arXiv subjects

A. H. MacDonald

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

At least 235 records · Page 13Linked to original sources

arXiv2002

Disorder and interactions in quantum Hall ferromagnets: effects of disorder in Skyrmion physics

We present a Hartree-Fock study of the competition between disorder and interactions in quantum Hall ferromagnets near $ν=1$. We find that the ground state at $ν=1$ evolves with increasing interaction strength from a quasi-metallic paramagnet, to a partially spin-polarized ferromagnetic Anderson insulator, and to a fully spin-polarized ferromagnet with a charge gap. Away from $ν=1$, the ground state evolves from a conventional Anderson insulator, to a conventional quasiparticle glass, and finally to a ferromagnetic Skyrmion quasiparticle glass. These different regimes can be measured in low-temperature transport and NMR experiments. We present calculations for the NMR spectra in different disorder regimes.

cond-mat.mes-hall↗

arXiv2002

Ferromagnetism in Diluted Magnetic Semiconductor Heterojunction Systems

Diluted magnetic semiconductors (DMSs), in which magnetic elements are substituted for a small fraction of host elements in a semiconductor lattice, can become ferromagnetic when doped. In this article we discuss the physics of DMS ferromagnetism in systems with semiconductor heterojunctions. We focus on the mechanism that cause magnetic and magnetoresistive properties to depend on doping profiles, defect distributions, gate voltage, and other system parameters that can in principle be engineered to yield desired results.

cond-mat.mtrl-sci↗

arXiv2002

Coupling between Edge and Bulk in Strong-Field Quantum Dots

The maximum-density-droplet (MDD) state of quantum-dot electrons becomes unstable at strong magnetic fields to the addition of interior holes. Using exact diagonalization, we demonstrate that the first hole is located at the center of the dot when the number of electrons $N$ is smaller than $\sim 14$ and is located away from the center for larger dots. The separation between field strengths at which additional holes are introduced becomes small for large dots, explaining recent observations of a rapid increase in dot area when the magnetic field is increased beyond the MDD stability limit. We comment on correlations between interior hole and collective edge fluctuations, and on the implications of these correlations for edge excitation models in bulk systems.

cond-mat.mes-hall↗

arXiv2002

Noncollinear Ferromagnetism in (III,Mn)V Semiconductors

We investigate the stability of the collinear ferromagnetic state in kinetic exchange models for (III,Mn)V semiconductors with randomly distributed Mn ions >. Our results suggest that {\em noncollinear ferromagnetism} is commom to these semiconductor systems. The instability of the collinear state is due to long-ranged fluctuations invloving a large fraction of the localized magnetic moments. We address conditions that favor the occurrence of noncollinear groundstates and discuss unusual behavior that we predict for the temperature and field dependence of its saturation magnetization.

cond-mat↗

arXiv2002

Metal-Insulator Transition and Ferromagnetism in Diluted Magnetic Semiconductors

We have investigated the interplay between the metal-insulator transition and ferromagnetism in $({\rm III}_{1-x},{\rm Mn}_x){\rm V}$ ferromagnetic semiconductors. Our study is based on a model in which $S=5/2$ Mn local moments are exchange-coupled to band electrons that interact via Coulomb interactions which each other, with ionized Mn acceptors, and with the antisite defects present in these materials. We find quasiparticle participation ratios that are consistent with a metal-insulator transition that occurs in the ferromagnetic state near $x \sim 0.01$. By evaluating the distribution of mean-field exchange coupling strengths at Mn moment sites, we provide evidence in favor of the applicability of small polaron and hole gas models on insulating and metallic sides of the phase transition respectively.

cond-mat↗

arXiv2002

Curie Temperature Trends in (III,Mn)V Ferromagnetic Semiconductors

We present a theoretical survey of ferromagnetic transition temperatures in cubic (III,Mn)V semiconductors based on a model with $S=5/2$ local moments exchange-coupled to itinerant holes in the host semiconductor valence band. Starting from the simplest mean-field theory of this model, we estimate the $T_c$ enhancement due to exchange and correlation in the itinerant-hole system, and the $T_c$ suppression due to collective fluctuations of the ordered moments. We show that high critical temperatures in these ferromagnetic semiconductors require both the large magnetic susceptibility contribution from the valence band's heavy holes and the large spin stiffness that results from strong spin-orbit coupling in the valence band. Our calculations demonstrate that this model for the ferromagnetism of these systems is fully consistent with the room-temperature ferromagnetism reported for Mn doped nitrides.

cond-mat.mtrl-sci↗

arXiv2001

Broken-Symmetry States in Quantum Hall Superlattices

We argue that broken-symmetry states with either spatially diagonal or spatially off-diagonal order are likely in the quantum Hall regime, for clean multiple quantum well (MQW) systems with small layer separations. We find that for MQW systems, unlike bilayers, charge order tends to be favored over spontaneous interlayer coherence. We estimate the size of the interlayer tunneling amplitude needed to stabilize superlattice Bloch minibands by comparing the variational energies of interlayer-coherent superlattice miniband states with those of states with charge order and states with no broken symmetries. We predict that when coherent miniband ground states are stable, strong interlayer electronic correlations will strongly enhance the growth-direction tunneling conductance and promote the possibility of Bloch oscillations.

cond-mat.mes-hall↗

arXiv2001

Theory of Ferromagnetism in Doped Excitonic Condensates

Nesting in a semimetal can lead to an excitonic insulator state with spontaneous coherence between conduction and valence bands and a gap for charged excitations. In this paper we present a theory of the ferromagnetic state that occurs when the density of electrons in the conduction band and holes in the valence band differ. We find an unexpectedly rich doping-field phase diagram and an unusual collective excitation spectrum that includes two gapless collective modes. We predict regions of doping and external field in which phase-separated condensates of electrons and holes with parallel spins and opposing spins coexist.

cond-mat.str-el↗

arXiv2001

Thermodynamic and Tunneling Density of States of the Integer Quantum Hall Critical State

We examine the long wave length limit of the self-consistent Hartree-Fock approximation irreducible static density-density response function by evaluating the charge induced by an external charge. Our results are consistent with the compressibility sum rule and inconsistent with earlier work that did not account for consistency between the exchange-local-field and the disorder potential. We conclude that the thermodynamic density of states is finite, in spite of the vanishing tunneling density of states at the critical energy of the integer quantum Hall transition.

cond-mat.mes-hall↗

arXiv2001

Magnetic domains in III-V magnetic semiconductors

Recent progress in theoretical understanding of magnetic anisotropy and stiffness in III-V magnetic semiconductors is exploited for predictions of magnetic domain characteristics and methods of their tuning. We evaluate the width and the energy of domain walls as well as the period of stripe domains in perpendicular films. The computed stripe width d = 1.1 um for Ga_0.957Mn_0.043As/In_0.16Ga_0.84As compares favorably to the experimental value 1.5 um, as determined by Shono et al. [Appl. Phys. Lett. 77, 1363 (2000)].

cond-mat.mtrl-sci↗

arXiv2001

Ferromagnetism in (III,Mn)V Semiconductors

In this review article we briefly summarize the main experimental properties of (III,Mn)V ferromagnets and outline several different but related approaches that have been explored in an effort to gain insight into these materials. The main body of the paper deals with the development of the semi-phenomenological model in which the low energy degrees of freedom are exchange-coupled valence-band holes and $S=5/2$ Mn local moments. We discuss physical predictions based on a mean-field treatment of the disorder-free model and demonstrate that it successfully describes a number of non-trivial properties of (Ga,Mn)As and (In,Mn)As ferromagnets. Study of collective excitations of these ferromagnets shows that the simple mean-field-theory is reasonably reliable for typical parameters of current samples but must fail at large carrier densities and also in the limit of very strong exchange coupling. Finally we discuss the results of Monte Carlo calculations that describe the effect of collective fluctuations of Mn moment orientations. The method can deal with some of the complications and additional physics, including the possibility of non-collinear ground states, that enters when disorder is added to the theoretical model.

cond-mat.mes-hall↗

arXiv2001

Anomalous Hall effect in ferromagnetic semiconductors

We present a theory of the anomalous Hall effect in ferromagnetic (Mn,III)V semiconductors. Our theory relates the anomalous Hall conductance of a homogeneous ferromagnet to the Berry phase acquired by a quasiparticle wavefunction upon traversing closed paths on the spin-split Fermi surface of a ferromagnetic state. It can be applied equally well to any itinerant electron ferromagnet. The quantitative agreement between our theory and experimental data in both (In,Mn)As and (Ga,Mn)As systems suggests that this disorder independent contribution to the anomalous Hall conductivity dominates in diluted magnetic semiconductors.

cond-mat.mes-hall↗

arXiv2001

Theory of Magnetic Properties and Spin-Wave Dispersion for Ferromagnetic (Ga,Mn)As

We present a microscopic theory of the long-wavelength magnetic properties of the ferromagnetic diluted magnetic semiconductor (Ga,Mn)As. Details of the host semiconductor band structure, described by a six-band Kohn-Luttinger Hamiltonian, are taken into account. We relate our quantum-mechanical calculation to the classical micromagnetic energy functional and determine anisotropy energies and exchange constants. We find that the exchange constant is substantially enhanced compared to the case of a parabolic heavy-hole-band model.

cond-mat.mtrl-sci↗

arXiv2001

Global phase diagram of bilayer quantum Hall ferromagnets

We present a microscopic study of the interlayer spacing d versus in-plane magnetic field $B_\parallel$ phase diagram for bilayer quantum Hall (QH) pseudo-ferromagnets. In addition to the interlayer charge balanced commensurate and incommensurate states analyzed previously, we address the corresponding interlayer charge unbalanced "canted" QH states. We predict a large anomaly in the bilayer capacitance at the canting transition and the formation of dipole stripe domains with periods exceeding 1 micron in the canted state.

cond-mat.mes-hall↗

arXiv2001

2D bands and electron-phonon interactions in polyacene plastic transistors

We present a simple tight-binding model for the two-dimensional energy bands of polyacene field-effect transistors and for the coupling of these bands to lattice vibrations of their host molecular crystal. We argue that the strongest electron-phonon interactions in these systems originate from the dependence of inter-molecule hopping amplitudes on collective molecular motion, and introduce a generalized Su-Schrieffer-Heeger model that accounts for all vibrations and is parameter-free once the band mass has been specified. We compute the electron-phonon spectral function $α^2F(ω)$ as a function of two-dimensional hole density, and are able to explain the onset of superconductivity near 2D carrier density $n_{2D} \sim 10^{14} cm^{-2}$, discovered in recent experiments by Schön {\it et al.} \onlinecite{Batlogg}.

cond-mat.supr-con↗

arXiv2001

Tomonaga-Luttinger parameters for quantum wires

The low-energy properties of a homogeneous one-dimensional electron system are completely specified by two Tomonaga-Luttinger parameters $K_ρ$ and $v_σ$. In this paper we discuss microscopic estimates of the values of these parameters in semiconductor quantum wires that exploit their relationship to thermodynamic properties. Motivated by the recognized similarity between correlations in the ground state of a one-dimensional electron liquid and correlations in a Wigner crystal, we evaluate these thermodynamic quantities in a self-consistent Hartree-Fock approximation. According to our calculations, the Hartree-Fock approximation ground state is a Wigner crystal at all electron densities and has antiferromagnetic order that gradually evolves from spin-density-wave to localized in character as the density is lowered. Our results for $K_ρ$ are in good agreement with weak-coupling perturbative estimates $K_ρ^{pert}$ at high densities, but deviate strongly at low densities, especially when the electron-electron interaction is screened at long distances. $K_ρ^{pert}\sim n^{1/2}$ vanishes at small carrier density $n$ whereas we conjecture that $K_ρ\to 1/2$ when $n\to 0$, implying that $K_ρ$ should pass through a minimum at an intermediate density. Observation of such a non-monotonic dependence on particle density would allow to measure the range of the microscopic interaction. In the spin sector we find that the spin velocity decreases with increasing interaction strength or decreasing $n$. Strong correlation effects make it difficult to obtain fully consistent estimates of $v_σ$ from Hartree-Fock calculations. We conjecture that $v_σ/\vf\propto n/V_0$ in the limit $n\to 0$ where $V_0$ is the interaction strength.

cond-mat.mes-hall↗

arXiv2001

Phase transition and spin-wave dispersion in quantum Hall bilayers at filling factor nu=1

We present an effective Hamiltonian for a bilayer quantum Hall system at filling factor $ν=1$ neglecting charge fluctuations. Our model is formulated in terms of spin and pseudospin operators and is an exact representation of the system within the above approximation. We analyze its low-lying excitations in terms of spin-wave theory. Moreover we add to previous first-principle exact-diagonalization studies concentrating on the quantum phase transition seen in this system.

cond-mat.mes-hall↗

arXiv2001

Monte Carlo Study of Ferromagnetism in (III,Mn)V Semiconductors

We report on Monte Carlo studies of the kinetic exchange model for (III,Mn)V ferromagnetic semiconductors in which S=5/2 local moments, representing Mn^{2+} ions, are exchange coupled to band electrons. We treat the Mn^{2+}$ spin orientations as classical degrees of freedom and use the Hybrid Monte Carlo algorithm to explore thermodynamically important Mn spin configurations. The critical temperature T_c of the model is unambiguously signalled in our finite-size simulations by pronounced peaks in fluctuations of both Mn and band carrier total spins. The T_c's we obtain are, over much of the model's parameter space, substantially smaller than those estimated using mean-field theory. When mean-field theory fails, short-range magnetic order and finite {\em local} carrier spin polarisation are present for temperatures substantially larger than T_{c}. For the simplest version of the model, which has a single parabolic band with effective mass m^*, the dependence of T_{c} on m^* is sublinear at large masses, in disagreement with the mean-field theory result T_{c} \propto m^*. For the carrier densities studied, results obtained with a six-band Luttinger model band Hamiltonian are in good agreement with those obtained for a model with a single parabolic band whose effective mass is equal to the Kohn-Luttinger model heavy-hole mass.

cond-mat↗