arXiv subjects

A. H. MacDonald

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

At least 289 records · Page 16Linked to original sources

arXiv1999

Broken Symmetry Ground States in ν=2 Bilayer Quantum Hall Systems

We report on a study of a bilayer two-dimensional electron gas at Landau level filling factor $ν=2$. The system exhibits both magnetic and spontaneous interlayer phase coherence broken symmetries. We propose a 3-parameter Slater determinant variational wavefunction which describes the ground state over the full range of bias potential ($Δ_V$) Zeeman coupling ($Δ_z$) and interlayer tunneling amplitude ($Δ_t$) strengths. Broken symmetry states occur inside a volume in this three-dimensional phase diagram near the $Δ_z^2=Δ_V^2+Δ_t^2$ surface. We have obtained analytic results for the intersections of the phase diagram with the $Δ_t=0$, $Δ_z=0$, and $Δ_V=0$, planes and show that the differential capacitance of the bilayer system is singular at the phase boundary.

cond-mat.mes-hall↗

arXiv1999

Pauli-Limited Superconductivity in Small Grains

We report on an exploration of the mean-field phase diagram for Pauli-limited superconductivity in small metallic grains. Emphasis is placed on the crossover from the ultra-small grain limit where superconductivity disappears to the bulk thin-film limit as the single-particle level spacing in the grain decreases. We find that the maximum Zeeman coupling strength compatible with superconductivity increases with decreasing grain size, in spite of a monotonically decreasing condensation energy per unit volume.

cond-mat.supr-con↗

arXiv1998

Itinerant Electron Ferromagnetism in the Quantum Hall Regime

We report on a study of the temperature and Zeeman-coupling-strength dependence of the one-particle Green's function of a two-dimensional (2D) electron gas at Landau level filling factor $ν=1$ where the ground state is a strong ferromagnet. Our work places emphasis on the role played by the itinerancy of the electrons, which carry the spin magnetization and on analogies between this system and conventional itinerant electron ferromagnets. We discuss the application to this system of the self-consistent Hartree-Fock approximation, which is analogous to the band theory description of metallic ferromagnetism and fails badly at finite temperatures because it does not account for spin-wave excitations. We go beyond this level by evaluating the one-particle Green's function using a self-energy, which accounts for quasiparticle spin-wave interactions. We report results for the temperature dependence of the spin magnetization, the nuclear spin relaxation rate, and 2D-2D tunneling conductances. Our calculations predict a sharp peak in the tunneling conductance at large bias voltages with strength proportional to temperature. We compare with experiment, where available, and with predictions based on numerical exact diagonalization and other theoretical approaches.

cond-mat.mes-hall↗

arXiv1998

Observability of counterpropagating modes at fractional-quantum-Hall edges

When the bulk filling factor is equal to 1 - 1/m with m odd, at least one counterpropagating chiral collective mode occurs simultaneously with magnetoplasmons at the edge of fractional-quantum-Hall samples. Initial experimental searches for an additional mode were unsuccessful. In this paper, we address conditions under which its observation should be expected in experiments where the electronic system is excited and probed by capacitive coupling. We derive realistic expressions for the velocity of the slow counterpropagating mode, starting from a microscopic calculation which is simplified by a Landau-Silin-like separation between long-range Hartree and residual interactions. The microscopic calculation determines the stiffness of the edge to long-wavelength neutral excitations, which fixes the slow-mode velocity, and the effective width of the edge region, which influences the magnetoplasmon dispersion.

cond-mat.mes-hall↗

arXiv1998

Magnons and skyrmions in fractional Hall ferromagnets

Recent experiments have established a qualitative difference between the magnetization temperature-dependences $M(T)$ of quantum Hall ferromagnets at integer and fractional filling factors. We explain this difference in terms of the relative energies of collective magnon and particle-hole excitations in the two cases. Analytic calculations for hard-core model systems are used to demonstrate that, in the fractional case, interactions suppress the magnetization at finite temperatures and that particle-hole excitations rather than long-wavelength magnons control $M(T)$ at low $T$.

cond-mat.mes-hall↗

arXiv1998

Properties of the Soliton-Lattice State in Double-Layer Quantum Hall Systems

Application of a sufficiently strong parallel magnetic field $B_\parallel > B_{c}$ produces a soliton-lattice (SL) ground state in a double-layer quantum Hall system. We calculate the ground-state properties of the SL state as a function of $B_\parallel$ for total filling factor $ν_{T}=1$, and obtain the total energy, anisotropic SL stiffness, Kosterlitz-Thouless melting temperature, and SL magnetization. The SL magnetization might be experimentally measurable, and the magnetic susceptibility diverges as $|B_\parallel - B_{c}|^{-1}$.

cond-mat.mes-hall↗

arXiv1998

Magnetic Anisotropy in Quantum Hall Ferromagnets

We show that the sign of magnetic anisotropy energy in quantum Hall ferromagnets is determined by a competition between electrostatic and exchange energies. Easy-axis ferromagnets tend to occur when Landau levels whose states have similar spatial profiles cross. We report measurements of integer QHE evolution with magnetic-field tilt. Reentrant behavior observed for the $ν= 4$ QHE at high tilt angles is attributed to easy-axis anisotropy. This interpretation is supported by a detailed calculation of the magnetic anisotropy energy.

cond-mat.mes-hall↗

arXiv1998

Circuit model for spin-bottleneck resistance in magnetic-tunnel-junction devices

Spin-bottlenecks are created in magnetic-tunnel-junction devices by spatial inhomogeneity in the relative resistances for up and down spins. We propose a simple electrical circuit model for these devices which incorporates spin-bottleneck effects and can be used to calculate their overall resistance and magnetoresistance. The model permits a simple understanding of the dependence of device magnetoresistance on spin diffusion lengths, tunneling magnetoresistance, and majority and minority spin resistivities in the ferromagnetic electrodes. The circuit model is in a good quantitative agreement with detailed transport calculations.

cond-mat↗

arXiv1998

Correlations weak and strong: divers guises of the two-dimensional electron gas

The three-dimensional electron-gas model has been a major focus for many-body theory applied to the electronic properties of metals and semiconductors. Because the model neglects band effects, whereas electronic systems are generally more strongly correlated in narrow band systems, it is most widely used to describe the qualitative physics of weakly correlated metals with unambiguous Fermi liquid properties. The model is more interesting in two space dimensions because it provides a quantitative description of electrons in quantum wells and because these can form strongly correlated many-particle states. We illustrate the range of possible many-particle behaviors by discussing the way correlations are manifested in 2D tunneling spectroscopy experiments.

cond-mat.str-el↗

arXiv1998

Shadow Bands and Tunneling Magnetoresistance in Itinerant Electron Ferromagnets

In itinerant electron ferromagnets spectral weight is transferred at finite temperatures from quasiparticle peaks located at majority and minority-spin band energies to shadow-band peaks. For a given Bloch wavevector and band index, the majority-spin shadow-band peak is located near the minority-spin quasiparticle energy and the minority-spin shadow-band peak is located near the majority-spin quasiparticle energy. This property can explain much of the temperature dependence seen in the magnetoresistance of magnetic tunnel junctions.

cond-mat↗

arXiv1997

Engineering Superfluidity in Electron-Hole Double Layers

We show that band-structure effects are likely to prevent superfluidity in semiconductor electron-hole double-layer systems. We suggest the possibility that superfluidity could be realized by the application of uniaxial pressure perpendicular to the electron and hole layers.

cond-mat↗

arXiv1997

Bulk charge distributions on integer and fractional quantum Hall plateaus

We discuss the charge distributions across the bulk of a two-dimensional electron gas system which is on an integer or fractional quantum Hall plateau. Our analysis is based on a relation, derived from the long wavelength limit of the bulk density-density response function, between the induced charge and leading derivatives of a slowly varying Hall potential. We use the Wiener-Hopf method to solve the coupled response and Poisson equations. Unlike the integer case, treated in previous work, the induced charge in the fractional case can alternate in sign.

cond-mat.mes-hall↗

arXiv1997

Mode Mixing in Antiferromagnetically Correlated Double Quantum Wells

We examine the robustness of a recently predicted exchange-induced zero-field magnetic phase in semiconductor double quantum wells in which each well is spin-polarized and the polarization vectors are antiparallel. Magnetic instabilities are a general feature of Coulombic double quantum well systems at low densities. We argue that this antiferromagnetic phase is stabilized relative to ferromagnetic ones by an effective superexchange interaction between the wells. Detailed self-consistent Hartree-Fock calculations using a point-contact model for the interaction show that the antiferromagnetic phase survives intra-subband repulsion matrix elements neglected in earlier work in a large portion of the model's parameter space. We also examine the role of asymmetry due to biasing or to differences in the widths of the two quantum wells. The asymmetry creates a mode coupling between the intra- and inter-subband collective spin-density excitations (SDEs) which changes the Raman spectroscopy signature of the phase transition from a complete softening of the inter-subband SDE to a cusp as the density is tuned through the transition. This cusp may be detectable in inelastic light scattering experiments in samples of sufficient quality at low enough temperatures and densities.

cond-mat.mes-hall↗

arXiv1997

A critical comparison of classical field theory and microscopic wavefunctions for skyrmions in quantum Hall ferromagnets

We report on a study of the classical field theory description of charged skyrmions in quantum Hall ferromagnets. The appropriate field theory is a non-linear $σ$ model generalized to include Coulomb and Zeeman interaction terms. We have tested the range of validity of the classical field theory by comparing with microscopic descriptions of the single skyrmion state based on the Hartree-Fock approximation, exact diagonalization calculations, and many-body trial wavefunctions. We find that the field theory description is accurate for skyrmions with moderate spin quantum numbers ($\gtrsim 10$) although, as expected, it fails qualitatively for small spin quantum numbers.

cond-mat.mes-hall↗

arXiv1997

Universal Phase Diagram for Vortex States of Layered Superconductors in Strong Magnetic Fields

We report on a Monte Carlo study of the lowest-Landau-level limit of the Lawrence-Doniach model for a layered superconductor. We have studied order parameter correlation functions for indications of the broken translational symmetry and the off-diagonal long range order present in the mean-field-theory vortex lattice. Our results are consistent with a single first order phase transition between a low temperature 3D vortex solid phase, with both broken translational symmetry and off-diagonal long range order, and a high temperature vortex liquid phase with no broken symmetries. We construct a universal phase diagram in terms of dimensionless parameters characterizing intra-layer and inter-layer couplings. The universal phase boundary extracted from our simulations and the associated latent heats and magnetization jumps are compared with experiment and with numerical results obtained for related models.

cond-mat.supr-con↗

arXiv1997

Collective Excitations, NMR, and Phase Transitions in Skyrme Crystals

At Landau level filling factors near nu =1, quantum Hall ferromagnets form a Skyrme crystal state with quasi-long-range translational and non-collinear magnetic order. We develop an effective low energy theory which explains the presence in these systems of magnetic excitations at low energies below the Larmor gap (Delta) and which predicts a dramatic enhancement of the nuclear spin relaxation rate by a factor of 1000. The effective theory predicts a rich set of quantum and classical phase transitions. Based in part on accurate time-dependent Hartree-Fock calculations of the ordered state collective excitation spectrum, we discuss aspects of the T-nu-Delta crystal phase diagram.

cond-mat.mes-hall↗

arXiv1997

Virial Expansions, Exclusion Statistics, and the Fractional Quantum Hall Effect

We report on a study of virial expansions for interacting electrons in the lowest Landau level of a two-dimensional electron gas. For hard-core-model interactions, we derive analytic results valid at low temperatures and filling factors smaller than 1/3 and comment on their relationship with virial expansions for exclusion statistics models. In the high temperature limit the virial coefficients reduce to those for a non-interacting electron system. For the first five virial coefficients, the crossover between low and high temperature limits has been studied numerically by using partition functions obtained from small system exact diagonalization calculations. Our results show that the exclusion statistics description of fractional Hall thermodynamics breaks down when the temperature exceeds a small fraction of the gap temperature.

cond-mat.mes-hall↗

arXiv1996

Hartree-Fock Theory of Skyrmions in Quantum Hall Ferromagnets

We report on a study of the charged-skyrmion or spin-texture excitations which occur in quantum Hall ferromagnets near odd Landau level filling factors. Particle-hole symmetry is used to relate the spin-quantum numbers of charged particle and hole excitations and neutral particle-hole pair excitations. Hartree-Fock theory is used to provide quantitative estimates of the energies of these excitations and their dependence on Zeeman coupling strength, Landau level quantum numbers, and the thicknesses of the two-dimensional electron layers. For the case of $ν$ near three we suggest the possibility of first order phase transitions with increasing Zeeman coupling strength from a many skyrmion state to one with many maximally spin-polarized quasiparticles.

cond-mat.mes-hall↗