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Bertrand I. Halperin

Publications and source records attributed to Bertrand I. Halperin.

At least 109 records · Page 6Linked to original sources
arXiv2003

Interference and zero-bias anomaly in tunneling between Luttinger-liquid wires

We present theoretical calculations and experimental measurements which reveal the Luttinger-liquid (LL) nature of elementary excitations in a system consisting of two quantum wires connected by a long narrow tunnel junction at the edge of a GaAs/AlGaAs bilayer heterostructure. The boundaries of the wires are important and lead to a characteristic interference pattern in measurements on short junctions. We show that the experimentally observed modulation of the conductance oscillation amplitude as a function of the voltage bias can be accounted for by spin-charge separation of the elementary excitations in the interacting wires. Furthermore, boundaries affect the LL exponents of the voltage and temperature dependence of the tunneling conductance at low energies. We show that the measured temperature dependence of the conductance zero-bias dip as well as the voltage modulation of the conductance oscillation pattern can be used to extract the electron interaction parameters in the wires.

cond-mat.mes-hall↗
arXiv2002

Finite-size effects in tunneling between parallel quantum wires

We present theoretical calculations and experimental measurements which reveal finite-size effects in the tunneling between two parallel quantum wires, fabricated at the cleaved edge of a GaAs/AlGaAs bilayer heterostructure. Observed oscillations in the differential conductance, as a function of bias voltage and applied magnetic field, provide direct information on the shape of the confining potential. Superimposed modulations indicate the existence of two distinct excitation velocities, as expected from spin-charge separation.

cond-mat.mes-hall↗
arXiv2002

Magnetoplasmon Excitations and Spin Density Instabilities in an Integer Quantum Hall System with a Tilted Magnetic Field

We study the magnetoplasmon collective mode excitations of integer quantum Hall systems in a parabolically confined quantum well nanostructure in the presence of a tilted magnetic field by using the time-dependent Hartree-Fock approximation. For even integer filling, we find that the dispersion of a spin density mode has a magneto-roton minimum at finite wavevectors, at a few times 10^6 cm^{-1} for parallel fields of order 1-10 Tesla, only in the direction perpendicular to the in-plane magnetic field, while the mode energy increases monotonously with wavevector parallel to the in-plane magnetic field. When the in-plane magnetic field is strong enough (well above 10 Tesla),we speculate that this roton minimum may reach zero energy, suggesting a possible second order phase transition to a state with broken translational and spin symmetries. We discuss the possibility for observing such parallel field-induced quantum phase transitions. We also derive an expression for the dielectric function within the time-dependent Hartree-Fock approximation and include screening effects in our magnetoplasmon calculation. We discuss several exotic symmetry-broken phases that may be stable in finite parallel fields, and propose that the transport anisotropy, observed recently in parallel field experiments, may be due to the formation of a skyrmion stripe phase predicted in our theory. Our predicted anisotropic finite wavevector suppression, perhaps even a mode-softening leading to the quantum phase transition to the anisotropic phase, in the collective spin excitation mode of the wide well system in the direction transverse to the applied parallel magnetic field should be directly experimentally observable via the inelastic light scattering spectroscopy.

cond-mat.mes-hall↗
arXiv2002

Spin battery operated by ferromagnetic resonance

Precessing ferromagnets are predicted to inject a spin current into adjacent conductors via Ohmic contacts, irrespective of a conductance mismatch with, for example, doped semiconductors. This opens the way to create a pure spin source spin battery by the ferromagnetic resonance. We estimate the spin current and spin bias for different material combinations.

cond-mat.mes-hall↗
arXiv2002

Kondo effect and STM spectra through ferromagnetic nanoclusters

Motivated by recent scanning tunneling microscope (STM) experiments on cobalt clusters adsorbed on single wall metallic nanotubes [Odom {\em et al.}, Science {\bf 290}, 1549 (2000)], we study theoretically the size dependence of STM spectra and spin-flip scattering of electrons from finite size ferromagnetic clusters adsorbed on metallic surfaces. We study two models of nanometer size ferromagnets: (i) An itinerant model with delocalized s, p and d electrons and (ii) a local moment model with both localized d-level spins and delocalized cluster electrons. The effective exchange coupling between the spin of the cluster and the conduction electrons of the metallic substrate depends on the specific details of the single particle density of states on the cluster. The calculated Kondo coupling is inversely proportional to the total spin of the ferromagnetic cluster in both models and thus the Kondo temperature is rapidly suppressed as the size of the cluster increases. Mesoscopic fluctuations in the charging energies and magnetization of nanoclusters can lead to large fluctuations in the Kondo temperatures and a very asymmetric voltage dependence of the STM spectra. We compare our results to the experiments.

cond-mat.str-el↗
arXiv2002

Wrapping Current versus Bulk Integer Quantum Hall Effect in Three Dimensions

Surface electron currents are studied for the integer quantum Hall effect in three dimensions(3D) proposed previously by Kohmoto et al. and by Koshino et al. We predict the current wraps the facets of the sample with its intensity and direction dictated by 3D Chern numbers which are just the quantized Hall conductivities in 3D ($σ_{xy}, σ_{zx}$), so a natural connection exists between the surface and bulk currents just as in 2D. An experiment to detect the 3D integer quantum Hall effect through the wrapping current is proposed.

cond-mat.mes-hall↗
arXiv2001

Spin, spin-orbit, and electron-electron interactions in mesoscopic systems

We review recent theoretical developments about the role of spins, electron-electron interactions, and spin-orbit coupling in metal nanoparticles and semiconductor quantum dots. For a closed system, in the absence of spin-orbit coupling or of an external magnetic field, electron-electron interactions make it possible to have ground states with spin $S > 1/2$. We review here a theoretical analysis which makes predictions for the probability of finding various values of spin $S$ for an irregular particle in the limit where the number of electron is large but finite. We also present results for the probability distribution of the spacing between successive groundstate energies in such a particle. In a metallic particle with strong spin-orbit interactions, for odd electron number, the groundstate has a Kramers' degeneracy, which is split linearly by a weak applied magnetic field. The splitting may be characterized by an effective $g$-tensor whose principal axes and eigenvalues vary from one level to another. Recent calculations have addressed the joint probability distribution, including the anisotropy, of the eigenvalues. The peculiar form of the spin-orbit coupling for a two-dimensional electron system in a GaAs heterostructure or quantum well leads to a strong suppression of spin-orbit effects when the electrons are confined in a small quantum dot. Spin-effects can be enhanced, however, in the presence of an applied magnetic field parallel to the layer, which may explain recent observations on fluctuations in the conductances through such dots. We also discuss possible explanations for the experimental observations by Davidovic and Tinkham of a multiplet splitting of the lowest resonance in the tunneling conductance through a gold nano-particle.

cond-mat.mes-hall↗
arXiv2001

Coulomb blockade of strongly coupled quantum dots studied via bosonization of a channel with a finite barrier

A pair of quantum dots, coupled through a point contact, can exhibit Coulomb blockade effects that reflect an oscillatory term in the dots' total energy whose value depends on whether the total number of electrons on the dots is even or odd. The effective energy associated with this even-odd alternation is reduced, relative to the bare Coulomb blockade energy for uncoupled dots, by a factor (1-f) that decreases as the interdot coupling is increased. When the transmission coefficient for interdot electronic motion is independent of energy and the same for all channels within the point contact (which are assumed uncoupled), the factor (1-f) takes on a universal value determined solely by the number of channels and the dimensionless conductance g of each individual channel. This paper studies corrections to the universal value of (1-f) that result when the transmission coefficent varies over energy scales of the size of the bare Coulomb blockade energy. We consider a model in which the point contact is described by a single orbital channel containing a parabolic barrier potential, and we calculate the leading correction to (1-f) for one-channel (spin-split) and two-channel (spin-degenerate) point contacts in the limit where the single orbital channel is almost completely open. By generalizing a previously used bosonization technique, we find that, for a given value of the dimensionless conductance g, the value of (1-f) is increased relative to its value for a zero-thickness barrier, but the absolute value of the increase is small in the region where our calculations apply.

cond-mat.mes-hall↗
arXiv2001

Striped States in Quantum Hall Effect: Deriving a Low Energy Theory from Hartree-Fock

There is growing experimental and theoretical evidence that very clean two dimensional electron systems form unidirectional charge density waves (UCDW) or ``striped'' states at low temperatures and at Landau level filling fractions of the form nu = M + x with 4 < M < 10 an integer and 0.4 < x < 0.6. Following previous work, we model the striped state using a Hartree-Fock approach. We construct the low energy excitations of the system by making smooth deformations of the stripe edges analogous to the construction of edge state excitations of quantum Hall droplets. These low energy excitations are described as a coupled Luttinger liquid theory, as discussed previously by MacDonald and Fisher (PRB61 5724 2000). Here, we extend that work and explicitly derive all of the parameters of this low energy theory using a Hartree Fock approach. We also make contact with the equivalent low energy hydrodynamic approach of Fogler and Vinokur (PRL84 5828 2000) and similarly derive the parameters of this theory. As examples of the use of these results, we explicitly calculate the low-energy excitation spectrum and study tunneling into the striped state.

cond-mat.mes-hall↗
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

Spin configurations of carbon nanotube in a nonuniform external potential

We study, theoretically, the ground state spin of a carbon nanotube in the presence of an external potential. We find that when the external potential is applied to a part of the nanotube, its variation changes the single electron spectrum significantly. This, in combination with Coulomb repulsion and the symmetry properties of a finite length armchair nanotube induces spin flips in the ground state when the external potential is varied. We discuss the possible application of our theory to recent measurements of Coulomb blocked peaks and their dependence on a weak magnetic field in armchair carbon nanotubes.

cond-mat.mes-hall↗
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

Drag and Hall drag in a bi-layer system with pinholes

The transresistance and the Hall transresistance of dirty two-dimensional bi-layer systems in the presence of tunneling bridges (pinholes) are studied theoretically. We find, at weak magnetic field, a non-zero Hall transresistance. In a geometry where the pinholes are concentrated in the middle of the sample, a quantum process gives the dominant contribution to both the ordinary transresistance and the Hall transresistance. Arising from the interplay between Coulomb repulsion, disorder and tunneling, the quantum contribution increases in a singular way as the temperature decreases.

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↗
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↗
arXiv1996

Consequences of a possible adiabatic transition between ν=1/3 and ν=1 quantum Hall states in a narrow wire

We consider the possibility of creating an adiabatic transition through a narrow neck, or point contact, between two different quantized Hall states that have the same number of edge modes, such as ν=1 and ν=1/3. We apply both the composite fermion and the Luttinger liquid formalism to analyze the transition. We suggest that using such adiabatic junctions one could build a DC step-up transformer, where the output voltage is higher than the input. Difficulties standing in the way of an experimental implementation of the adiabatic junction are addressed.

cond-mat.mes-hall↗
arXiv1996

Corrections to the universal behavior of the Coulomb-blockade peak splitting for quantum dots separated by a finite barrier

Building upon earlier work on the relation between the dimensionless interdot channel conductance g and the fractional Coulomb-blockade peak splitting f for two electrostatically equivalent dots, we calculate the leading correction that results from an interdot tunneling barrier that is not a delta-function but, rather, has a finite height V and a nonzero width xi and can be approximated as parabolic near its peak. We develop a new treatment of the problem for g much less than 1 that starts from the single-particle eigenstates for the full coupled-dot system. The finiteness of the barrier leads to a small upward shift of the f-versus-g curve at small values of g. The shift is a consequence of the fact that the tunneling matrix elements vary exponentially with the energies of the states connected. Therefore, when g is small, it can pay to tunnel to intermediate states with single-particle energies above the barrier height V. The correction to the zero-width behavior does not affect agreement with recent experimental results but may be important in future experiments.

cond-mat.mes-hall↗