arXiv subjects

A. H. MacDonald

Publications and source records attributed to A. H. MacDonald.

At least 271 records · Page 15Linked to original sources

arXiv2000

Spin and Charge Luttinger-Liquid Parameters of the One-Dimensional Electron Gas

Low-energy properties of the homogeneous electron gas in one dimension are completely described by the group velocities of its charge (plasmon) and spin collective excitations. Because of the long range of the electron-electron interaction, the plasmon velocity is dominated by an electrostatic contribution and can be estimated accurately. In this Letter we report on Quantum Monte Carlo simulations which demonstrate that the spin velocity is substantially decreased by interactions in semiconductor quantum wire realizations of the one-dimensional electron liquid.

cond-mat.mes-hall↗

arXiv2000

Disorder and interactions in quantum Hall ferromagnets near $ν=1$

We report on a finite-size Hartree-Fock study of the competition between disorder and interactions in a two-dimensional electron gas near Landau level filling factor $ν=1$. The ground state at $ν=1$ evolves with increasing disorder from a fully spin-polarized ferromagnet with a charge gap, to a partially spin-polarized ferromagnetic Anderson insulator, to a quasi-metallic paramagnet at the critical point between $i=0$ and $i=2$ quantum Hall plateaus. Away from $ν=1$, the ground state evolves from a ferromagnetic Skyrmion quasiparticle glass, to a conventional quasiparticle glass, and finally to a conventional Anderson insulator. We comment on signatures of these different regimes in low-temperature transport and NMR lineshape and peak position data.

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arXiv2000

Reorientation of Anisotropy in a Square Well Quantum Hall Sample

We have measured magnetotransport at half-filled high Landau levels in a quantum well with two occupied electric subbands. We find resistivities that are {\em isotropic} in perpendicular magnetic field but become strongly {\em anisotropic} at $ν$ = 9/2 and 11/2 on tilting the field. The anisotropy appears at an in-plane field, $B_{ip} \sim$ 2.5T, with the easy-current direction {\em parallel} to $B_{ip}$ but rotates by 90$^{\circ}$ at $B_{ip} \sim$ 10T and points now in the same direction as in single-subband samples. This complex behavior is in quantitative agreement with theoretical calculations based on a unidirectional charge density wave state model.

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arXiv2000

Pseudospin Anisotropy Classification of Quantum Hall Ferromagnets

Broken symmetry ground states with uniform electron density are common in quantum Hall systems when two Landau levels simultaneously approach the chemical potential at integer filling factor $ν$. The close analogy between these two-dimensional electron system states and conventional itinerant electron ferromagnets can be emphasized by using a pseudospin label to distinguish the two Landau levels. As in conventional ferromagnets, the evolution of the system's state as external field parameters are varied is expected to be sensitive to the dependence of ground state energy on pseudospin orientation. We discuss the predictions of Hartree-Fock theory for the dependence of the sign and magnitude of the pseudospin anisotropy energy on the nature of the crossing Landau levels. We build up a classification scheme for quantum Hall ferromagnets that applies for single layer and bilayer systems with two aligned Landau levels distinguished by any combination of real-spin, orbit-radius, or growth direction degree-of-freedom quantum numbers. The possibility of in-situ tuning between easy-axis and easy-plane quantum Hall ferromagnets is discussed for biased bilayer systems with total filling factors $ν=3$ or $ν=4$. Detailed predictions are made for the bias dependence of pseudospin reversal properties in $ν=3$ bilayer systems.

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arXiv2000

Bilayer Quantum Hall Systems at Filling Factor ν=2: An Exact Diagonalisation Study

We present an exact diagonalisation study of bilayer quantum Hall systems at a filling factor of two in the spherical geometry. We find the high-Zeeman-coupling phase boundary of the broken symmetry canted antiferromagnet is given exactly by previous Hartree-Fock mean-field theories, but that the state's stability at weak Zeeman coupling has been qualitatively overestimated. In the absence of interlayer tunneling, degeneracies occur between total spin multiplets due to the Hamiltonian's invariance under independent spin-rotations in top and bottom two-dimensional electron layers.

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arXiv2000

Phonon-mediated drag at $ν=1/2$: A test of the Chern-Simons composite fermion theory

We report on a study of the phonon-mediated frictional drag between two-dimensional electron layers at the Landau level filling factor $ν=1/2$ predicted by Chern-Simons composite-fermion theory. Frictional drag between widely spaced layers is dominated by the phonon-mediated interaction and is altered from its zero-field form because of the large composite-fermion effective mass and, less strongly, by magnetic local-field effects. We find that the reduced drag resistance $ρ(T)/T^2$ peaks at a larger temperature and has a different density dependence than at zero field. We compare these results with recent experimental findings.

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arXiv2000

Interlayer coupling in ferromagnetic semiconductor superlattices

We develop a mean-field theory of carrier-induced ferromagnetism in diluted magnetic semiconductors. Our approach represents an improvement over standard RKKY model allowing spatial inhomogeneity of the system, free-carrier spin polarization, finite temperature, and free-carrier exchange and correlation to be accounted for self-consistently. As an example, we calculate the electronic structure of a Mn$_x$Ga$_{1-x}$As/GaAs superlattice with alternating ferromagnetic and paramagnetic layers and demonstrate the possibility of semiconductor magnetoresistance systems with designed properties.

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arXiv2000

Field-tilt Anisotropy Energy in Quantum Hall Stripe States

Recently reported giant anisotropy in the longitudinal resistivity of a 2D electron system with valence Landau level index $N \ge 2$ has been interpreted as a signal of unidirectional charge density wave (UCDW) ground states. We report on detailed Hartree-Fock calculations of the UCDW orientation energy induced by a tilted magnetic field. We find that for current experimental samples stripes are oriented perpendicular to the in-plane field, consistent with experiment. For wider 2D electron systems we predict tilt-induced stripe states with variable anisotropy energy sign.

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arXiv2000

Theory of Ferromagnetism in Diluted Magnetic Semiconductor Quantum Wells

We present a mean field theory of ferromagnetism in diluted magnetic semiconductor quantum wells. When subband mixing due to exchange interactions between quantum well free carriers and magnetic impurities is neglected, analytic result can be obtained for the dependence of the critical temperature and the spontaneous magnetization on the distribution of magnetic impurities and the quantum well width. The validity of this approximate theory has been tested by comparing its predictions with those from numerical self-consistent field calculations. Interactions among free carriers, accounted for using the local-spin-density approximation, substantially enhance the critical temperature. We demonstrate that an external bias potential can tune the critical temperature through a wide range.

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arXiv2000

Non-linear Transport in Quantum-Hall Smectics

Recent transport experiments have established that two-dimensional electron systems with high-index partial Landau level filling, $ν^{*} =ν- \lbrack ν\rbrack$, have ground states with broken orientational symmetry. In a mean-field theory, the broken symmetry state consists of electron stripes with local filling factor $\lbrack ν\rbrack + 1 $, separated by hole stripes with filling factor $\lbrack ν\rbrack$. We have recently developed a theory of these states in which the electron stripes are treated as one-dimensional electron systems coupled by interactions and described by using a Luttinger liquid model. Among other things, this theory predicts non-linearities of opposite sign in easy and hard direction resistivities. In this article we briefly review our theory, focusing on its predictions for the dependence of non-linear transport exponents on the separation $d$ between the two-dimensional electron system and a co-planar screening layer.

cond-mat↗

arXiv1999

Spintronic Spin Accumulation and Thermodynamics

The spin degree of freedom can play an essential role in determining the electrical transport properties of spin-polarized electron systems in metals or semiconductors. In this article, I address the dependence of spin-subsystem chemical potentials on accumulated spin-densities. I discuss both approaches which can be used to measure this fundamental thermodynamic quantity and the microscopic physics which determines its value in several different systems.

cond-mat↗

arXiv1999

Effect of Order-Parameter Suppression on Scattering by Isolated Impurities in Asymmetric Bands

The single-impurity problem in d-wave superconductors with asymmetric bands is discussed. The effect of local order parameter suppression near the impurity is to shift the quasiparticle resonance. Contrary to previous work [A. Shnirman et al., Phys. Rev. B 60, 7517 (1999)] we find that the direction of the shift is not universally towards the strong scattering limit.

cond-mat.supr-con↗

arXiv1999

Umklapp scattering at reconstructed quantum-Hall edges

We study the low-lying excitations of a quantum-Hall sample that has undergone edge reconstruction such that there exist three branches of chiral edge excitations. Among the interaction processes that involve electrons close to the three Fermi points is a new type of Umklapp-scattering process which has not been discussed before. Using bosonization and a refermionization technique, we obtain exact results for electronic correlation functions and discuss the effect Umklapp scattering has on the Luttinger-liquid properties of quantum-Hall edges.

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arXiv1999

Quantum Theory of Quantum-Hall Smectics

We propose a quantum stripe (smectic) coupled-Luttinger-liquid model for the anisotropic states which occur in two-dimensional electron systems with high-index partial Landau level filling, $ν^{*} = ν- \lbrackν\rbrack$. Perturbative renormalization group calculations establish that interaction terms neglected in this model are relevant - probably driving the system into an anisotropic Wigner crystal---but for $0.4 \lesssim ν^{*} \lesssim 0.6$ only below temperatures which are outside of the experimentally accessible range. We argue that the Hall conductance of the ground state flows toward $\lbrackν\rbrack e^{2}/h$ and $(\lbrackν\rbrack + 1) e^{2}/h$ respectively, on the low and high filling factor sides of this range, consistent with recent observations. A semiclassical theory of smectic state transport properties, which incorporates Luttinger liquid effects in the evaluation of scattering amplitudes, accounts for the magnitude of the dissipative resistivities at $ν^{*}=1/2$, for their $ν^{*}$-dependence, and for the observation of non-linearities of opposite sign in easy and hard direction resistivities.

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arXiv1999

Spin Bottlenecks in the Quantum Hall Regim

We present a theory of time-dependent tunneling between a metal and a partially spin-polarized two-dimensional electron system (2DES). We find that the leakage current which flows to screen an electric field between the metal and the 2DES is the sum of two exponential contributions whose relative weights depend on spin-dependent tunneling conductances, on quantum corrections to the electrostatic capacitance of the tunnel junction, and on the rate at which the 2DES spin-polarization approaches equilibrium. For high-mobility and homogeneous 2DES's at Landau level filling factor $ν=1$, we predict a ratio of the fast and slow leakage rates equal to $(2K+1)^2$ where $K$ is the number of reversed spins in the skyrmionic elementary charged excitations.

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arXiv1999

Fractional-quantum-Hall edges at filling factor 1-1/m

We consider the edge of a two-dimensional electron system that is in the quantum-Hall-effect regime at filling factor 1-1/m with m being an odd integer, where microscopic theory explaining the occurrence of the quantum Hall effect in the bulk predicts the existence of two counterpropagating edge-excitation modes. These two modes are the classical edge-magnetoplasmon mode and a slow-moving neutral mode. Assuming the electrons to be confined by a coplanar neutralizing background of positive charges, and taking careful account of long-range Coulomb interactions, we determine microscopically the velocity of the neutral mode and the edge width. Our results are intended to guide experimental efforts aimed at verifying the existence of the neutral mode, which would provide a powerful confirmation of the current microscopic understanding of quantum-Hall physics at the simplest hierarchical filling Factors 1-1/m.

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arXiv1999

Electrodynamics of a Clean Vortex Lattice

We report on a microscopic evaluation of electrodynamic response for the vortex lattice state of a model s-wave superconductor. Our calculation accounts self-consistently for both quasiparticle and order parameter response and establishes the collective nature of linear response in the clean limit. We discuss the effects of homogeneous and inhomogeneous pinning on the optical conductivity and the penetration depth, and comment on the relationship between macroscopic and local penetration depths. We find unexpected relationships between pinning arrangements and conductivity due to the strongly non-local response.

cond-mat.supr-con↗

arXiv1999

Periphery deformations and tunneling at correlated quantum-Hall edges

We argue that, at any filling factor, correlated quantum-Hall systems possess a set of chiral boson excitations which are generated by electronically rigid deformations of the system's periphery. We submit that tunneling electrons can be accommodated, at low energies, in these systems only by periphery-deformation excitations. This property would explain the recent observation of a tunneling density of states at the edge which does not exhibit a strong dependence on the occurrence or absence of the quantum Hall effect and has a power-law dependence on energy with exponent (inverse filling factor)-1.

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