574 research outputs found
Interactions of multi-scale heterogeneity in the lithosphere: Australia
Understanding the complex heterogeneity of the continental lithosphere involves a wide variety of spatial scales and the synthesis of multiple classes of information. Seismic surface waves and multiply reflected body waves provide the main constraints on broad-scale structure, and bounds on the extent of the lithosphere-asthenosphere transition (LAT) can be found from the vertical gradients of S wavespeed. Information on finer-scale structures comes through body wave studies, including detailed seismic tomography and P-wave reflectivity extracted from stacked autocorrelograms of continuous component records. With the inclusion of deterministic large-scale structure and realistic medium-scale stochastic features fine-scale variations are subdued. The resulting multi-scale heterogeneity model for the Australian region gives a good representation of the character of observed seismograms and their geographic variations and matches the observations of P-wave reflectivity. P reflections in the 0.5–3.0 Hz band in the uppermost mantle suggest variations on vertical scales of a few hundred metres with amplitudes of the order of 1%. Interference of waves reflected or converted at sequences of such modest variations in physical properties produce relatively simple behaviour for lower frequencies, which can suggest simpler structures than are actually present. Vertical changes in the character of fine-scale heterogeneity can produce apparent discontinuities. In Central Australia a ‘mid-lithospheric discontinuity’ can be tracked via changes in frequency content of station reflectivity, with links to the broad-scale pattern of wavespeed gradients and, in particular, the gradients of radial anisotropy. Comparisons with xenolith results from southeastern Australia indicate a strong tie between geochemical stratification and P-wave reflectivity.Part of this study was supported by JSPS KAKENHI Grant Number
26400443 to K.Y., with additional support from the AuScope AuSREM
project. The Earth Simulator Center of JAMSTEC is thanked for providing
CPU time on the Earth Simulator
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
Bound Magnetic Polaron Interactions in Insulating Doped Diluted Magnetic Semiconductors
The magnetic behavior of insulating doped diluted magnetic semiconductors
(DMS) is characterized by the interaction of large collective spins known as
bound magnetic polarons. Experimental measurements of the susceptibility of
these materials have suggested that the polaron-polaron interaction is
ferromagnetic, in contrast to the antiferromagnetic carrier-carrier
interactions that are characteristic of nonmagnetic semiconductors. To explain
this behavior, a model has been developed in which polarons interact via both
the standard direct carrier-carrier exchange interaction (due to virtual
carrier hopping) and an indirect carrier-ion-carrier exchange interaction (due
to the interactions of polarons with magnetic ions in an interstitial region).
Using a variational procedure, the optimal values of the model parameters were
determined as a function of temperature. At temperatures of interest, the
parameters describing polaron-polaron interactions were found to be nearly
temperature-independent. For reasonable values of these constant parameters, we
find that indirect ferromagnetic interactions can dominate the direct
antiferromagnetic interactions and cause the polarons to align. This result
supports the experimental evidence for ferromagnetism in insulating doped DMS.Comment: 11 pages, 7 figure
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
Neutrino emission via the plasma process in a magnetized plasma
Neutrino emission via the plasma process using the vertex formalism for QED
in a strongly magnetized plasma is considered. A new vertex function is
introduced to include the axial vector part of the weak interaction. Our
results are compared with previous calculations, and the effect of the axial
vector coupling on neutrino emission is discussed. The contribution from the
axial vector coupling can be of the same order as or greater than the vector
vector coupling under certain plasma conditions.Comment: 20 pages, 3 figure
Indirect exchange in GaMnAs bilayers via spin-polarized inhomogeneous hole gas: Monte Carlo simulation
The magnetic order resulting from an indirect exchange between magnetic
moments provided by spin-polarized hole gas in the metallic phase of a GaMnAs
double layer structure is studied via Monte Carlo simulation. The coupling
mechanism involves a perturbative calculation in second order of the
interaction between the magnetic moments and carriers (holes). We take into
account a possible polarization of the hole gas due to the existence of an
average magnetization in the magnetic layers, establishing, in this way, a
self-consistency between the magnetic order and the electronic structure. That
interaction leads to an internal ferromagnetic order inside each layer, and a
parallel arrangement between their magnetizations, even in the case of thin
layers. This fact is analyzed in terms of the inter- and intra-layer
interactions.Comment: 17 pages and 14 figure
Dissipative effects on quantum glassy systems
We discuss the behavior of a quantum glassy system coupled to a bath of
quantum oscillators. We show that the system localizes in the absence of
interactions when coupled to a subOhmic bath. When interactions are switched on
localization disappears and the system undergoes a phase transition towards a
glassy phase. We show that the position of the critical line separating the
disordered and the ordered phases strongly depends on the coupling to the bath.
For a given type of bath, the ordered glassy phase is favored by a stronger
coupling. Ohmic, subOhmic and superOhmic baths lead to different transition
lines. We draw our conclusions from the analysis of the partition function
using the replicated imaginary-time formalism and from the study of the
real-time dynamics of the coupled system using the Schwinger-Keldysh closed
time-path formalism.Comment: 39 pages, 13 figures, RevTe
Ferromagnetism in a dilute magnetic semiconductor -- Generalized RKKY interaction and spin-wave excitations
Carrier-mediated ferromagnetism in a dilute magnetic semiconductor has been
studied using i) a single-impurity based generalized RKKY approach which goes
beyond linear response theory, and ii) a mean-field-plus-spin-fluctuation
(MF+SF) approach within a (purely fermionic) Hubbard-model representation of
the magnetic impurities, which incorporates dynamical effects associated with
finite frequency spin correlations in the ordered state. Due to a competition
between the magnitude of the carrier spin polarization and its oscillation
length scale, the ferromagnetic spin coupling is found to be optimized with
respect to both hole doping concentration and impurity-carrier spin coupling
energy (or equivalently ). The ferromagnetic transition temperature
, deteremined within the spin-fluctuation theory, corresponds closely with
the observed values. Positional disorder of magnetic impurities causes
significant stiffening of the high-energy magnon modes. We also explicitly
study the stability/instability of the mean-field ferromagnetic state, which
highlights the role of competing AF interactions causing spin twisting and
noncollinear ferromagnetic ordering.Comment: 10 pages, 12 figure
Two-component approach for thermodynamic properties in diluted magnetic semiconductors
We examine the feasibility of a simple description of Mn ions in III-V
diluted magnetic semiconductors (DMSs) in terms of two species (components),
motivated by the expectation that the Mn-hole exchange couplings are widely
distributed, expecially for low Mn concentrations. We find, using distributions
indicated by recent numerical mean field studies, that the thermodynamic
properties (magnetization, susceptibility, and specific heat) cannot be fit by
a single coupling as in a homogeneous model, but can be fit well by a
two-component model with a temperature dependent number of ``strongly'' and
``weakly'' coupled spins. This suggests that a two-component description may be
a minimal model for the interpretation of experimental measurements of
thermodynamic quantities in III-V DMS systems.Comment: 10 pages, 9 figures, 1 new figure, substantial revision
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