463 research outputs found
Strong electron-lattice coupling as the mechanism behind charge density wave transformations in transition-metal dichalcogenides
We consider single band of conduction electrons interacting with
displacements of the transitional ions. In the classical regime strong enough
coupling transforms the harmonic elastic energy for an ion to the one of the
well with two deep minima, so that the system is described in terms of Ising
spins. Inter-site interactions order spins at lower tempratures. Extention to
the quantum regime is discussed. Below the CDW-transition the energy spectrum
of electrons remains metallic because the structural vector Q and the FS sizes
are not related. Large values of the CDW gap seen in the tunneling experiments
correspond to the energy of the minima in the electron-ion two-well complex.
The gap is defined through the density of states (DOS) inside the electronic
bands below the CDW transition. We focus mainly on electronic properties of
transition-metal dichalcogenides.Comment: new references added; accepted for publication in Physical Review B.
arXiv admin note: substantial text overlap with arXiv:1110.043
Characteristic features of anharmonic effects in the lattice dynamics of fcc metals
The dispersion in the entire Brillouin zone and the temperature dependence
(right up to the melting temperature) of the anharmonic frequency shift and
phonon damping in a number of fcc metals is investigated on the basis of
microscopic calculations. It is found that the anharmonic effects depend
sharply on the wave vector in the directions -X, X-W, and -L
and, in contrast to bcc metals, the magnitude of the effects is not due to the
softness of the initial phonon spectrum. It is shown that the relative
frequency shifts and the phonon damping near melting do not exceed 10-20%. The
relative role of various anharmonic processes is examined, and the relation
between the results obtained and existing experimental data is discussed.Comment: 4 pages, 5 figures, LaTe
Transition from a one-dimensional to a quasi-one-dimensional state in interacting quantum wires
Upon increasing the electron density in a quantum wire, the one-dimensional
electron system undergoes a transition to a quasi-one-dimensional state. In the
absence of interactions between electrons, this corresponds to filling up the
second subband of transverse quantization, and there are two gapless excitation
modes above the transition. On the other hand, strongly interacting
one-dimensional electrons form a Wigner crystal, and the transition corresponds
to it splitting into two chains (zigzag crystal). The two chains are locked, so
their relative motion is gapped, and only one gapless mode remains. We study
the evolution of the system as the interaction strength changes, and show that
only one gapless mode exists near the transition at any interaction strength.Comment: 4 pages, 2 figure
Statistical Derivation of Basic Equations of Diffusional Kinetics in Alloys with Application to the Description of Diffusion of Carbon in Austenite
Basic equations of diffusional kinetics in alloys are statistically derived
using the master equation approach. To describe diffusional transformations in
substitution alloys, we derive the "quasi-equilibrium" kinetic equation which
generalizes its earlier versions by taking into account possible "interaction
renormalization" effects. For the interstitial alloys Me-X, we derive the
explicit expression for the diffusivity D of an interstitial atom X which
notably differs from those used in previous phenomenological treatments. This
microscopic expression for D is applied to describe the diffusion of carbon in
austenite basing on some simple models of carbon-carbon interaction. The
results obtained enable us to make certain conclusions about the real form of
these interactions, and about the scale of the "transition state entropy" for
diffusion of carbon in austenite.Comment: 26 pages, 5 postscript figures, LaTe
Studies of concentration and temperature dependencies of precipitation kinetics in iron-copper alloys using kinetic monte carlo and stochastic statistical simulations
The earlier-developed ab initio model and the kinetic Monte Carlo method
(KMCM) are used to simulate precipitation in a number of iron-copper alloys
with different copper concentrations x and temperatures T. The same simulations
are also made using the improved version of the earlier-suggested stochastic
statistical method (SSM). The results obtained enable us to make a number of
general conclusions about the dependencies of the decomposition kinetics in
Fe-Cu alloys on x and T. We also show that the SSM describes the precipitation
kinetics in a fair agreement with the KMCM, and employing the SSM in
conjunction with the KMCM enables us to extend the KMC simulations to the
longer evolution times. The results of simulations seem to agree with available
experimental data for Fe-Cu alloys within statistical errors of simulations and
the scatter of experimental results. Comparison of results of simulations to
experiments for some multicomponent Fe-Cu-based alloys enables us to make
certain conclusions about the influence of alloying elements in these alloys on
the precipitation kinetics at different stages of evolution.Comment: 18 pages, 17 postscript figures, LaTe
Fluctuations, Higher Order Anharmonicities, and Landau Expansion for Barium Titanate
Correct phenomenological description of ferroelectric phase transitions in
barium titanate requires accounting for eighth-order terms in the free energy
expansion, in addition to the conventional sixth-order contributions. Another
unusual feature of BaTiO_3 crystal is that the coefficients B_1 and B_2 of the
terms P_x^4 and P_x^2*P_y^2 in the Landau expansion depend on the temperature.
It is shown that the temperature dependence of B_1 and B_2 may be caused by
thermal fluctuations of the polarization, provided the fourth-order
anharmonicity is anomalously small, i. e. the nonlinearity of P^4 type and
higher-order ones play comparable roles. Non-singular (non-critical)
fluctuation contributions to B_1 and B_2 are calculated in the first
approximation in sixth-order and eighth-order anharmonic constants. Both
contributions increase with the temperature, which is in agreement with
available experimental data. Moreover, the theory makes it possible to
estimate, without any additional assumptions, the ratio of fluctuation
(temperature dependent) contributions to coefficients B_1 and B_2. Theoretical
value of B_1/B_2 appears to be close to that given by experiments.Comment: 5 pages, 1 figur
Semi-fermionic representation for spin systems under equilibrium and non-equilibrium conditions
We present a general derivation of semi-fermionic representation for spin
operators in terms of a bilinear combination of fermions in real and imaginary
time formalisms. The constraint on fermionic occupation numbers is fulfilled by
means of imaginary Lagrange multipliers resulting in special shape of
quasiparticle distribution functions. We show how Schwinger-Keldysh technique
for spin operators is constructed with the help of semi-fermions. We
demonstrate how the idea of semi-fermionic representation might be extended to
the groups possessing dynamic symmetries (e.g. singlet/triplet transitions in
quantum dots). We illustrate the application of semi-fermionic representations
for various problems of strongly correlated and mesoscopic physics.Comment: Review article, 40 pages, 11 figure
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