arXiv subjects

T. Jungwirth

Publications and source records attributed to T. Jungwirth.

At least 181 records · Page 10Linked to original sources

arXiv2001

An Investigation of Orientational Symmetry-Breaking Mechanisms in High Landau Levels

The principal axes of the recently discovered anisotropic phases of 2D electron systems at high Landau level occupancy are consistently oriented relative to the crystal axes of the host semiconductor. The nature of the native rotational symmetry breaking field responsible for this preferential orientation remains unknown. Here we report on experiments designed to investigate the origin and magnitude of this symmetry breaking field. Our results suggest that neither micron-scale surface roughness features nor the precise symmetry of the quantum well potential confining the 2D system are important factors. By combining tilted field transport measurements with detailed self-consistent calculations we estimate that the native anisotropy energy, whatever its origin, is typically ~ 1 mK per electron.

cond-mat.mes-hall↗

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

Novel critical field in magneto-resistance oscillation of 2DEG in asymmetric GaAs/AlGaAs double wells measured as a function of the in-plane magnetic field

We have investigated the magnetoresistance of strongly asymmetric double-well structures formed by a thin AlGaAs barrier grown far from the interface in the GaAs buffer of standard heterostructures. In magnetic fields oriented parallel to the electron layers, the magnetoresistance exhibits an oscillation associated with the depopulation of the higher occupied subband and with the field-induced transition into a decoupled bilayer. In addition, the increasing field transfers electrons from the triangular to rectangular well and, at high enough field value, the triangular well is emptied. Consequently, the electronic system becomes a single layer which leads to a sharp step in the density of electron states and to an additional minimum in the magnetoresistance curve.

cond-mat.mes-hall↗

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↗

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

Magnetoresistance and electronic structure of asymmetric GaAs/AlGaAs double quantum wells in the in-plane/tilted magnetic field

Bilayer two-dimensional electron systems formed by a thin barrier in the GaAs buffer of a standard heterostructure were investigated by magnetotransport measurements. In magnetic fields oriented parallel to the electron layers, the magnetoresistance exhibits an oscillation associated with the depopulation of the higher occupied subband and the field-induced transition into a decoupled bilayer. Shubnikov-de Haas oscillations in slightly tilted magnetic fields allow to reconstruct the evolution of the electron concentration in the individual subbands as a function of the in-plane magnetic field. The characteristics of the system derived experimentally are in quantitative agreement with numerical self-consistent-field calculations of the electronic structure.

cond-mat.mes-hall↗

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.

cond-mat.mes-hall↗

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.

cond-mat.mes-hall↗

arXiv2000

Skyrmion Dynamics and NMR Line Shapes in QHE Ferromagnets

The low energy charged excitations in quantum Hall ferromagnets are topological defects in the spin orientation known as skyrmions. Recent experimental studies on nuclear magnetic resonance spectral line shapes in quantum well heterostructures show a transition from a motionally narrowed to a broader `frozen' line shape as the temperature is lowered at fixed filling factor. We present a skyrmion diffusion model that describes the experimental observations qualitatively and shows a time scale of $\sim 50 μ{\rm sec}$ for the transport relaxation time of the skyrmions. The transition is characterized by an intermediate time regime that we demonstrate is weakly sensitive to the dynamics of the charged spin texture excitations and the sub-band electronic wave functions within our model. We also show that the spectral line shape is very sensitive to the nuclear polarization profile along the z-axis obtained through the optical pumping technique.

cond-mat.mes-hall↗

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.

cond-mat.mes-hall↗

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.

cond-mat.mes-hall↗

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.

cond-mat.mes-hall↗

arXiv1999

Finite Temperature Behavior of the $ν=1$ Quantum Hall Effect in Bilayer Electron Systems

An effective field theoretic description of $ν=1$ bilayer electron systems stabilized by Coulomb repulsion in a single wide quantum well is examined using renormalization group techniques. The system is found to undergo a crossover from a low temperature strongly correlated quantum Hall state to a high temperature compressible state. This picture is used to account for the recent experimental observation of an anomalous transition in bilayer electron systems (T. S. Lay, {\em et al.} Phys. Rev. B {\bf 50}, 17725 (1994)). An estimate for the crossover temperature is provided, and it is shown that its dependence on electron density is in reasonable agreement with i the experiment.

cond-mat↗

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↗

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↗