "Do Intradot Electron-Electron Interactions Induce Dephasing?"

In a recent Letter, Jiang, Sun, Xie and Wang [Phys. Rev. Lett. 93, 076802 (2004), cond-mat/0408261] study transport through an interacting quantum dot embedded in one arm of an Aharonov-Bohm interferometer. Based on a theoretical analysis of the Aharonov-Bohm oscillation amplitude, Jiang {\it et al.} claim, contrary to earlier work by two of us, that at finite temperature the intradot interaction will {\em not} lead to any dephasing. Likewise, they claim that the theoretically predicted and experimentally verified asymmetry of the Aharonov-Bohm oscillation amplitude is {\em not} associated with dephasing. In this Comment, we point out severe inconsistencies in the analysis of the Letter by Jian {\it et al.}, and show that their conclusions are ill-founded.

cond-mat.mes-hall↗

arXiv2005

The Interplay of Spin and Charge Channels in Zero Dimensional Systems

We present a full fledged quantum mechanical treatment of the interplay between the charge and the spin zero-mode interactions in quantum dots. Quantum fluctuations of the spin-mode suppress the Coulomb blockade and give rise to non-monotonic behavior near this point. They also greatly enhance the dynamic spin susceptibility. Transverse fluctuations become important as one approaches the Stoner instability. The non-perturbative effects of zero-mode interaction are described in terms of charge (U(1)) and spin (SU(2)) gauge bosons.

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↗

arXiv2005

Geometric nature of the environment-induced Berry phase and geometric dephasing

We investigate the geometric phase or Berry phase (BP) acquired by a spin-half which is both subject to a slowly varying magnetic field and weakly-coupled to a dissipative environment (either quantum or classical). We study how this phase is modified by the environment and find that the modification is of a geometric nature. While the original BP (for an isolated system) is the flux of a monopole-field through the loop traversed by the magnetic field, the environment-induced modification of the BP is the flux of a quadrupole-like field. We find that the environment-induced phase is complex, and its imaginary part is a geometric contribution to dephasing. Its sign depends on the direction of the loop. Unlike the BP, this geometric dephasing is gauge invariant for open paths of the magnetic field.

cond-mat.mes-hall↗

arXiv2004

Theory of charge sensing in quantum-dot structures

Charge sensing in quantum-dot structures is studied by an exactly solvable reduced model and numerical density-matrix renormalization group methods. Charge sensing is characterized by the repeated cycling of the occupation of current-carrying states due to the capacitive coupling to trap states which are weakly coupled to the leads. In agreement with recent experiments, it results in a variety of characteristic behaviors ranging from asymmetric Coulomb-blockade peaks to sawtooth- and dome-like structures. Temperature introduces distinct asymmetric smearing of these features and correlations in the conductance provide a fingerprint of charge-sensing behavior.

cond-mat.mes-hall↗

arXiv2004

Nonmonotonic charge occupation in double dots

We study the occupation of two electrostatically-coupled single-level quantum dots with spinless electrons as a function of gate voltage. While the total occupation of the double-dot system varies monotonically with gate voltage, we predict that the competition between tunneling and Coulomb interaction can give rise to a nonmonotonic filling of the individual quantum dots. This non-monotonicity is a signature of the correlated nature of the many-body wavefunction in the reduced Hilbert space of the dots. We identify two mechanisms for this nonmonotonic behavior, which are associated with changes in the spectral weights and the positions, respectively, of the excitation spectra of the individual quantum dots. An experimental setup to test these predictions is proposed.

cond-mat.mes-hall↗

arXiv2004

Kinetic theory of fluctuations in conducting systems

We propose an effective field theory describing the time dependent fluctuations of electrons in conducting systems, generalizing the well known kinetic theory of fluctuations. On several examples, we show its equivalence, (when quantum corrections are neglected) to a microscopic quantum mechanical non-linear $σ$-model theory. We apply then the theory to analyze the effects of strong electron-electron and electron-phonon scattering on the statistics of current fluctuations. We find that if the electron-electron scattering length is much shorter than the transport mean free path the higher cumulants of current are parametrically enhanced.

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

Berry phase with environment: classical versus quantum

We discuss the concept of the Berry phase in a dissipative system. We show that one can identify a Berry phase in a weakly-dissipative system and find the respective correction to this quantity, induced by the environment. This correction is expressed in terms of the symmetrized noise power and is therefore insensitive to the nature of the noise representing the environment, namely whether it is classical or quantum mechanical. It is only the spectrum of the noise which counts. We analyze a model of a spin-half (qubit) anisotropically coupled to its environment and explicitly show the coincidence between the effect of a quantum environment and a classical one.

cond-mat.mes-hall↗

arXiv2003

Fluctuations and response in financial markets: the subtle nature of `random' price changes

Using Trades and Quotes data from the Paris stock market, we show that the random walk nature of traded prices results from a very delicate interplay between two opposite tendencies: long-range correlated market orders that lead to super-diffusion (or persistence), and mean reverting limit orders that lead to sub-diffusion (or anti-persistence). We define and study a model where the price, at any instant, is the result of the impact of all past trades, mediated by a non constant `propagator' in time that describes the response of the market to a single trade. Within this model, the market is shown to be, in a precise sense, at a critical point, where the price is purely diffusive and the average response function almost constant. We find empirically, and discuss theoretically, a fluctuation-response relation. We also discuss the fraction of truly informed market orders, that correctly anticipate short term moves, and find that it is quite small.

cond-mat.stat-mech↗

arXiv2003

Statistical Properties of The First Excited State of an Interacting Many Particle Disordered System

The distribution $P_1$ of the first many-body excitation energy of a weakly and moderately interacting electron gas in a finite conductor (in the diffusive regime) is calculated. As the interaction is increased, $P_1$ crosses over from Wigner-Dyson to Poisson. We characterize this transition through the inverse participation ratio in Hilbert space, and examine its manifestation in a projected 2-dimensional space.

cond-mat.dis-nn↗

arXiv2003

Berry phase in a non-isolated system

We investigate the effect of the environment on a Berry phase measurement involving a spin-half. We model the spin+environment using a biased spin-boson Hamiltonian with a time-dependent magnetic field. We find that, contrary to naive expectations, the Berry phase acquired by the spin can be observed, but only on timescales which are neither too short nor very long. However this Berry phase is not the same as for the isolated spin-half. It does not have a simple geometric interpretation in terms of the adiabatic evolution of either bare spin-states or the dressed spin-resonances that remain once we have traced out the environment. This result is crucial for proposed Berry phase measurements in superconducting nanocircuits as dissipation there is known to be significant.

cond-mat.mes-hall↗

arXiv2002

High cumulants of current fluctuations out of equilibrium

We consider high order current cumulants in disordered systems out of equilibrium. They are interesting and reveal information which is not easily exposed by the traditional shot noise. Despite the fact that the dynamics of the electrons is classical, the standard kinetic theory of fluctuations needs to be modified to account for those cumulants. We perform a quantum-mechanical calculation using the Keldysh technique and analyze its relation to the quasi classical Boltzmann-Langevin scheme. We also consider the effect of inelastic scattering. Strong electron-phonon scattering renders the current fluctuations Gaussian, completely suppressing the $n>2$ cumulants. Under strong electron-electron scattering the current fluctuations remain non-Gaussian.

cond-mat.mes-hall↗

arXiv2002

Shot Noise at High Temperatures

We consider the possibility of measuring non-equilibrium properties of the current correlation functions at high temperatures (and small bias). Through the example of the third cumulant of the current (${\cal{S}}_3$) we demonstrate that odd order correlation functions represent non-equilibrium physics even at small external bias and high temperatures. We calculate ${\cal{S}}_3=y(eV/T) e^2 I$ for a quasi-one-dimensional diffusive constriction. We calculate the scaling function $y$ in two regimes: when the scattering processes are purely elastic and when the inelastic electron-electron scattering is strong. In both cases we find that $y$ interpolates between two constants. In the low (high) temperature limit $y$ is strongly (weakly) enhanced (suppressed) by the electron-electron scattering.

cond-mat.mes-hall↗

arXiv2002

How backscattering off a point impurity can enhance the current and make the conductance greater than e^2/h per channel

It is well known that while forward scattering has no effect on the conductance of one-dimensional systems, backscattering off a static impurity suppresses the current. We study the effect of a time-dependent point impurity on the conductance of a one-channel quantum wire. At strong repulsive interaction (Luttinger liquid parameter g<1/2), backscattering renders the linear conductance greater than its value e^2/h in the absence of the impurity. A possible experimental realization of our model is a constricted quantum wire or a constricted Hall bar at fractional filling factors nu=1/(2n+1) with a time-dependent voltage at the constriction.

cond-mat.str-el↗

arXiv2002

Quantum Interferometry with Electrons: Outstanding Challenges

Recent experiments involving semiconducting quantum dots embedded in Aharonov-Bohm interferometry setups suggest that information concerning the phase of electron wavefunctions can be obtained from transport measurements. Here we review the basics of the theory of electron interferometry, some of the relevant experimental results, and recent theoretical developments attempting to shed light on the outstanding dilemmas.

cond-mat.mes-hall↗