arXiv subjects

Yuval Oreg

Publications and source records attributed to Yuval Oreg.

At least 145 records · Page 8Linked to original sources

arXiv2009

A new universality class of aging in glassy systems

Glassy systems are ubiquitous in nature, and are characterized by slow relaxations to equilibrium without a typical timescale, aging and memory effects. Understanding these is a long-standing problem in physics. We study the aging of the electron glass, a system showing remarkable slow relaxations in the conductance. We find that the broad distribution of relaxation rates leads to a universal relaxation of the form log(1+t_w/t) for the usual aging protocol, where t_w is the length of time the perturbation driving the system out-of-equilibrium was on, and t the time of measurement. These results agree well with several experiments performed on different glasses, and examining different physical observables, for times ranging from seconds to several hours. The suggested theoretical framework appears to offer a paradigm for aging in a broad class of glassy materials.

cond-mat.dis-nn↗

arXiv2008

Superconductor insulator transition in thin films driven by an orbital parallel magnetic field effect

We study theoretically orbital effects of a parallel magnetic field applied to a disordered superconducting film. We find that the field reduces the phase stiffness and leads to strong quantum phase fluctuations driving the system into an insulating behavior. This microscopic model shows that the critical field decreases with the sheet resistance, in agreement with recent experimental results. The predictions of this model can be used to discriminate spin and orbital effects. We find that experiments conducted by A. Johansson \textit{et al.} are more consistent with the orbital mechanism.

cond-mat.supr-con↗

arXiv2008

Interplay of mesoscopic and Kondo effects for transmission amplitude of few-level quantum dots

The magnitude and phase of the transmission amplitude of a multi-level quantum dot is calculated for the mesoscopic regime of level spacing large compared to level width. The interplay between Kondo correlations and the influence by neighboring levels is discussed. As in the single-level case, the Kondo plateaus of magnitude and phase disappear with increasing temperature. At certain gate voltages, ``stationary'' points are found at which the transmission phase is independent of temperature. Depending on the mesoscopic parameters of the adjacent levels (like relative sign and magnitude of tunneling matrix elements), the stationary points are shifted to or repelled by the neighboring level.

cond-mat.str-el↗

arXiv2008

Superconductor-to-normal transition in finite nanowires

In this paper we discuss the interplay of quantum fluctuations and dissipation in uniform superconducting nanowires. We consider a phenomenological model with superconducting and normal components, and a finite equilibration rate between these two-fluids. We find that phase-slip dipoles proliferate in the wire, and decouple the two-fluids within its bulk. This implies that the the normal fluid only couples to the superconductor fluid through the leads at the edges of the wire, and the {\it local} dissipation is unimportant. Therefore, while long wires have a superconductor-metal transition tuned by local properties of the superconducting fluid, short wires have a transition when the {\it total} resistance is $R_{total}=R_Q=h/4e^2$.

cond-mat.supr-con↗

arXiv2007

A mean-field model for the electron glass dynamics

We study a microscopic mean-field model for the dynamics of the electron glass, near a local equilibrium state. Phonon-induced tunneling processes are responsible for generating transitions between localized electronic sites, which eventually lead to the thermalization of the system. We find that the decay of an excited state to a locally stable state is far from being exponential in time, and does not have a characteristic time scale. Working in a mean-field approximation, we write rate equations for the average occupation numbers, and describe the return to the locally stable state using the eigenvalues of a rate matrix, A, describing the linearized time-evolution of the occupation numbers. Analyzing the probability distribution of the eigenvalues of A we find that, under certain physically reasonable assumptions, it takes the form $P(λ) \sim \frac{1}{|λ|}$, leading naturally to a logarithmic decay in time. While our derivation of the matrix A is specific for the chosen model, we expect that other glassy systems, with different microscopic characteristics, will be described by random rate matrices belonging to the same universality class of A. Namely, the rate matrix has elements with a very broad distribution, i.e., exponentials of a variable with nearly uniform distribution.

cond-mat.dis-nn↗

arXiv2007

Sharp Superconductor-Insulator Transition in Short Wires

Recent experiments on short MoGe nanowires show a sharp superconductor-insulator transition tuned by the normal state resistance of the wire, with a critical resistance of $R_c\approx R_Q= h/(4e^2)$. These results are at odds with a broad range of theoretical work on Josephson-like systems that predicts a smooth transition, tuned by the value of the resistance that shunts the junction. We develop a self-consistent renormalization group treatment of interacting phase-slips and their dual counterparts, correlated cooper pair tunneling, beyond the dilute approximation. This analysis leads to a very sharp transition with a critical resistance of $R_Q$. The addition of the quasi-particles' resistance at finite temperature leads to a quantitative agreement with the experimental results. This self-consistent renormalization group method should also be applicable to other physical systems that can be mapped onto similar sine-Gordon models, in the previously inaccessible intermediate-coupling regime.

cond-mat.supr-con↗

arXiv2007

Decays in Quantum Hierarchical Models

We study the dynamics of a simple model for quantum decay, where a single state is coupled to a set of discrete states, the pseudo continuum, each coupled to a real continuum of states. We find that for constant matrix elements between the single state and the pseudo continuum the decay occurs via one state in a certain region of the parameters, involving the Dicke and quantum Zeno effects. When the matrix elements are random several cases are identified. For a pseudo continuum with small bandwidth there are weakly damped oscillations in the probability to be in the initial single state. For intermediate bandwidth one finds mesoscopic fluctuations in the probability with amplitude inversely proportional to the square root of the volume of the pseudo continuum space. They last for a long time compared to the non-random case.

quant-ph↗

arXiv2007

Electron Pair Resonance in the Coulomb Blockade

We study many-body corrections to the cotunneling current via a localized state with energy $ε_d$ at large bias voltages $V$. We show that the transfer of {\em electron pairs}, enabled by the Coulomb repulsion in the localized level, results in ionization resonance peaks in the third derivative of the current with respect to $V$, centered at $eV=\pm 2ε_d/3$. Our results predict the existence of previously unnoticed structure within Coulomb-blockade diamonds.

cond-mat.mes-hall↗

arXiv2006

Supplementary Information for cond-mat/0610721: Potok et al. "Observation of the two-channel Kondo effect"

This document provides detailed descriptions of data acquisition and data analysis in support of the accompanying Article, cond-mat/0610721: Observation of the two-channel Kondo effect. Some of the most intriguing problems in solid state physics arise when the motion of one electron dramatically affects the motion of surrounding electrons. Traditionally, such highly-correlated electron systems have been studied mainly in materials with complex transition metal chemistry. Over the past decade, researchers have learned to confine one or a few electrons within a nanoscale semiconductor "artificial atom", and to understand and control this simple system in exquisite detail. In the accompanying Article, we combine such individually well-understood components to create a novel highly-correlated electron system within a nano-engineered semiconductor structure. We tune the system in situ through a quantum phase transition between two distinct states, one familiar and one subtly new. The boundary between these states is a quantum critical point: the exotic and previously elusive two-channel Kondo state, in which electrons in two reservoirs are entangled through their interaction with a single localized spin.

cond-mat.mes-hall↗

arXiv2006

Observation of the two-channel Kondo effect

Some of the most intriguing problems in solid state physics arise when the motion of one electron dramatically affects the motion of surrounding electrons. Traditionally, such highly-correlated electron systems have been studied mainly in materials with complex transition metal chemistry. Over the past decade, researchers have learned to confine one or a few electrons within a nanoscale semiconductor "artificial atom", and to understand and control this simple system in exquisite detail. In this Article, we combine such individually well-understood components to create a novel highly-correlated electron system within a nano-engineered semiconductor structure. We tune the system in situ through a quantum phase transition between two distinct states, one familiar and one subtly new. The boundary between these states is a quantum critical point: the exotic and previously elusive two-channel Kondo state, in which electrons in two reservoirs are entangled through their interaction with a single localized spin.

cond-mat.mes-hall↗

arXiv2006

Nonequilibrium charge-Kondo transport through negative-U molecules

Low-temperature transport through molecules with effectively negative charging energy U exhibits a charge-Kondo effect. We explore this regime analytically by establishing an exact mapping between the negative-U and the positive-U Anderson models, which is suitable for the description of nonequilibrium transport. We employ this mapping to demonstrate the intimate relation between nonequilibrium tranport in the spin-Kondo and charge-Kondo regimes, and derive analytical expressions for the nonlinear current-voltage chracteristics as well as the shot noise in the latter regime. Applying the mapping in the opposite direction, we elucidate the finding of super-Poissonian noise in the positive-U Anderson model at high temperatures, by relating the correlations between spin flips to pair-tunneling processes at negative U.

cond-mat.mes-hall↗

arXiv2006

Fractional Shot Noise in the Kondo Regime

Low temperature transport through a quantum dot in the Kondo regime proceeds by a universal combination of elastic and inelastic processes, as dictated by the low-energy Fermi-liquid fixed point. We show that as a result of inelastic processes, the charge detected by a shot-noise experiment is enhanced relative to the noninteracting situation to a universal fractional value, $e^*=5/3 e$. Thus, shot noise reveals that the Kondo effect involves many-body features even at low energies, despite its Fermi-liquid nature. We discuss the influence of symmetry breaking perturbations.

cond-mat.str-el↗

arXiv2005

Charge oscillations in Quantum Dots: Renormalization group and Hartree method calculations

We analyze the local level occupation of a spinless, interacting two-level quantum dot coupled to two leads by means of Wilson's numerical renormalization group method. A gate voltage sweep, causing a rearrangement of the charge such that the system's energy is minimized, leads to oscillations, and sometimes even inversions, in the level occupations. These charge oscillations can be understood qualitatively by a simple Hartree analysis. By allowing a relative sign in one tunneling matrix element between dot and leads, we extend our findings to more generic models.

cond-mat.mes-hall↗

arXiv2005

Superconductor-to-Metal Transitions in Dissipative Chains of Mesoscopic Grains and Nanowires

The interplay of quantum fluctuations and dissipation in chains of mesoscopic superconducting grains is analyzed, and the results are also applied to nanowires. It is shown that in 1-d arrays of resistively shunted Josephson junctions, the superconducting-normal charge relaxation within the grains plays an important role. At zero temperature, two superconducting phases can exist, depending primarily on the strength of the dissipation. In the fully superconducting phase (FSC), each grain acts superconducting, and the coupling to the dissipative conduction is important. In the SC* phase, the dissipation is irrelevant at long wavelengths. The phase transitions between these two superconducting phases and the normal metallic phase may be either local or global, and possess rich and complex critical properties. These are inferred from both weak and strong coupling renormalization group analyses. At intermediate temperatures, near either superconductor-to-normal phase transition, there are regimes of super-metallic behavior, in which the resistivity first decreases gradually with decreasing temperature before eventually increasing as temperature is lowered further. The results on chains of Josephson junctions are extended to continuous superconducting nanowires and the subtle issue of whether these can exhibit an FSC phase is considered. Potential relevance to superconductor-metal transitions in other systems is also discussed.

cond-mat.supr-con↗

arXiv2005

Adiabatic Pumping in Interacting Systems

A dc current can be pumped through an interacting system by periodically varying two independent parameters such as magnetic field and a gate potential. We present a formula for the adiabatic pumping current in general interacting systems, in terms of instantaneous properties of the system, and find the limits for its applicability. This formula generalizes the scattering approach for noninteracting pumps. We study the pumped spin in a system that exhibits the two-channel Kondo effect as an application of the adiabatic pumping formula. We find that a quantized spin of $\hbar$ is transferred between the two channels as the temperature approaches zero, and discuss the non-Fermi liquid features of this system at finite temperatures.

cond-mat.str-el↗

arXiv2005

Multiple Particle Scattering in Quantum Point Contacts

Recent experiments performed on weakly pinched quantum point contacts, have shown a resistance that tend to decrease at low source drain voltage. We show that enhanced Coulomb interactions, prompt by the presence of the point contact, may lead to anomalously large multiple-particle scattering at finite bias voltage. These processes tend to decrease at low voltage, and thus may account for the observed reduction of the resistance. We concentrate on the case of a normal point contact, and model it by a spinfull interacting Tomonaga-Luttinger liquid, with a single impurity, connected to non interacting leads. We find that sufficiently strong Coulomb interactions enhance two-electron scattering, so as these dominate the conductance. Our calculation shows that the effective charge, probed by the shot noise of such a system, approaches a value proportional to e* = 2e at sufficiently large backscattering current. This distinctive hallmark may be tested experimentally. We discuss possible applications of this model to experiments conducted on Hall bars.

cond-mat.str-el↗

arXiv2004

Thermopower of Single-Molecule Devices

We investigate the thermopower of single molecules weakly coupled to metallic leads. We model the molecule in terms of the relevant electronic orbitals coupled to phonons corresponding to both internal vibrations and to oscillations of the molecule as a whole. The thermopower is computed by means of rate equations including both sequential-tunneling and cotunneling processes. Under certain conditions, the thermopower allows one to access the electronic and phononic excitation spectrum of the molecule in a linear-response measurement. In particular, we find that the phonon features are more pronounced for weak lead-molecule coupling. This way of measuring the excitation spectrum is less invasive than the more conventional current-voltage characteristic, which, by contrast, probes the system far from equilibrium.

cond-mat.mes-hall↗

arXiv2004

Theory of Non-Coherent Spin Pumps

We study electron pumps in the absence of interference effects paying attention to the spin degree of freedom. Electron-electron exchange interactions combined with a variation of external parameters, such as magnetic field and gate potentials, affect the compressibility-spin-tensor whose components determine the non-coherent parts of the charge and spin pumped currents. An appropriate choice of the trajectory in the parameter space generates an arbitrary ratio of spin to charge pumped currents. After showing that the addition of dephasing leads to a full quantum coherent system diminishes the interference contribution, but leaves the non-coherent (classical) contribution intact, we apply the theory of the classical term for several examples. We show that when exchange interactions are included one can construct a source of pure spin current, with a constant magnetic field and a periodic variation of gate potentials only. We discuss the possibility to observe it experimentally in GaAs heterostructures.

cond-mat.str-el↗