arXiv subjects

A. H. MacDonald

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

At least 253 records · Page 14Linked to original sources

arXiv2001

Field-domain spintronics in magnetic semiconductor multiple quantum wells

We develop a theory of non-linear growth direction transport in magnetically doped II-VI compound semiconductor multiple-quantum-well systems. We find that the formation of electric field domains can be controlled by manipulating the space dependence of the band electron spin-polarization, using its exchange coupling to local moments. We emphasize the importance of band electron spin relaxation in limiting the strength of these effects.

cond-mat↗

arXiv2001

Field Effect Magnetization Reversal in Ferromagnetic Semiconductor Quantum Wells

We predict that a novel bias-voltage assisted magnetization reversal process will occur in Mn doped II-VI semiconductor quantum wells or heterojunctions with carrier induced ferromagnetism. The effect is due to strong exchange-coupling induced subband mixing that leads to electrically tunable hysteresis loops. Our model calculations are based on the mean-field theory of carrier induced ferromagnetism in Mn-doped quantum wells and on a semi-phenomenological description of the host II-VI semiconductor valence bands.

cond-mat.mtrl-sci↗

arXiv2001

Resistance spikes and domain wall loops in Ising quantum Hall ferromagnets

We explain the recent observation of resistance spikes and hysteretic transport properties in Ising quantum Hall ferromagnets in terms of the unique physics of their domain walls. Self-consistent RPA/Hartree-Fock theory is applied to microscopically determine properties of the ground state and domain-wall excitations. In these systems domain wall loops support one-dimensional electron systems with an effective mass comparable to the bare electron mass and may carry charge. Our theory is able to account quantitatively for the experimental Ising critical temperature and to explain characteristics of the resistive hysteresis loops.

cond-mat.mes-hall↗

arXiv2001

Quantum Hall Stripe States in a Tilted Magnetic Field

A strong anisotropy in the longitudinal resistivity of a 2D electron system has been observed at half-filled high Landau levels. We report on detailed Hartree-Fock calculations of the unidirectional charge density wave (UCDW) orientation energy induced by a tilted magnetic field. We find that stripes can orient both parallel or perpendicular to the in-plane field depending on the sample parameters and field strength. The close agreement between complex experimental data on different sample geometries and our theoretical results strongly support the UCDW picture as the origin of the observed anisotropies in high Landau levels.

cond-mat.mes-hall↗

arXiv2001

Hole-hole correlation effects on magnetic properties of Mn$_x$III$_{1-x}$V diluted magnetic semiconductors

The mean-field theory represents a useful starting point for studying carrier-induced ferromagnetism in Mn$_x$III$_{1-x}$V diluted magnetic semiconductors. A detail description of these systems requires to include correlations in the many-body hole system. We discuss the effects of correlations among itinerant carriers on magnetic properties of bulk Mn$_x$III$_{1-x}$V and magnetic semiconductor quantum wells. Presented results were obtained using parabolic band approximation and we also derive a many-body perturbation technique that allows to account for hole-hole correlations in realistic semiconductor valence bands.

cond-mat.mtrl-sci↗

arXiv2001

Mean-field theory of magnetic properties of Mn$_x$III$_{1-x}$V semiconductors

We present a mean-field theory of carrier-induced ferromagnetism in Mn$_x$III$_{1-x}$V diluted magnetic semiconductors with a special emphasis placed on the magnetic anisotropy. The valence band holes are described using the six band Kohn-Luttinger model. We find that the magnetic anisotropy is a complicated function of sample parameters such as hole density or strain. Results of our numerical simulations are in agreement with magnetic anisotropy measurements on samples with both compressive and tensile strains.

cond-mat.mtrl-sci↗

arXiv2001

Incommensurate ground state of double-layer quantum Hall systems

Double-layer quantum Hall systems possess interlayer phase coherence at sufficiently small layer separations, even without interlayer tunneling. When interlayer tunneling is present, application of a sufficiently strong in-plane magnetic field $B_\parallel > B_c$ drives a commensurate-incommensurate (CI) transition to an incommensurate soliton-lattice (SL) state. We calculate the Hartree-Fock ground-state energy of the SL state for all values of $B_\parallel$ within a gradient approximation, and use it to obtain the anisotropic SL stiffness, the Kosterlitz-Thouless melting temperature for the SL, and the SL magnetization. The in-plane differential magnetic susceptibility diverges as $(B_\parallel - B_c)^{-1}$ when the CI transition is approached from the SL state.

cond-mat.mes-hall↗

arXiv2001

Hartree-Fock Theory of Hole Stripe States

We report on Hartree-Fock theory results for stripe states of two-dimensional hole systems in quantum wells grown on GaAs (311)A substrates. We find that the stripe orientation energy has a rich dependence on hole density, and on in-plane field magnitude and orientation. Unlike the electron case, the orientation energy is non-zero for zero in-plane field, and the ground state orientation can be either parallel or perpendicular to a finite in-plane field. We predict an orientation reversal transition in in-plane fields applied along the $\lbrack\bar{2}33\rbrack$ direction.

cond-mat.mes-hall↗

arXiv2001

Nondissipative Drag Conductance as a Topological Quantum Number

We show in this paper that the boundary condition averaged nondissipative drag conductance of two coupled mesoscopic rings with no tunneling, evaluated in a particular many-particle eigenstate, is a topological invariant characterized by a Chern integer. Physical implications of this observation are discussed.

cond-mat.mes-hall↗

arXiv2001

Strong Correlation to Weak Correlation Phase Transition in Bilayer Quantum Hall Systems

At small layer separations, the ground state of a nu=1 bilayer quantum Hall system exhibits spontaneous interlayer phase coherence and has a charged-excitation gap E_g. The evolution of this state with increasing layer separation d has been a matter of controversy. In this letter we report on small system exact diagonalization calculations which suggest that a single phase transition, likely of first order, separates coherent incompressible (E_g >0) states with strong interlayer correlations from incoherent compressible states with weak interlayer correlations. We find a dependence of the phase boundary on d and interlayer tunneling amplitude that is in very good agreement with recent experiments.

cond-mat.mes-hall↗

arXiv2000

Theory of Tunneling Spectroscopy in Ferromagnetic Nanoparticles

We present a theory of the low-energy excitations of a ferromagnetic metal nanoparticle. In addition to the particle-hole excitations, which occur in a paramagnetic metal nanoparticle, we predict a branch of excitations involving the magnetization-orientation collective coordinate. Tunneling matrix elements are in general sizable for several different collective states associated with the same band configuration. We point out that the average change in ground state spin per added electron differs from non-interacting quasiparticle expectations, and that the change in the spin-polarization, due to Zeeman coupling, is strongly influenced by Coulomb blockade physics.

cond-mat.mes-hall↗

arXiv2000

Quantum Dynamics of Pseudospin Solitons in Double-Layer Quantum Hall Systems

Pseudospin solitons in double-layer quantum Hall systems can be introduced by a magnetic field component coplanar with the electrons and can be pinned by applying voltages to external gates. We estimate the temperature below which depinning occurs predominantly via tunneling and calculate low-temperature depinning rates for realistic geometries. We discuss the local changes in charge and current densities and in spectral functions that can be used to detect solitons and observe their temporal evolution.

cond-mat.mes-hall↗

arXiv2000

Double-Occupancy Errors, Adiabaticity, and Entanglement of Spin-Qubits in Quantum Dots

Quantum gates that temporarily increase singlet-triplet splitting in order to swap electronic spins in coupled quantum dots, lead inevitably to a finite double-occupancy probability for both dots. By solving the time-dependent Schrödinger equation for a coupled dot model, we demonstrate that this does not necessarily lead to quantum computation errors. Instead, the coupled dot ground state evolves quasi-adiabatically for typical system parameters so that the double-occupancy probability at the completion of swapping is negligibly small. We introduce a measure of entanglement which explicitly takes into account the possibilty of double occupancies and provides a necessary and sufficient criterion for entangled states.

cond-mat.mes-hall↗

arXiv2000

The nature of ergodicity breaking in Ising spin glasses as revealed by correlation function spectral properties

In this work we address the nature of broken ergodicity in the low temperature phase of Ising spin glasses by examining spectral properties of spin correlation functions $C_{ij}\equiv< S_i S_j>$. We argue that more than one extensive (i.e., O(N)) eigenvalue in this matrix signals replica symmetry breaking. Monte-Carlo simulations of the infinite-range Ising spin-glass model, above and below the Almeida-Thouless line, support this conclusion. Exchange Monte-Carlo simulations for the short-range model in four dimensions find a single extensive eigenvalue and a large subdominant eigenvalue consistent with droplet model expectations.

cond-mat.dis-nn↗

arXiv2000

Inhomogeneous order-parameter suppression in disordered d-wave superconductors

We report on a numerical study of disorder effects in 2D d-wave BCS superconductors. We compare exact numerical solutions of the Bogoliubov-deGennes (BdG) equations for the density of states $ρ(E)$ with the standard T-matrix approximation. Local suppression of the order parameter near impurity sites, which occurs in self-consistent solutions of the BdG equations, leads to apparent power law behavior $ρ(E) \sim |E|^α$ with non-universal $α$ over an energy scale comparable to the single impurity resonance energy $Ω_0$. We show that the novel effects arise from static spatial correlations between the order parameter and the impurity distribution.

cond-mat.supr-con↗

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

Theory of Magnetic Anisotropy in III_{1-x}Mn_{x}V Ferromagnets

We present a theory of magnetic anisotropy in ${\rm III}_{1-x}{\rm Mn}_{x}{\rm V}$ diluted magnetic semiconductors with carrier-induced ferromagnetism. The theory is based on four and six band envelope functions models for the valence band holes and a mean-field treatment of their exchange interactions with ${\rm Mn}^{++}$ ions. We find that easy-axis reorientations can occur as a function of temperature, carrier density $p$, and strain. The magnetic anisotropy in strain-free samples is predicted to have a $p^{5/3}$ hole-density dependence at small $p$, a $p^{-1}$ dependence at large $p$, and remarkably large values at intermediate densities. An explicit expression, valid at small $p$, is given for the uniaxial contribution to the magnetic anisotropy due to unrelaxed epitaxial growth lattice-matching strains. Results of our numerical simulations are in agreement with magnetic anisotropy measurements on samples with both compressive and tensile strains. We predict that decreasing the hole density in current samples will lower the ferromagnetic transition temperature, but will increase the magnetic anisotropy energy and the coercivity.

cond-mat.mtrl-sci↗

arXiv2000

Details of disorder matter in 2D d-wave superconductors

We demonstrate that discrepancies between predicted low-energy quasiparticle properties in disordered 2D d-wave superconductors occur because of the unanticipated importance of disorder model details and normal-state particle-hole symmetry. This conclusion follows from numerically exact evaluations of the quasiparticle density-of-states predicted by the Bogoliubov-deGennes (BdG) mean field equations for both binary alloy and random site energy disorder models. For the realistic case, which is best described by a binary alloy model without particle-hole symmetry, we predict density-of-states suppression below an energy scale which appears to be correlated with the corresponding single-impurity resonance.

cond-mat.supr-con↗