arXiv subjects

Ady Stern

Publications and source records attributed to Ady Stern.

179 records · Page 10Linked to original sources

arXiv1999

Conductivity tensor of striped quantum Hall phases

We study the transport properties of pinned striped quantum Hall phases. We show that under quite general assumptions, the macroscopic conductivity tensor satisfies a semicircle law. In particular, this result is valid for both smectic and nematic stripe phases, independent of the presence of topological defects such as dislocations and grain boundaries. As a special case, our results explain the experimental validity of a product rule for the dissipative part of the resistivity tensor, which was previously derived by MacDonald and Fisher for a perfect stripe structure.

cond-mat.mes-hall↗

arXiv1999

Dissipationless transport in low density bilayer systems

In a bilayer electronic system the layer index may be viewed as the z-component of an isospin-1/2. An XY isospin-ordered ferromagnetic phase was observed in quantum Hall systems and is predicted to exist at zero magnetic field at low density. This phase is a superfluid for opposite currents in the two layers. At B=0 the system is gapless but superfluidity is not destroyed by weak disorder. In the quantum Hall case, weak disorder generates a random gauge field which probably does not destroy superfluidity. Experimental signatures include Coulomb drag and collective mode measurements.

cond-mat.mes-hall↗

arXiv1999

On the Relation between Interband Scattering and the "Metallic Phase" of Two Dimensional Holes in GaAs/AlGaAs

The "metallic" regime of holes in GaAs/AlGaAs heterostructures corresponds to densities where two splitted heavy hole bands exist at a zero magnetic field. Using Landau fan diagrams and weak field magnetoresistance curves we extract the carrier density in each band and the interband scattering rates. The measured inelastic rates depend Arrheniusly on temperature with an activation energy similar to that characterizing the longitudinal resistance. The "metallic" characteristics, namely, the resistance increase with temperature, is hence traced to the activation of inelastic interband scattering. The data are used to extract the bands dispersion relations as well as the two particle-hole excitation continua. It is then argued that acoustic plasmon mediated Coulomb scattering might be responsible for the Arrhenius dependence on temperature. The absence of standard Coulomb scattering characterized by a power law dependence upon temperature is pointed out.

cond-mat.mes-hall↗

arXiv1999

The half-filled Landau level - composite fermions and dipoles

The composite-fermion approach as formulated in the fermion Chern-Simons theory has been very successful in describing the physics of the lowest Landau level near Landau level filling factor 1/2. Recent work has emphasized the fact that the true quasiparticles at these filling factors are electrically neutral and carry an electric dipole moment. In a previous work, we discussed at length two formulations in terms of dipolar quasiparticles. Here we briefly review one approach - termed electron-centered quasiparticles - and show how it can be extended from 1/2 to nearby filling factors where the quasiparticles carry both an electric dipole moment and an overall charge.

cond-mat.mes-hall↗

arXiv1999

The Different Effect of Electron-Electron Interaction on the Spectrum of Atoms and Quantum Dots

The electron-electron scattering rate of single particle excitations in atoms is estimated and compared with the corresponding rate in quantum dots. It is found that in alkali atoms single particle excitations do not acquire a width due to electron-electron interaction, while in complex atoms they may. This width is typically smaller than the single particle level spacing, and hence does not affect the number of discrete single particle excitations resolved below the ionization threshold. This situation is contrasted with that of quantum dots where electron-electron interaction severely limits the number of resolved excitations. Unlike the case of quantum dots, the scattering rate in atoms is found to decrease with increasing excitation energy. The different effect of electron-electron interaction on the spectrum of quantum dots and atoms is traced to the different confining potentials in the two systems.

cond-mat↗

arXiv1998

Coulomb drag at ν= 1/2: Composite fermion pairing fluctuations

We consider the Coulomb drag between two two-dimensional electron layers at filling factor ν= 1/2 each, using a strong coupling approach within the composite fermion picture. Due to an attractive interlayer interaction, composite fermions are expected to form a paired state below a critical temperature T_c. We find that above T_c pairing fluctuations make the longitudinal transresistivity ρ_D increase with decreasing temperature. The pairing mechanism we study is very sensitive to density variations in the two layers, and to an applied current. We discuss possible relation to an experiment by Lilly et al. [Phys. Rev. Lett. 80, 1714 (1998)].

cond-mat.mes-hall↗

arXiv1998

Composite Fermions in Modulated Structures: Transport and Surface Acoustic Waves

Motivated by a recent experiment of Willett et al. [Phys. Rev. Lett. 78, 4478 (1997)], we employ semiclassical composite-fermion theory to study the effect of a periodic density modulation on a quantum Hall system near Landau level filling factor nu=1/2. We show that even a weak density modulation leads to dramatic changes in surface-acoustic-wave (SAW) propagation, and propose an explanation for several key features of the experimental observations. We predict that properly arranged dc transport measurements would show a structure similar to that seen in SAW measurements.

cond-mat.mes-hall↗

arXiv1998

Quantum limitations on superluminal propagation

Unstable systems such as media with inverted atomic population have been shown to allow the propagation of analytic wavepackets with group velocity faster than that of light, without violating causality. We illuminate the important role played by unstable modes in this propagation, and show that the quantum fluctuations of these modes, and their unitary time evolution, impose severe restrictions on the observation of superluminal phenomena.

quant-ph↗

arXiv1998

Compressibility of a quantum Hall system at $ν= 1/2$

Despite the fact that composite fermions may be regarded as electrically neutral objects at Landua-level filling factor $ν=1/2$, carrying only an electric dipole moment, we argue that the long-wavelength compressibility of the system is finite.

cond-mat.mes-hall↗

arXiv1997

Coulomb drag in the quantum Hall $ν= 1/2$ state: Role of disorder

We consider Coulomb drag between two layers of two-dimensional electron gases subject to a strong magnetic field, with the Landau level filling factor in each layer being 1/2. We find $ρ_D$ to be very large, as compared to the zero magnetic field case. We attribute this enhancement to the slow decay of density fluctuations in a strong magnetic field. For a clean system, the linear $q$-dependence of the longitudinal conductivity, characteristic of the $ν=1/2$ state, leads a unique temperature dependence---$\rd \propto T^{4/3}$. Within a semiclassical approximation, disorder leads to a decrease of the transresistivity as compared with the clean case, and a temperature dependence of $T^2\log T$ at low temperatures.

cond-mat.mes-hall↗

arXiv1997

Polymer Winding Numbers and Quantum Mechanics

The winding of a single polymer in thermal equilibrium around a repulsive cylindrical obstacle is perhaps the simplest example of statistical mechanics in a multiply connected geometry. As shown by S.F. Edwards, this problem is closely related to the quantum mechanics of a charged particle interacting with a Aharonov-Bohm flux. In another development, Pollock and Ceperley have shown that boson world lines in 2+1 dimensions with periodic boundary conditions, regarded as ring polymers on a torus, have a mean square winding number given by $ = 2n_s\hbar^2/mk_BT$, where $m$ is the boson mass and $n_s$ is the superfluid number density. Here, we review the mapping of the statistical mechanics of polymers with constraints onto quantum mechanics, and show that there is an interesting generalization of the Pollock-Ceperley result to directed polymer melts interacting with a repulsive rod of radius $a$. When translated into boson language, the mean square winding number around the rod for a system of size $R$ perpendicular to the rod reads $ = {n_s\hbar^2\over 2πmk_BT}\ln(R/a)$. This result is directly applicable to vortices in Type II superconductors in the presence of columnar defects. An external current passing through the rod couples directly to the winding number in this case.

cond-mat.stat-mech↗

arXiv1997

Coulomb drag in compressible quantum Hall states

We consider the Coulomb drag between two layers of two-dimensional electronic gases subject to a strong magnetic field. We first focus on the case in which the electronic density is such that the Landau level filling fraction $ν$ in each layer is at, or close to, $ν=1/2$. Discussing the coupling between the layers in purely electronic terms, we show that the unique dependence of the longitudinal conductivity on wave-vector, observed in surface acoustic waves experiments, leads to a very slow decay of density fluctuations. Consequently, it has a crucial effect on the Coulomb drag, as manifested in the transresistivity $ρ_D$. We find that the transresistivity is very large compared to its typical values at zero magnetic field, and that its temperature dependence is unique -- $ρ_D \propto T^{4/3}$. For filling factors at or close to $1/4$ and $3/4$ the transresistivity has the same $T$-dependence, and is larger than at $ν= 1/2$. We calculate $ρ_D$ for the $ν=3/2$ case and propose that it might shed light on the spin polarization of electrons at $ν=3/2$. We compare our results to recent calculations of $ρ_D$ at $ν=1/2$ where a composite fermion approach was used and a $T^{4/3}$-dependence was obtained. We conclude that what appears in the composite fermion language to be drag induced by Chern-Simons interaction is, physically, electronic Coulomb drag.

cond-mat.mes-hall↗

arXiv1996

Current noise, electron-electron interactions, and quantum interference

It is shown that electron-electron interactions lead to a novel quantum-interference contribution to non-equilibrium current noise in mesoscopic conductors. The corresponding noise spectrum is obtained in detail for diffusive systems. It is found to be quadratic in the applied voltage and to exhibit a power-law frequency spectrum whose exponent depends on the dimensionality of the sample.

cond-mat.mes-hall↗

arXiv1996

Composite Fermions with Orbital Magnetization

For quantum Hall systems, in the limit of large magnetic field (or equivalently small electron band mass $m_b$), the static response of electrons to a spatially varying magnetic field is largely determined by kinetic energy considerations. This response is not correctly given in existing approximations based on the Fermion Chern-Simons theory of the partially filled Landau level. We remedy this problem by attaching an orbital magnetization to each fermion to separate the current into magnetization and transport contributions, associated with the cyclotron and guiding center motions respectively. This leads to a Chern-Simons Fermi liquid description of the $ν=\frac{1}{2m}$ state which correctly predicts the $m_b$ dependence of the static and dynamic response in the limit $m_b \rightarrow 0$.

cond-mat↗

arXiv1995

The Physical Significance of Singularities in the Chern--Simons Fermi Liquid Description of a Partially Filled Landau Level

We analyze the linear response of a half filled Landau level to long wavelength and low frequency driving forces, using Fermi liquid theory for composite fermions. This response is determined by the composite fermions quasi--particle effective mass, $m^*$, and quasi--particle Landau interaction function $f(θ-θ')$. Analyzing infra--red divergences of perturbation theory, we get an exact expression for $m^*$, and conjecture the form of the $f(θ-θ')$. We then conclude that in the limit of infinite cyclotron frequency, and small ${\bf q},ω$, the composite fermion excitation spectrum is continuous for $0<ω<γ\frac{e^2}{εh}q$, with $γ$ an unknown number. For fractional quantum Hall states near a half filled Landau level, we derive an exact expression for the energy gap.

cond-mat↗

arXiv1995

Singularities in the Fermi liquid description of a partially filled Landau level and the energy gaps of fractional quantum Hall states

We consider a two dimensional electron system in an external magnetic field at and near an even denominator Landau level filling fraction. Using a fermionic Chern--Simons approach we study the description of the system's low energy excitations within an extension of Landau's Fermi liquid theory. We calculate perturbatively the effective mass and the quasi--particle interaction function characterizing this description. We find that at an even denominator filling fraction the fermion's effective mass diverges logarithmically at the Fermi level, and argue that this divergence allows for an {\it exact} calculation of the energy gaps of the fractional quantized Hall states asymptotically approaching these filling fractions. We find that the quasi--particle interaction function approaches a delta function. This singular behavior leads to a cancelation of the diverging effective mass from the long wavelength low frequency linear response functions at even denominator filling fractions.

cond-mat↗

arXiv1994

Quantum Hall Fluid of Vortices in a Two Dimensional Array of Josephson Junctions

A two dimensional array of Josephson junctions in a magnetic field is considered. It is shown that the dynamics of the vortices in the array resembles that of electrons on a two--dimensional lattice put in a magnetic field perpendicular to the lattice. Under appropriate conditions, this resemblance results in the formation of a quantum Hall fluid of vortices. The bosonic nature of vortices and their long range logarithmic interaction make some of the properties of the vortices' quantum Hall fluid different from those of the electronic one. Some of these differences are studied in detail. Finally, it is shown that a quantum Hall fluid of vortices manifests itself in a quantized Hall electronic transport in the array.

cond-mat↗