19 research outputs found
Spin Waves in Disordered III-V Diluted Magnetic Semiconductors
We propose a new scheme for numerically computing collective-mode spectra for
large-size systems, using a reformulation of the Random Phase Approximation. In
this study, we apply this method to investigate the spectrum and nature of the
spin-waves of a (III,Mn)V Diluted Magnetic Semiconductor. We use an impurity
band picture to describe the interaction of the charge carriers with the local
Mn spins. The spin-wave spectrum is shown to depend sensitively on the
positional disorder of the Mn atoms inside the host semiconductor. Both
localized and extended spin-wave modes are found. Unusual spin and charge
transport is implied.Comment: 14 pages, including 11 figure
Exchange anisotropy, disorder and frustration in diluted, predominantly ferromagnetic, Heisenberg spin systems
Motivated by the recent suggestion of anisotropic effective exchange
interactions between Mn spins in GaMnAs (arising as a result of
spin-orbit coupling), we study their effects in diluted Heisenberg spin
systems. We perform Monte Carlo simulations on several phenomenological model
spin Hamiltonians, and investigate the extent to which frustration induced by
anisotropic exchanges can reduce the low temperature magnetization in these
models and the interplay of this effect with disorder in the exchange. In a
model with low coordination number and purely ferromagnetic (FM) exchanges, we
find that the low temperature magnetization is gradually reduced as exchange
anisotropy is turned on. However, as the connectivity of the model is
increased, the effect of small-to-moderate anisotropy is suppressed, and the
magnetization regains its maximum saturation value at low temperatures unless
the distribution of exchanges is very wide. To obtain significant suppression
of the low temperature magnetization in a model with high connectivity, as is
found for long-range interactions, we find it necessary to have both
ferromagnetic and antiferromagnetic (AFM) exchanges (e.g. as in the RKKY
interaction). This implies that disorder in the sign of the exchange
interaction is much more effective in suppressing magnetization at low
temperatures than exchange anisotropy.Comment: 9 pages, 8 figure
Influence of disorder on the ferromagnetism in diluted magnetic semiconductors
Influence of disorder on the ferromagnetic phase transition in diluted
(III,Mn)V semiconductors is investigated analytically. The regime of small
disorder is addressed, and the enhancement of the critical temperature by
disorder is found both in the mean field approximation and from the analysis of
the zero temperature spin stiffness. Due to disorder, the spin wave
fluctuations around the ferromagnetically ordered state acquire a finite mass.
At large charge carrier band width, the spin wave mass squared becomes
negative, signaling the breakdown of the ferromagnetic ground state and the
onset of a noncollinear magnetic order.Comment: Replaced with revised version. 10 pages, 3 figure
Magnetic spin excitations in Mn doped GaAs : A model study
We provide a quantitative theoretical model study of the dynamical magnetic
properties of optimally annealed GaMnAs. This model has already
been shown to reproduce accurately the Curie temperatures for
GaMnAs. Here we show that the calculated spin stiffness are in
excellent agreement with those which were obtained from ab-initio based
studies. In addition, an overall good agreement is also found with available
experimental data. We have also evaluated the magnon density of states and the
typical density of states from which the "mobility edge", separating the
extended from localized magnon states, was determined. The power of the model
lies in its ability to be generalized for a broad class of diluted magnetic
semiconductor materials, thus it bridges the gap between first principle
calculations and model based studies.Comment: 5 pages, 5 figures, Text and some figures revised to match the
accepted versio
A Dynamical Study of the Quantum p=2 Spherical Model
We present a dynamical study of the disordered quantum p=2 spherical model at
long times. Its phase behavior as a function of spin-bath coupling, strength of
quantum fluctuations and temperature is characterized, and we identify
different paramagnetic and coarsened regions. A quantum critical point at zero
temperature in the limit of vanishing dissipation is also found. Furthermore we
show analytically that the fluctuation-dissipation theorem is obeyed in the
stationary regime.Comment: 13 pages, 4 figures; published versio
Monte Carlo simulations of an impurity band model for III-V diluted magnetic semiconductors
We report the results of a Monte Carlo study of a model of (III,Mn)V diluted
magnetic semiconductors which uses an impurity band description of carriers
coupled to localized Mn spins and is applicable for carrier densities below and
around the metal-insulator transition. In agreement with mean field studies, we
find a transition to a ferromagnetic phase at low temperatures. We compare our
results for the magnetic properties with the mean field approximation, as well
as with experiments, and find favorable qualitative agreement with the latter.
The local Mn magnetization below the Curie temperature is found to be spatially
inhomogeneous, and strongly correlated with the local carrier charge density at
the Mn sites. The model contains fermions and classical spins and hence we
introduce a perturbative Monte Carlo scheme to increase the speed of our
simulations.Comment: 17 pages, 24 figures, 2 table
Hard Loop Approach to Anisotropic Systems
Anisotropic systems of quarks and gluons, which at least for sufficiently
short space-time intervals can be treated as homogeneous and static, are
considered. The gluon polarization tensor of such a system is explicitly
computed within the semiclassical kinetic and Hard Loop diagrammatic theories.
The equivalence of the two approaches is demonstrated. The quark self energy is
computed as well, and finally, the dispersion relations of quarks and gluons in
the anisotropic medium are discussed.Comment: 10 pages, revised to appear in Phys. Rev.
Temperature-dependent magnetization in diluted magnetic semiconductors
We calculate magnetization in magnetically doped semiconductors assuming a
local exchange model of carrier-mediated ferromagnetic mechanism and using a
number of complementary theoretical approaches. In general, we find that the
results of our mean-field calculations, particularly the dynamical mean field
theory results, give excellent qualitative agreement with the experimentally
observed magnetization in systems with itinerant charge carriers, such as
Ga_{1-x}Mn_xAs with 0.03 < x < 0.07, whereas our percolation-theory-based
calculations agree well with the existing data in strongly insulating
materials, such as Ge_{1-x}Mn_x. We comment on the issue of non-mean-field like
magnetization curves and on the observed incomplete saturation magnetization
values in diluted magnetic semiconductors from our theoretical perspective. In
agreement with experimental observations, we find the carrier density to be the
crucial parameter determining the magnetization behavior. Our calculated
dependence of magnetization on external magnetic field is also in excellent
agreement with the existing experimental data.Comment: 17 pages, 15 figure
Spatially heterogeneous ages in glassy dynamics
We construct a framework for the study of fluctuations in the nonequilibrium
relaxation of glassy systems with and without quenched disorder. We study two
types of two-time local correlators with the aim of characterizing the
heterogeneous evolution: in one case we average the local correlators over
histories of the thermal noise, in the other case we simply coarse-grain the
local correlators. We explain why the former describe the fingerprint of
quenched disorder when it exists, while the latter are linked to noise-induced
mesoscopic fluctuations. We predict constraints on the pdfs of the fluctuations
of the coarse-grained quantities. We show that locally defined correlations and
responses are connected by a generalized local out-of-equilibrium
fluctuation-dissipation relation. We argue that large-size heterogeneities in
the age of the system survive in the long-time limit. The invariance of the
theory under reparametrizations of time underlies these results. We relate the
pdfs of local coarse-grained quantities and the theory of dynamic random
manifolds. We define a two-time dependent correlation length from the spatial
decay of the fluctuations in the two-time local functions. We present numerical
tests performed on disordered spin models in finite and infinite dimensions.
Finally, we explain how these ideas can be applied to the analysis of the
dynamics of other glassy systems that can be either spin models without
disorder or atomic and molecular glassy systems.Comment: 47 pages, 60 Fig
Probing Fermi surface anisotropies in layered metals with AMRO
A layered crystal structure and strong interactions between electrons are common features of many interesting electronic materials. The coherence of interlayer charge transport and origin of intralayer anisotropies in such materials are important questions. We show that angle dependent magnetoresistance oscillations (AMRO) are sensitive to anisotropies around an intralayer Fermi surface, and discuss their asymptotic behaviour in the high field limit