2,288 research outputs found
On possible superconductivity in the doped ladder compound La_(1-x)Sr_xCuO_2.5
LaCuO_2.5 is a system of coupled, two-chain, cuprate ladders which may be
doped systematically by Sr substitution. Motivated by the recent synthesis of
single crystals, we investigate theoretically the possibility of
superconductivity in this compound. We use a model of spin fluctuation-mediated
superconductivity, where the pairing potential is strongly peaked at \pi in the
ladder direction. We solve the coupled gap equations on the bonding and
antibonding ladder bands to find superconducting solutions across the range of
doping, and discuss their relevance to the real material.Comment: RevTex, 4 pages, 7 figure
Activated sampling in complex materials at finite temperature: the properly-obeying-probability activation-relaxation technique
While the dynamics of many complex systems is dominated by activated events,
there are very few simulation methods that take advantage of this fact. Most of
these procedures are restricted to relatively simple systems or, as with the
activation-relaxation technique (ART), sample the conformation space
efficiently at the cost of a correct thermodynamical description. We present
here an extension of ART, the properly-obeying-probability ART (POP-ART), that
obeys detailed balance and samples correctly the thermodynamic ensemble.
Testing POP-ART on two model systems, a vacancy and an interstitial in
crystalline silicon, we show that this method recovers the proper
thermodynamical weights associated with the various accessible states and is
significantly faster than MD in the diffusion of a vacancy below 700 K.Comment: 10 pages, 3 figure
Effect of iron content and potassium substitution in AFeSe (A = K, Rb, Tl) superconductors: a Raman-scattering investigation
We have performed Raman-scattering measurements on high-quality single
crystals of the superconductors KFeSe ( = 32 K),
TlKFeSe ( = 29 K), and
TlRbFeSe ( = 31 K), as well as of the
insulating compound KFeSe. To interpret our results, we have made
first-principles calculations for the phonon modes in the ordered iron-vacancy
structure of KFeSe. The modes we observe can be assigned
very well from our symmetry analysis and calculations, allowing us to compare
Raman-active phonons in the AFeSe compounds. We find a clear frequency
difference in most phonon modes between the superconducting and
non-superconducting potassium crystals, indicating the fundamental influence of
iron content. By contrast, substitution of K by Tl or Rb in
AFeSe causes no substantial frequency shift for any modes
above 60 cm, demonstrating that the alkali-type metal has little effect
on the microstructure of the FeSe layer. Several additional modes appear below
60 cm in Tl- and Rb-substituted samples, which are vibrations of heavier
Tl and Rb ions. Finally, our calculations reveal the presence of "chiral"
phonon modes, whose origin lies in the chiral nature of the
KFeSe structure.Comment: 11 pages, 10 figures and 2 table
Self-vacancies in Gallium Arsenide: an ab initio calculation
We report here a reexamination of the static properties of vacancies in GaAs
by means of first-principles density-functional calculations using localized
basis sets. Our calculated formation energies yields results that are in good
agreement with recent experimental and {\it ab-initio} calculation and provide
a complete description of the relaxation geometry and energetic for various
charge state of vacancies from both sublattices. Gallium vacancies are stable
in the 0, -, -2, -3 charge state, but V_Ga^-3 remains the dominant charge state
for intrinsic and n-type GaAs, confirming results from positron annihilation.
Interestingly, Arsenic vacancies show two successive negative-U transitions
making only +1, -1 and -3 charge states stable, while the intermediate defects
are metastable. The second transition (-/-3) brings a resonant bond relaxation
for V_As^-3 similar to the one identified for silicon and GaAs divacancies.Comment: 14 page
SsODNet: The Solar system Open Database Network
The sample of Solar system objects has dramatically increased over the last
decade. The amount of measured properties (e.g., diameter, taxonomy, rotation
period, thermal inertia) has grown even faster. However, this wealth of
information is spread over a myriad of articles, under many different
designations per object. We provide a solution to the identification of Solar
system objects from any of their multiple names or designations. We also
compile and rationalize their properties to provide an easy access to them. We
aim to continuously update the database as new measurements become available.
We built a Web Service, SsODNet, that offers four access points, each
corresponding to an identified necessity in the community: name resolution
(quaero), compilation of a large corpus of properties (datacloud),
determination of the best estimate among compiled values (ssoCard), and
statistical description of the population (ssoBFT). The SsODNet interfaces are
fully operational and freely accessible to everyone. The name resolver quaero
translates any of the ~5.3 million designations of objects into their current
official designation. The datacloud compiles about 105 million parameters
(osculating and proper elements, pair and family membership, diameter, albedo,
mass, density, rotation period, spin coordinates, phase function parameters,
colors, taxonomy, thermal inertia, and Yarkovsky drift) from over 3,000
articles (and growing). For each of the known asteroids and dwarf planets (~1.2
million), a ssoCard providing a single best-estimate for each parameter is
available. The SsODNet service provides these resources in a fraction of second
upon query. Finally, the large ssoBFT table compiles all the best-estimates in
a single table for population-wide studies
Boundary conditions associated with the Painlev\'e III' and V evaluations of some random matrix averages
In a previous work a random matrix average for the Laguerre unitary ensemble,
generalising the generating function for the probability that an interval at the hard edge contains eigenvalues, was evaluated in terms of
a Painlev\'e V transcendent in -form. However the boundary conditions
for the corresponding differential equation were not specified for the full
parameter space. Here this task is accomplished in general, and the obtained
functional form is compared against the most general small behaviour of
the Painlev\'e V equation in -form known from the work of Jimbo. An
analogous study is carried out for the the hard edge scaling limit of the
random matrix average, which we have previously evaluated in terms of a
Painlev\'e \IIId transcendent in -form. An application of the latter
result is given to the rapid evaluation of a Hankel determinant appearing in a
recent work of Conrey, Rubinstein and Snaith relating to the derivative of the
Riemann zeta function
Block orthogonal polynomials: I. Definition and properties
Constrained orthogonal polynomials have been recently introduced in the study
of the Hohenberg-Kohn functional to provide basis functions satisfying particle
number conservation for an expansion of the particle density. More generally,
we define block orthogonal (BO) polynomials which are orthogonal, with respect
to a first Euclidean scalar product, to a given -dimensional subspace of polynomials associated with the constraints. In addition, they are
mutually orthogonal with respect to a second Euclidean scalar product. We
recast the determination of these polynomials into a general problem of finding
particular orthogonal bases in an Euclidean vector space endowed with distinct
scalar products. An explicit two step Gram-Schmidt orthogonalization (G-SO)
procedure to determine these bases is given. By definition, the standard block
orthogonal (SBO) polynomials are associated with a choice of equal
to the subspace of polynomials of degree less than . We investigate their
properties, emphasizing similarities to and differences from the standard
orthogonal polynomials. Applications to classical orthogonal polynomials will
be given in forthcoming papers.Comment: This is a reduced version of the initial manuscript, the number of
pages being reduced from 34 to 2
Calculation of some determinants using the s-shifted factorial
Several determinants with gamma functions as elements are evaluated. This
kind of determinants are encountered in the computation of the probability
density of the determinant of random matrices. The s-shifted factorial is
defined as a generalization for non-negative integers of the power function,
the rising factorial (or Pochammer's symbol) and the falling factorial. It is a
special case of polynomial sequence of the binomial type studied in
combinatorics theory. In terms of the gamma function, an extension is defined
for negative integers and even complex values. Properties, mainly composition
laws and binomial formulae, are given. They are used to evaluate families of
generalized Vandermonde determinants with s-shifted factorials as elements,
instead of power functions.Comment: 25 pages; added section 5 for some examples of application
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