80 research outputs found
Cross effect of Coulomb correlation and hybridization in the occurrence of ferromagnetism in two shifted band transition metals
In this work we discuss the occurrence of ferromagnetism in transition-like
metals. The metal is represented by two hybridized() and shifted
) bands one of which includes Hubbard correlation whereas the
other is uncorrelated. The starting point is to transform the original
Hamiltonian into an effective one. Only one site retains the full correlation
(U) while in the others the correlations are represented by an effective field,
the self-energy(single-site approximation). This field is self-consistently
determined by imposing the translational invariance of the problem. Thereby one
gets an exchange split quasi-particle density of states and then an
electron-spin polarization for some values of the parameters , being the ratio of the effective masses of the two bands
and of the occupation number .Comment: 4 pages, 10 figure
Lifetime distributions in the methods of non-equilibrium statistical operator and superstatistics
A family of non-equilibrium statistical operators is introduced which differ
by the system age distribution over which the quasi-equilibrium (relevant)
distribution is averaged. To describe the nonequilibrium states of a system we
introduce a new thermodynamic parameter - the lifetime of a system.
Superstatistics, introduced in works of Beck and Cohen [Physica A \textbf{322},
(2003), 267] as fluctuating quantities of intensive thermodynamical parameters,
are obtained from the statistical distribution of lifetime (random time to the
system degeneracy) considered as a thermodynamical parameter. It is suggested
to set the mixing distribution of the fluctuating parameter in the
superstatistics theory in the form of the piecewise continuous functions. The
distribution of lifetime in such systems has different form on the different
stages of evolution of the system. The account of the past stages of the
evolution of a system can have a substantial impact on the non-equilibrium
behaviour of the system in a present time moment.Comment: 18 page
Extended Clausius Relation and Entropy for Nonequilibrium Steady States in Heat Conducting Quantum Systems
Recently, in their attempt to construct steady state thermodynamics (SST),
Komatsu, Nakagwa, Sasa, and Tasaki found an extension of the Clausius relation
to nonequilibrium steady states in classical stochastic processes. Here we
derive a quantum mechanical version of the extended Clausius relation. We
consider a small system of interest attached to large systems which play the
role of heat baths. By only using the genuine quantum dynamics, we realize a
heat conducting nonequilibrium steady state in the small system. We study the
response of the steady state when the parameters of the system are changed
abruptly, and show that the extended Clausius relation, in which "heat" is
replaced by the "excess heat", is valid when the temperature difference is
small. Moreover we show that the entropy that appears in the relation is
similar to von Neumann entropy but has an extra symmetrization with respect to
time-reversal. We believe that the present work opens a new possibility in the
study of nonequilibrium phenomena in quantum systems, and also confirms the
robustness of the approach by Komtatsu et al.Comment: 19 pages, 2 figure
Neel Temperature for Quasi-Two-Dimensional Dipolar Antiferromagnets
We calculate the N\'eel temperature for two-dimensional isotropic
dipolar Heisenberg antiferromagnets via linear spin-wave theory and a high
temperature expansion, employing the method of Callen. The theoretical
predictions for for KMnF, RbMnF, RbMnCl and
(CHNH)MnCl are in good agreement with the measured values.Comment: 12 pages, REVTEX, TUM-CP-93-0
Phase transitions in two-dimensional anisotropic quantum magnets
We consider quantum Heisenberg ferro- and antiferromagnets on the square
lattice with exchange anisotropy of easy-plane or easy-axis type. The
thermodynamics and the critical behaviour of the models are studied by the
pure-quantum self-consistent harmonic approximation, in order to evaluate the
spin and anisotropy dependence of the critical temperatures. Results for
thermodynamic quantities are reported and comparison with experimental and
numerical simulation data is made. The obtained results allow us to draw a
general picture of the subject and, in particular, to estimate the value of the
critical temperature for any model belonging to the considered class.Comment: To be published on Eur. Phys. J.
Representation of nonequilibrium steady states in large mechanical systems
Recently a novel concise representation of the probability distribution of
heat conducting nonequilibrium steady states was derived. The representation is
valid to the second order in the ``degree of nonequilibrium'', and has a very
suggestive form where the effective Hamiltonian is determined by the excess
entropy production. Here we extend the representation to a wide class of
nonequilibrium steady states realized in classical mechanical systems where
baths (reservoirs) are also defined in terms of deterministic mechanics. The
present extension covers such nonequilibrium steady states with a heat
conduction, with particle flow (maintained either by external field or by
particle reservoirs), and under an oscillating external field. We also simplify
the derivation and discuss the corresponding representation to the full order.Comment: 27 pages, 3 figure
An optimization principle for deriving nonequilibrium statistical models of Hamiltonian dynamics
A general method for deriving closed reduced models of Hamiltonian dynamical
systems is developed using techniques from optimization and statistical
estimation. As in standard projection operator methods, a set of resolved
variables is selected to capture the slow, macroscopic behavior of the system,
and the family of quasi-equilibrium probability densities on phase space
corresponding to these resolved variables is employed as a statistical model.
The macroscopic dynamics of the mean resolved variables is determined by
optimizing over paths of these probability densities. Specifically, a cost
function is introduced that quantifies the lack-of-fit of such paths to the
underlying microscopic dynamics; it is an ensemble-averaged, squared-norm of
the residual that results from submitting a path of trial densities to the
Liouville equation. The evolution of the macrostate is estimated by minimizing
the time integral of the cost function. The value function for this
optimization satisfies the associated Hamilton-Jacobi equation, and it
determines the optimal relation between the statistical parameters and the
irreversible fluxes of the resolved variables, thereby closing the reduced
dynamics. The resulting equations for the macroscopic variables have the
generic form of governing equations for nonequilibrium thermodynamics, and they
furnish a rational extension of the classical equations of linear irreversible
thermodynamics beyond the near-equilibrium regime. In particular, the value
function is a thermodynamic potential that extends the classical dissipation
function and supplies the nonlinear relation between thermodynamics forces and
fluxes
Time evolution in linear response: Boltzmann equations and beyond
In this work a perturbative linear response analysis is performed for the
time evolution of the quasi-conserved charge of a scalar field. One can find
two regimes, one follows exponential damping, where the damping rate is shown
to come from quantum Boltzmann equations. The other regime (coming from
multiparticle cuts and products of them) decays as power law. The most
important, non-oscillating contribution in our model comes from a 4-particle
intermediate state and decays as 1/t^3. These results may have relevance for
instance in the context of lepton number violation in the Early Universe.Comment: 19 page
Fano resonances and Aharonov-Bohm effects in transport through a square quantum dot molecule
We study the Aharonov-Bohm effect in a coupled 22 quantum dot array
with two-terminals. A striking conductance dip arising from the Fano
interference is found as the energy levels of the intermediate dots are
mismatched, which is lifted in the presence of a magnetic flux. A novel five
peak structure is observed in the conductance for large mismatch. The
Aharonov-Bohm evolution of the linear conductance strongly depends on the
configuration of dot levels and interdot and dot-lead coupling strengths. In
addition, the magnetic flux and asymmetry between dot-lead couplings can induce
the splitting and combination of the conductance peak(s).Comment: 15 pages, 7 figures, Revtex, to be published in Phys. Rev.
Phonon-drag effects on thermoelectric power
We carry out a calculation of the phonon-drag contribution to the
thermoelectric power of bulk semiconductors and quantum well structures for the
first time using the balance equation transport theory extended to the weakly
nonuniform systems. Introducing wavevector and phonon-mode dependent relaxation
times due to phonon-phonon interactions, the formula obtained can be used not
only at low temperatures where the phonon mean free path is determined by
boundary scattering, but also at high temperatures. In the linear transport
limit, is equivalent to the result obtained from the Boltzmann equation
with a relaxation time approximation. The theory is applied to experiments and
agreement is found between the theoretical predictions and experimental
results. The role of hot-electron effects in is discussed. The importance
of the contribution of to thermoelectric power in the hot-electron
transport condition is emphasized.Comment: 8 pages, REVTEX 3.0, 7 figures avilable upon reques
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