12,744 research outputs found
The expressions for the 2nd-order mixed partial derivatives of Slater-Koster matrix elements at spherical coordinate singularities
In a recent publication it has been shown how to generate derivatives with
respect to atom coordinates of Slater-Koster matrix elements for the tight
binding (TB) modelling of a system. For the special case of a mixed second
partial derivative at coordinate singularities only the results were stated in
that publication. In this work, the derivation of these results is given in
detail. Though it may seem rather `technical' and only applicable to a very
special case, atomic configurations where the connecting vector between the two
atoms involved in a two-centre matrix element is aligned along the z-axis (in
the usual approach) require results for precisely this case. The expressions
derived in this work have been implemented in the DINAMO code.Comment: 9 pages, no figure
On microscopic origins of generalized gradient structures
Classical gradient systems have a linear relation between rates and driving
forces. In generalized gradient systems we allow for arbitrary relations
derived from general non-quadratic dissipation potentials. This paper describes
two natural origins for these structures.
A first microscopic origin of generalized gradient structures is given by the
theory of large-deviation principles. While Markovian diffusion processes lead
to classical gradient structures, Poissonian jump processes give rise to
cosh-type dissipation potentials.
A second origin arises via a new form of convergence, that we call
EDP-convergence. Even when starting with classical gradient systems, where the
dissipation potential is a quadratic functional of the rate, we may obtain a
generalized gradient system in the evolutionary -limit. As examples we
treat (i) the limit of a diffusion equation having a thin layer of low
diffusivity, which leads to a membrane model, and (ii) the limit of diffusion
over a high barrier, which gives a reaction-diffusion system.Comment: Keywords: Generalized gradient structure, gradient system,
evolutionary \Gamma-convergence, energy-dissipation principle, variational
evolution, relative entropy, large-deviation principl
Quantum size effect in Pb(100) films: the role of symmetry and implication for film growth
We show from density-functional calculations that Pb(100) thin films exhibit
quantum size effect with a bilayer periodicity in film energies, film
relaxations, and work functions, which originate from different symmetry of the
stacking geometry of odd and even layer films. The bilayer periodicity of the
film energy is argued to survive on a semiconductor substrate, which should
allow the growth of ``magically'' thick even-layer Pb(100) films. Furthermore,
it is found that the quantum well states in a simple metal film can be
classified into -bonded and -bonded states, which quantize
independently
Two-channel point-contact tunneling theory of superconductors
We introduce a two-channel tunneling model to generalize the widely used BTK
theory of point-contact conductance between a normal metal contact and
superconductor. Tunneling of electrons can occur via localized surface states
or directly, resulting in a Fano resonance in the differential conductance
. We present an analysis of within the two-channel model when
applied to soft point-contacts between normal metallic silver particles and
prototypical heavy-fermion superconductors CeCoIn and CeRhIn at high
pressures. In the normal state the Fano line shape of the measured is well
described by a model with two tunneling channels and a large
temperature-independent background conductance. In the superconducting state a
strongly suppressed Andreev reflection signal is explained by the presence of
the background conductance. We report Andreev signal in CeCoIn consistent
with standard -wave pairing, assuming an equal mixture of
tunneling into [100] and [110] crystallographic interfaces. Whereas in
CeRhIn at 1.8 and 2.0 GPa the signal is described by a -wave
gap with reduced nodal region, i.e., increased slope of the gap opening on the
Fermi surface. A possibility is that the shape of the high-pressure Andreev
signal is affected by the proximity of a line of quantum critical points that
extends from 1.75 to 2.3 GPa, which is not accounted for in our description of
the heavy-fermion superconductor.Comment: 13 pages, 13 figure
Automatic Generation of Matrix Element Derivatives for Tight Binding Models
Tight binding (TB) models are one approach to the quantum mechanical many
particle problem. An important role in TB models is played by hopping and
overlap matrix elements between the orbitals on two atoms, which of course
depend on the relative positions of the atoms involved. This dependence can be
expressed with the help of Slater-Koster parameters, which are usually taken
from tables. Recently, a way to generate these tables automatically was
published. If TB approaches are applied to simulations of the dynamics of a
system, also derivatives of matrix elements can appear. In this work we give
general expressions for first and second derivatives of such matrix elements.
Implemented in a computer program they obviate the need to type all the
required derivatives of all occuring matrix elements by hand.Comment: 11 pages, 2 figure
Proton Stopping Power of Different Density Profile Plasmas
In this work, the stopping power of a partially ionized plasma is analyzed by
means of free electron stopping and bound electron stopping. For the first one,
the RPA dielectric function is used, and for the latter one, an interpolation
of high and low projectile velocity formulas is used. The dynamical energy loss
of an ion beam inside a plasma is estimated by using an iterative scheme of
calculation. The Abel inversion is also applied when we have a plasma with
radial symmetry. Finally, we compare our methods with two kind of plasmas. In
the first one, we estimate the energy loss in a plasma created by a laser
prepulse, whose density is approximated by a piecewise function. For the latter
one, a radial electron density is supposed and the stopping is obtained as
function of radius from the calculated lateral points. In both cases, the
dependence with the density profile is observed.Comment: 5 pages, 7 figure
CO oxidation on Pd(100) at technologically relevant pressure conditions: A first-principles kinetic Monte Carlo study
The possible importance of oxide formation for the catalytic activity of
transition metals in heterogenous oxidation catalysis has evoked a lively
discussion over the recent years. On the more noble transition metals (like Pd,
Pt or Ag) the low stability of the common bulk oxides suggests primarily
sub-nanometer thin oxide films, so-called surface oxides, as potential
candidates that may be stabilized under gas phase conditions representative of
technological oxidation catalysis. We address this issue for the Pd(100) model
catalyst surface with first-principles kinetic Monte Carlo (kMC) simulations
that assess the stability of the well-characterized (sqrt{5} x sqrt{5})R27
surface oxide during steady-state CO oxidation. Our results show that at
ambient pressure conditions the surface oxide is stabilized at the surface up
to CO:O2 partial pressure ratios just around the catalytically most relevant
stoichiometric feeds (p(CO):p(O2) = 2:1). The precise value depends sensitively
on temperature, so that both local pressure and temperature fluctuations may
induce a continuous formation and decomposition of oxidic phases during
steady-state operation under ambient stoichiometric conditions.Comment: 13 pages including 5 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
Mirror Inversion of Quantum States in Linear Registers
Transfer of data in linear quantum registers can be significantly simplified
with pre-engineered but not dynamically controlled inter-qubit couplings. We
show how to implement a mirror inversion of the state of the register in each
excitation subspace with respect to the centre of the register. Our
construction is especially appealing as it requires no dynamical control over
individual inter-qubit interactions. If, however, individual control of the
interactions is available then the mirror inversion operation can be performed
on any substring of qubits in the register. In this case a sequence of mirror
inversions can generate any permutation of a quantum state of the involved
qubits.Comment: 4 page
Nonequilibrium fluctuation dissipation relations of interacting Brownian particles driven by shear
We present a detailed analysis of the fluctuation dissipation theorem (FDT)
close to the glass transition in colloidal suspensions under steady shear using
mode coupling approximations. Starting point is the many-particle Smoluchowski
equation. Under shear, detailed balance is broken and the response functions in
the stationary state are smaller at long times than estimated from the
equilibrium FDT. An asymptotically constant relation connects response and
fluctuations during the shear driven decay, restoring the form of the FDT with,
however, a ratio different from the equilibrium one. At short times, the
equilibrium FDT holds. We follow two independent approaches whose results are
in qualitative agreement. To discuss the derived fluctuation dissipation
ratios, we show an exact reformulation of the susceptibility which contains not
the full Smoluchowski operator as in equilibrium, but only its well defined
Hermitian part. This Hermitian part can be interpreted as governing the
dynamics in the frame comoving with the probability current. We present a
simple toy model which illustrates the FDT violation in the sheared colloidal
system.Comment: 21 pages, 13 figures, submitted to Phys. Rev.
Boundary fermion currents and subleading order chiral anomaly in the AdS/CFT correspondence
We construct a wave-functional whose argument couples to boundary fermion
currents in the AdS/CFT correspondence. Using this we calculate the
contributions from bulk fermions to the chiral anomaly that give the subleading
order term in the exact -dependence of the chiral anomaly of
SYM. The result agrees with the calculation of Bilal & Chu.Comment: 6 page
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