285 research outputs found
Derivation of phenomenological expressions for transition matrix elements for electron-phonon scattering
In the literature on electron-phonon scatterings very often a
phenomenological expression for the transition matrix element is used which was
derived in the textbooks of Ashcroft/Mermin and of Czycholl. There are various
steps in the derivation of this expression. In the textbooks in part different
arguments have been used in these steps, but the final result is the same. In
the present paper again slightly different arguments are used which motivate
the procedure in a more intuitive way. Furthermore, we generalize the
phenomenological expression to describe the dependence of the matrix elements
on the spin state of the initial and final electron state
Electronic correlations in vanadium chalcogenides: BaVSe3 versus BaVS3
Albeit structurally and electronically very similar, at low temperature the
quasi-one-dimensional vanadium sulfide BaVS3 shows a metal-to-insulator
transition via the appearance of a charge-density-wave state, while BaVSe3
apparently remains metallic down to zero temperature. This different behavior
upon cooling is studied by means of density functional theory and its
combination with the dynamical mean-field theory and the rotationally-invariant
slave-boson method. We reveal several subtle differences between these
chalcogenides that provide indications for the deviant behavior of BaVSe3 at
low temperature. In this regard, a smaller Hubbard U in line with an increased
relevance of the Hund's exchange J plays a vital role.Comment: 16 pages, 11 figures, published versio
Scaling Analysis of the Site-Diluted Ising Model in Two Dimensions
A combination of recent numerical and theoretical advances are applied to
analyze the scaling behaviour of the site-diluted Ising model in two
dimensions, paying special attention to the implications for multiplicative
logarithmic corrections. The analysis focuses primarily on the odd sector of
the model (i.e., that associated with magnetic exponents), and in particular on
its Lee-Yang zeros, which are determined to high accuracy. Scaling relations
are used to connect to the even (thermal) sector, and a first analysis of the
density of zeros yields information on the specific heat and its corrections.
The analysis is fully supportive of the strong scaling hypothesis and of the
scaling relations for logarithmic corrections.Comment: 15 pages, 3 figures. Published versio
Magnetism in systems with various dimensionality: A comparison between Fe and Co
A systematic ab initio study is performed for the spin and orbital moments
and for the validity of the sum rules for x-ray magnetic circular dichroism for
Fe systems with various dimensionality (bulk, Pt-supported monolayers and
monatomic wires, free-standing monolayers and monatomic wires). Qualitatively,
the results are similar to those for the respective Co systems, with the main
difference that for the monatomic Fe wires the term in the spin sum rule
is much larger than for the Co wires. The spin and orbital moments induced in
the Pt substrate are also discussed.Comment: 4 page
Effective critical behaviour of diluted Heisenberg-like magnets
In agreement with the Harris criterion, asymptotic critical exponents of
three-dimensional (3d) Heisenberg-like magnets are not influenced by weak
quenched dilution of non-magnetic component. However, often in the experimental
studies of corresponding systems concentration- and temperature-dependent
exponents are found with values differing from those of the 3d Heisenberg
model.
In our study, we use the field--theoretical renormalization group approach to
explain this observation and to calculate the effective critical exponents of
weakly diluted quenched Heisenberg-like magnet. Being non-universal, these
exponents change with distance to the critical point as observed
experimentally. In the asymptotic limit (at ) they equal to the critical
exponents of the pure 3d Heisenberg magnet as predicted by the Harris
criterion.Comment: 15 pages, 4 figure
Random Exchange Quantum Heisenberg Chains
The one-dimensional quantum Heisenberg model with random bonds is
studied for and . The specific heat and the zero-field
susceptibility are calculated by using high-temperature series expansions and
quantum transfer matrix method. The susceptibility shows a Curie-like
temperature dependence at low temperatures as well as at high temperatures. The
numerical results for the specific heat suggest that there are anomalously many
low-lying excitations. The qualitative nature of these excitations is discussed
based on the exact diagonalization of finite size systems.Comment: 13 pages, RevTex, 12 figures available on request ([email protected]
Calculation of magnetic anisotropy energy in SmCo5
SmCo5 is an important hard magnetic material, due to its large magnetic
anisotropy energy (MAE). We have studied the magnetic properties of SmCo5 using
density functional theory (DFT) calculations where the Sm f-bands, which are
difficult to include in DFT calculations, have been treated within the LDA+U
formalism. The large MAE comes mostly from the Sm f-shell anisotropy, stemming
from an interplay between the crystal field and the spin-orbit coupling. We
found that both are of similar strengths, unlike some other Sm compounds,
leading to a partial quenching of the orbital moment (f-states cannot be
described as either pure lattice harmonics or pure complex harmonics), an
optimal situation for enhanced MAE. A smaller portion of the MAE can be
associated with the Co-d band anisotropy, related to the peak in the density of
states at the Fermi energy. Our result for the MAE of SmCo5, 21.6 meV/f.u.,
agrees reasonably with the experimental value of 13-16 meV/f.u., and the
calculated magnetic moment (including the orbital component) of 9.4 mu_B agrees
with the experimental value of 8.9 mu_B.Comment: Submitted to Phys. Rev.
Universality, frustration and conformal invariance in two-dimensional random Ising magnets
We consider long, finite-width strips of Ising spins with randomly
distributed couplings. Frustration is introduced by allowing both ferro- and
antiferromagnetic interactions. Free energy and spin-spin correlation functions
are calculated by transfer-matrix methods. Numerical derivatives and
finite-size scaling concepts allow estimates of the usual critical exponents
, and to be obtained, whenever a second-order
transition is present. Low-temperature ordering persists for suitably small
concentrations of frustrated bonds, with a transition governed by pure--Ising
exponents. Contrary to the unfrustrated case, subdominant terms do not fit a
simple, logarithmic-enhancement form. Our analysis also suggests a vertical
critical line at and below the Nishimori point. Approaching this point along
either the temperature axis or the Nishimori line, one finds non-diverging
specific heats. A percolation-like ratio is found upon analysis of
the uniform susceptibility at the Nishimori point. Our data are also consistent
with frustration inducing a breakdown of the relationship between
correlation-length amplitude and critical exponents, predicted by conformal
invariance for pure systems.Comment: RevTeX code for 10 pages, 9 eps figures, to appear in Physical Review
B (September 1999
Critical properties of random anisotropy magnets
The problem of critical behaviour of three dimensional random anisotropy
magnets, which constitute a wide class of disordered magnets is considered.
Previous results obtained in experiments, by Monte Carlo simulations and within
different theoretical approaches give evidence for a second order phase
transition for anisotropic distributions of the local anisotropy axes, while
for the case of isotropic distribution such transition is absent. This outcome
is described by renormalization group in its field theoretical variant on the
basis of the random anisotropy model. Considerable attention is paid to the
investigation of the effective critical behaviour which explains the
observation of different behaviour in the same universality class.Comment: 41 pages, 10 figure
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