840 research outputs found
Hierarchical Equations of Motion Approach to Quantum Thermodynamics
We present a theoretical framework to investigate quantum thermodynamic
processes under non-Markovian system-bath interactions on the basis of the
hierarchical equations of motion (HEOM) approach, which is convenient to carry
out numerically "exact" calculations. This formalism is valuable because it can
be used to treat not only strong system-bath coupling but also system-bath
correlation or entanglement, which will be essential to characterize the heat
transport between the system and quantum heat baths. Using this formalism, we
demonstrated an importance of the thermodynamic effect from the tri-partite
correlations (TPC) for a two-level heat transfer model and a three-level
autonomous heat engine model under the conditions that the conventional quantum
master equation approaches are failed. Our numerical calculations show that TPC
contributions, which distinguish the heat current from the energy current, have
to be take into account to satisfy the thermodynamic laws.Comment: 9 pages, 4 figures. As a chapter of: F. Binder, L. A. Correa, C.
Gogolin, J. Anders, and G. Adesso (eds.), "Thermodynamics in the quantum
regime - Recent Progress and Outlook", (Springer International Publishing
On quantum mechanics with a magnetic field on R^n and on a torus T^n, and their relation
We show in elementary terms the equivalence in a general gauge of a
U(1)-gauge theory of a scalar charged particle on a torus T^n = R^n/L to the
analogous theory on R^n constrained by quasiperiodicity under translations in
the lattice L. The latter theory provides a global description of the former:
the quasiperiodic wavefunctions defined on R^n play the role of sections of the
associated hermitean line bundle E on T^n, since also E admits a global
description as a quotient. The components of the covariant derivatives
corresponding to a constant (necessarily integral) magnetic field B = dA
generate a Lie algebra g_Q and together with the periodic functions the algebra
of observables O_Q . The non-abelian part of g_Q is a Heisenberg Lie algebra
with the electric charge operator Q as the central generator; the corresponding
Lie group G_Q acts on the Hilbert space as the translation group up to phase
factors. Also the space of sections of E is mapped into itself by g in G_Q . We
identify the socalled magnetic translation group as a subgroup of the
observables' group Y_Q . We determine the unitary irreducible representations
of O_Q, Y_Q corresponding to integer charges and for each of them an associated
orthonormal basis explicitly in configuration space. We also clarify how in the
n = 2m case a holomorphic structure and Theta functions arise on the associated
complex torus. These results apply equally well to the physics of charged
scalar particles on R^n and on T^n in the presence of periodic magnetic field B
and scalar potential. They are also necessary preliminary steps for the
application to these theories of the deformation procedure induced by Drinfel'd
twists.Comment: Latex2e file, 22 pages. Final version appeared in IJT
Direct observation of the proliferation of ferroelectric loop domains and vortex-antivortex pairs
We discovered "stripe" patterns of trimerization-ferroelectric domains in
hexagonal REMnO3 (RE=Ho, ---, Lu) crystals (grown below ferroelectric
transition temperatures (Tc), reaching up to 1435 oC), in contrast with the
vortex patterns in YMnO3. These stripe patterns roughen with the appearance of
numerous loop domains through thermal annealing just below Tc, but the stripe
domain patterns turn to vortex-antivortex domain patterns through a freezing
process when crystals cross Tc even though the phase transition appears not to
be Kosterlitz-Thouless-type. The experimental systematics are compared with the
results of our six-state clock model simulation and also the Kibble-Zurek
Mechanism for trapped topological defects
An evaporation-based model of thermal neutron induced ternary fission of plutonium
Ternary fission probabilities for thermal neutron induced fission of
plutonium are analyzed within the framework of an evaporation-based model where
the complexity of time-varying potentials, associated with the neck collapse,
are included in a simplistic fashion. If the nuclear temperature at scission
and the fission-neck-collapse time are assumed to be ~1.2 MeV and ~10^-22 s,
respectively, then calculated relative probabilities of ternary-fission
light-charged-particle emission follow the trends seen in the experimental
data. The ability of this model to reproduce ternary fission probabilities
spanning seven orders of magnitude for a wide range of light-particle charges
and masses implies that ternary fission is caused by the coupling of an
evaporation-like process with the rapid re-arrangement of the nuclear fluid
following scission.Comment: 25 pages, 12 figures, accepted for publication in IJMP
Coupled-channels analysis of the O+Pb fusion barrier distribution
Analyses using simplified coupled-channels models have been unable to
describe the shape of the previously measured fusion barrier distribution for
the doubly magic O+Pb system. This problem was investigated by
re-measuring the fission excitation function for O+Pb with
improved accuracy and performing more exact coupled-channels calculations,
avoiding the constant-coupling and first-order coupling approximations often
used in simplified analyses. Couplings to the single- and 2-phonon states of
Pb, correctly taking into account the excitation energy and the phonon
character of these states, particle transfers, and the effects of varying the
diffuseness of the nuclear potential, were all explored. However, in contrast
to other recent analyses of precise fusion data, no satisfactory simultaneous
description of the shape of the experimental barrier distribution and the
fusion cross-sections for O+Pb was obtained.Comment: RevTex, 29 pages, 7 postscript figures, to appear in PR
First-principles calculations of the self-trapped exciton in crystalline NaCl
The atomic and electronic structure of the lowest triplet state of the
off-center (C2v symmetry) self-trapped exciton (STE) in crystalline NaCl is
calculated using the local-spin-density (LSDA) approximation. In addition, the
Franck-Condon broadening of the luminescence peak and the a1g -> b3u absorption
peak are calculated and compared to experiment. LSDA accurately predicts
transition energies if the initial and final states are both localized or
delocalized, but 1 eV discrepancies with experiment occur if one state is
localized and the other is delocalized.Comment: 4 pages with 4 embeddded figure
Modelling charge self-trapping in wide-gap dielectrics: Localization problem in local density functionals
We discuss the adiabatic self-trapping of small polarons within the density
functional theory (DFT). In particular, we carried out plane-wave
pseudo-potential calculations of the triplet exciton in NaCl and found no
energy minimum corresponding to the self-trapped exciton (STE) contrary to the
experimental evidence and previous calculations. To explore the origin of this
problem we modelled the self-trapped hole in NaCl using hybrid density
functionals and an embedded cluster method. Calculations show that the
stability of the self-trapped state of the hole drastically depends on the
amount of the exact exchange in the density functional: at less than 30% of the
Hartree-Fock exchange, only delocalized hole is stable, at 50% - both
delocalized and self-trapped states are stable, while further increase of exact
exchange results in only the self-trapped state being stable. We argue that the
main contributions to the self-trapping energy such as the kinetic energy of
the localizing charge, the chemical bond formation of the di-halogen quasi
molecule, and the lattice polarization, are represented incorrectly within the
Kohn-Sham (KS) based approaches.Comment: 6 figures, 1 tabl
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