arXiv subjects

Ady Stern

Publications and source records attributed to Ady Stern.

At least 163 records · Page 9Linked to original sources

arXiv2005

The physical picture behind the oscillating sign of drag in high Landau levels

We consider the oscillating sign of the drag resistivity and its anomalous temperature dependence discovered experimentally in a bi-layer system in the regime of the integer quantum Hall effect. We attribute the oscillating sign to the effect of disorder on the relation between an adiabatic momentum transfer to an electron and the displacement of its position. While in the absence of any Landau level mixing a momentum transfer $\hbar \bf q$ implies a displacement of $ql_H^2$ (with $l_H$ being the magnetic length), Landau level mixing induced by short range disorder adds a potentially large displacement that depends on the electron's energy, with the sign being odd with respect to the distance of that energy from the center of the Landau level. We show how the oscillating sign of drag disappears when the disorder is smooth and when the electronic states are localized.

cond-mat.mes-hall↗

arXiv2005

Electronic transport in an array of quasi-particles in the nu=5/2 non-abelian quantum Hall state

The Moore-Read Pfaffian $ν=5/2$ quantum Hall state is a p-wave super-conductor of composite fermions. Small deviations from $ν=5/2$ result in the formation of an array of vortices within this super-conductor, each supporting a Majorana zero mode near its core. Here we consider how tunneling between these cores is reflected in the electronic response to an electric field of non-zero wave vector $\bf q$ and frequency $ω$. We find a mechanism for dissipative transport at frequencies below the $ν=5/2$ gap, and calculate the ${\bf q},ω$ dependence of the dissipative conductivity. The contributions we find depend exponentially on $|ν-5/2|^{-1/2}$.

cond-mat.mes-hall↗

arXiv2005

Proposed experiments to probe the non-abelian ν=5/2 quantum Hall state

We propose several experiments to test the non-abelian nature of quasi-particles in the fractional quantum Hall state of ν=5/2. One set of experiments studies interference contribution to back-scattering of current, and is a simplified version of an experiment suggested recently. Another set looks at thermodynamic properties of a closed system. Both experiments are only weakly sensitive to disorder-induced distribution of localized quasi-particles.

cond-mat.mes-hall↗

arXiv2005

Local Measurements of Magnetization in Mn12 Crystals

The spatial profile of the magnetization of Mn12 crystals in a swept magnetic field applied along the easy axis is determined from measurements of the local magnetic induction along the sample surface using an array of Hall sensors. We find that the magnetization is not uniform inside the sample, but rather shows some spatial oscillations which become more prominent around the resonance field values. Moreover, it appears that different regions of the sample are at resonance at different values of the applied field and that the sweep rate of the internal magnetic induction is spatially non-uniform. We present a model which describes the evolution of the non-uniformities as a function of the applied field. Finally we show that the degree of non-uniformity can be manipulated by sweeping the magnetic field back and forth through part of the resonance.

cond-mat.mes-hall↗

arXiv2005

Localized excitations at the Mott Insulator-Superfluid interfaces for confined Bose-Einstein condensates

In this paper we derive the dispersion relation of the surface waves at the interfaces between Mott insulating and Superfluid phases for confined Bose-Einstein Condensates. We then calculate their contribution to the heat capacity of the system and show how its low temperature scaling allows an experimental test of the existence and properties of Mott Insulator-Superfluid domains.

cond-mat.other↗

arXiv2004

Coulomb drag between quantum wires with different electron densities

We study the way back-scattering electron--electron interaction generates Coulomb drag between quantum wires with different densities. At low temperature $T$ the system can undergo a commensurate-- incommensurate transition as the potential difference $|W|$ between the two wires passes a critical value $Δ$, and this transition is reflected in a marked change in the dependence of drag resistivity on $W$ and $T$. At high temperature a density difference between the wires suppresses Coulomb drag induced by back scattering, and we use the Tomonaga--Luttinger model to study this suppression in detail.

cond-mat.mes-hall↗

arXiv2003

Geometric phases and quantum entanglement as building blocks for nonabelian quasiparticle statistics

Some models describing unconventional fractional quantum Hall states predict quasiparticles that obey nonabelian quantum statistics. The most prominent example is the Moore-Read model for the $ν=5/2$ state, in which the ground state is a superconductor of composite fermions, and the charged excitations are vortices in that superconductor. In this paper we develop a physical picture of the nonabelian statistics of these vortices. Considering first the positions of the vortices as fixed, we define a set of single-particle states at and near the core of each vortex, and employ general properties of the corresponding Bogolubov-deGennes equations to write the ground states in the Fock space defined by these single-particle states. We find all ground states to be entangled superpositions of all possible occupations of the single-particle states near the vortex cores, in which the probability for all occupations is equal, and the relative phases vary from one ground state to another. Then, we examine the evolution of the ground states as the positions of the vortices are braided. We find that as vortices move, they accumulate a geometric phase that depends on the occupations of the single-particle states near the cores of other vortices. Thus, braiding of vortices changes the relative phase between different components of a superposition, in which the occupations of the single-particle states differ, and hence transform the system from one ground state to another. These transformations are the source of the nonabelian statistics. Finally, by exploring a "self-similar" form of the many-body wave functions of the ground states, we show the equivalence of our picture and pictures derived previously, in which vortex braiding seemingly affects the occupations of states in the cores of the vortices.

cond-mat.mes-hall↗

arXiv2003

A dc voltage step-up transformer based on a bi-layer ν=1 quantum Hall system

A bilayer electron system in a strong magnetic field at low temperatures, with total Landau level filling factor nu =1, can enter a strongly coupled phase, known as the (111) phase or the quantum Hall pseudospin-ferromagnet. In this phase there is a large quantized Hall drag resistivity between the layers. We consider here structures where regions of (111) phase are separated by regions in which one of the layers is depleted by means of a gate, and various of the regions are connected together by wired contacts. We note that with suitable designs, one can create a DC step-up transformer where the output voltage is larger than the input, and we show how to analyze the current flows and voltages in such devices.

cond-mat.mes-hall↗

arXiv2002

Frictional drag in dilute bilayer 2D hole systems

We develop a theory for frictional drag between two 2D hole layers in a dilute bilayer GaAs hole system, including effects of hole-hole and hole-phonon interactions. Our calculations suggest significant enhancement of hole drag transresistivity over the corresponding electron drag results. This enhancement originates from the exchange induced renormalization of the single layer compressibility and the strong dependence of single layer conductivity on density. We also address the effect of hole-phonon interaction on the drag temperature dependence. Our calculated results are in reasonable quantitative agreement with recent experimental observations.

cond-mat.str-el↗

arXiv2001

Strong enhancement of drag and dissipation at the weak- to strong- coupling phase transition in a bi-layer system at a total Landau level filling nu=1

We consider a bi-layer electronic system at a total Landau level filling factor nu =1, and focus on the transition from the regime of weak inter-layer coupling to that of the strongly coupled (1,1,1) phase (or ''quantum Hall ferromagnet''). Making the assumption that in the transition region the system is made of puddles of the (1,1,1) phase embedded in a bulk of the weakly coupled state, we show that the transition is accompanied by a strong increase in longitudinal Coulomb drag, that reaches a maximum of approximately $h/2e^{2}$. In that regime the longitudinal drag is increased with decreasing temperature.

cond-mat.mes-hall↗

arXiv2001

Two band transport and the question of a metal-insulator transition in GaAs/GaAlAs two dimensional holes

The magnetotransport of two dimensional holes in a GaAs/AlGaAs heterostructure is studied experimentally and theoretically. Spin-orbit splitting of the heavy hole band is manifested at high carrier densities in two Shubnikov-de Haas frequencies, classical positive magnetoresistance, and weak antilocalization. The latter effect combined with inelastic scattering between the spin-orbit split bands lead to metallic characteristics, namely resistance increase with temperature. At lower densities, when splitting is smaller than the inverse elastic scattering time, the two bands effectively merge to yield the expected insulating characteristics and negative magnetoresistance due to weak localization and interaction corrections to the conductivity. The "metal to insulator" transition at intermediate densities is found to be a smooth crossover between the two regimes rather than a quantum phase transition. Two band calculations of conventional interference and interaction effects account well for the data in the whole parameter range.

cond-mat.mes-hall↗

arXiv2001

Oscillating sign of drag in high Landau levels

Motivated by experiments, we study the sign of the Coulomb drag voltage in a double layer system in a strong magnetic field. We show that the commonly used Fermi Golden Rule approach implicitly assumes a linear dependence of intra-layer conductivity on density, and is thus inadequate in strong magnetic fields. Going beyond this approach, we show that the drag voltage commonly changes sign with density difference between the layers. We predict that in the Quantum Hall regime the Hall and longitudinal drag resistivities are comparable. Our results are also relevant for pumping and acoustoelectric experiments.

cond-mat.mes-hall↗

arXiv2000

Mesoscopic fluctuations of the Coulomb drag at $ν$=1/2

We consider mesoscopic fluctuations of Coulomb drag transresistivity between two layers at a Landau level filling factor $ν=1/2$ each. We find that at low temperature sample to sample fluctuations exceed both the ensemble average and the corresponding fluctuations at B=0. At the experimentally relevant temperatures, the variance of the transresistivity is proportional to $T^{-1/2}$. We find the dependence of this variance on density and magnetic field to reflect the attachment of two flux quanta to each electron.

cond-mat.mes-hall↗

arXiv2000

Spin orbit effects in a GaAs quantum dot in a parallel magnetic field

We analyze the effects of spin-orbit coupling on fluctuations of the conductance of a quantum dot fabricated in a GaAs heterostructure. We argue that spin-orbit effects may become important in the presence of a large parallel magnetic field B_{||}, even if they are negligble for B_{||}=0. This should be manifest in the level repulsion of a closed dot, and in reduced conductance fluctuations in dots with a small number of open channels in each lead, for large B_{||}. Our picture is consistent with the experimental observations of Folk et al.

cond-mat.mes-hall↗

arXiv2000

The Evolution of Quasiparticle Charge in the Fractional Quantum Hall Regime

The charge of quasiparticles in a fractional quantum Hall (FQH) liquid, tunneling through a partly reflecting constriction with transmission t, was determined via shot noise measurements. In the nu=1/3 FQH state, a charge smoothly evolving from e*=e/3 for t=1 to e*=e for t<<1 was determined, agreeing with chiral Luttinger liquid theory. In the nu=2/5 FQH state the quasiparticle charge evolves smoothly from e*=e/5 at t=1 to a maximum charge less than e*=e/3 at t<<1. Thus it appears that quasiparticles with an approximate charge e/5 pass a barrier they see as almost opaque.

cond-mat.mes-hall↗

arXiv2000

Theory of interlayer tunneling in bi-layer quantum Hall ferromagnets

Spielman et al. have recently observed a large zero-bias peak in the tunnel conductance of a bi-layer system in a quantum Hall ferromagnet state. We argue that disorder-induced topological defects in the pseudospin order parameter limit the peak size and destroy the predicted Josephson effect. We predict that the peak would be split and shifted by an in-plane magnetic field in a way that maps the dispersion relation of the ferromagnet's Goldstone mode. We also predict resonant structures in the DC I-V characteristic under bias by an {\em ac} electric field.

cond-mat↗

arXiv2000

Striped quantum Hall phases

Recent experiments seem to confirm predictions that interactions lead to charge density wave ground states in higher Landau levels. These new ``correlated'' ground states of the quantum Hall system manifest themselves for example in a strongly anisotropic resistivity tensor. We give a brief introduction and overview of this new and emerging field.

cond-mat.mes-hall↗

arXiv1999

Coulomb Drag between Quantum Wires

We study Coulomb drag in a pair of parallel one-dimensional electron systems within the framework of the Tomanaga-Luttinger model. We find that Coulomb coupling has a much stronger effect on one dimensional wires than on two-dimensional layers: At zero temperature the trans-resistivity {\em diverges}, due to the formation of locked charge density waves. At temperature well above a cross-over temperature $T^*$ the trans-resistivity follows a power law $ρ\propto T^x$, where the interaction-strength dependent exponent $x$ is determined by the Luttinger Liquid parameter $K_{c-}$ of the relative charge mode. At temperature below $T^*$ relative charge displacements are enabled by solitonic excitations, reflected by an exponential temperature dependence. The cross-over temperature $T^*$ depends sensitively on the wire width, inter-wire distance, Fermi wavelength and the effective Bohr radius. For wire distances $\bar{d} \gg k_F^{-1}$ it is exponentially suppressed with $T/E_F \sim \exp[ - \bar{d} k_F / (1-K_{c-}) ]$. The behavior changes drastically if each of the two wires develop spin gaps. In this case we find that the trans-resistivity {\em vanishes} at zero temperature. We discuss our results in view of possible experimental realizations in GaAs-AlGaAs semiconductor structures.

cond-mat.mes-hall↗