1,500 research outputs found
Low-temperature electrical resistivity in paramagnetic spinel LiV2O4
The 3d electron spinel compound LiV2O4 exhibits heavy fermion behaviour below
30K which is related to antiferromagnetic spin fluctuations strongly enhanced
in an extended region of momentum space. This mechanism explains enhanced
thermodynamic quantities and nearly critical NMR relaxation in the framework of
the selfconsistent renormalization (SCR) theory. Here we show that the low-T
Fermi liquid behaviour of the resistivity and a deviation from this behavior
for higher T may also be understood within that context. We calculate the
temperature dependence of the electrical resistivity \rho(T) assuming that two
basic mechanisms of the quasiparticle scattering, resulting from impurities and
spin-fluctuations, operate simultaneously at low temperature. The calculation
is based on the variational principle in the form of a perturbative series
expansion for \rho(T). A peculiar behavior of \rho(T) in LiV2O4 is related to
properties of low-energy spin fluctuations whose T-dependence is obtained from
SCR theory.Comment: 10 pages, 3 figures, to appear in Phys. Rev.
Electronic states and pairing symmetry in the two-dimensional 16 band d-p model for iron-based superconductor
The electronic states of the FeAs plane in iron-based superconductors are
investigated on the basis of the two-dimensional 16-band d-p model, where the
tight-binding parameters are determined so as to fit the band structure
obtained by the density functional calculation for LaFeAsO. The model includes
the Coulomb interaction on a Fe site: the intra- and inter-orbital direct terms
U and U', the exchange coupling J and the pair-transfer J'. Within the random
phase approximation (RPA), we discuss the pairing symmetry of possible
superconducting states including s-wave and d-wave pairing on the U'-J plane.Comment: 2 pages, 4 figures; Proceedings of the Int. Symposium on
Fe-Oxipnictide Superconductors (Tokyo, 28-29th June 2008
Adaptive Regret Minimization in Bounded-Memory Games
Online learning algorithms that minimize regret provide strong guarantees in
situations that involve repeatedly making decisions in an uncertain
environment, e.g. a driver deciding what route to drive to work every day.
While regret minimization has been extensively studied in repeated games, we
study regret minimization for a richer class of games called bounded memory
games. In each round of a two-player bounded memory-m game, both players
simultaneously play an action, observe an outcome and receive a reward. The
reward may depend on the last m outcomes as well as the actions of the players
in the current round. The standard notion of regret for repeated games is no
longer suitable because actions and rewards can depend on the history of play.
To account for this generality, we introduce the notion of k-adaptive regret,
which compares the reward obtained by playing actions prescribed by the
algorithm against a hypothetical k-adaptive adversary with the reward obtained
by the best expert in hindsight against the same adversary. Roughly, a
hypothetical k-adaptive adversary adapts her strategy to the defender's actions
exactly as the real adversary would within each window of k rounds. Our
definition is parametrized by a set of experts, which can include both fixed
and adaptive defender strategies.
We investigate the inherent complexity of and design algorithms for adaptive
regret minimization in bounded memory games of perfect and imperfect
information. We prove a hardness result showing that, with imperfect
information, any k-adaptive regret minimizing algorithm (with fixed strategies
as experts) must be inefficient unless NP=RP even when playing against an
oblivious adversary. In contrast, for bounded memory games of perfect and
imperfect information we present approximate 0-adaptive regret minimization
algorithms against an oblivious adversary running in time n^{O(1)}.Comment: Full Version. GameSec 2013 (Invited Paper
Basis set effects on the hyperpolarizability of CHCl_3: Gaussian-type orbitals, numerical basis sets and real-space grids
Calculations of the hyperpolarizability are typically much more difficult to
converge with basis set size than the linear polarizability. In order to
understand these convergence issues and hence obtain accurate ab initio values,
we compare calculations of the static hyperpolarizability of the gas-phase
chloroform molecule (CHCl_3) using three different kinds of basis sets:
Gaussian-type orbitals, numerical basis sets, and real-space grids. Although
all of these methods can yield similar results, surprisingly large, diffuse
basis sets are needed to achieve convergence to comparable values. These
results are interpreted in terms of local polarizability and
hyperpolarizability densities. We find that the hyperpolarizability is very
sensitive to the molecular structure, and we also assess the significance of
vibrational contributions and frequency dispersion
An efficient algorithm for learning with semi-bandit feedback
We consider the problem of online combinatorial optimization under
semi-bandit feedback. The goal of the learner is to sequentially select its
actions from a combinatorial decision set so as to minimize its cumulative
loss. We propose a learning algorithm for this problem based on combining the
Follow-the-Perturbed-Leader (FPL) prediction method with a novel loss
estimation procedure called Geometric Resampling (GR). Contrary to previous
solutions, the resulting algorithm can be efficiently implemented for any
decision set where efficient offline combinatorial optimization is possible at
all. Assuming that the elements of the decision set can be described with
d-dimensional binary vectors with at most m non-zero entries, we show that the
expected regret of our algorithm after T rounds is O(m sqrt(dT log d)). As a
side result, we also improve the best known regret bounds for FPL in the full
information setting to O(m^(3/2) sqrt(T log d)), gaining a factor of sqrt(d/m)
over previous bounds for this algorithm.Comment: submitted to ALT 201
Multipole correlations in low-dimensional f-electron systems
By using a density matrix renormalization group method, we investigate the
ground-state properties of a one-dimensional three-orbital Hubbard model on the
basis of a j-j coupling scheme. For , where is a parameter
to control cubic crystalline electric field effect, one orbital is itinerant,
while other two are localized. Due to the competition between itinerant and
localized natures, we obtain orbital ordering pattern which is sensitive to
, leading to a characteristic change of quadrupole state
into an incommensurate structure. At , all the three orbitals are
degenerate, but we observe a peak at in quadrupole
correlation, indicating a ferro-orbital state, and the peak at in
dipole correlation, suggesting an antiferromagnetic state. We
also discuss the effect of octupole on magnetic anisotropy.Comment: 4 pages, 3 figures, Proceedings of ASR-WYP-2005 (September 27-29,
2005, Tokai
Application of compressed sensing to the simulation of atomic systems
Compressed sensing is a method that allows a significant reduction in the
number of samples required for accurate measurements in many applications in
experimental sciences and engineering. In this work, we show that compressed
sensing can also be used to speed up numerical simulations. We apply compressed
sensing to extract information from the real-time simulation of atomic and
molecular systems, including electronic and nuclear dynamics. We find that for
the calculation of vibrational and optical spectra the total propagation time,
and hence the computational cost, can be reduced by approximately a factor of
five.Comment: 7 pages, 5 figure
Effects of degenerate orbitals on the Hubbard model
Stability of a metallic state in the two-orbital Hubbard model at
half-filling is investigated. We clarify how spin and orbital fluctuations are
enhanced to stabilize the formation of quasi-particles by combining dynamical
mean field theory with the quantum Monte Carlo simulations. These analyses shed
some light on the reason why the metallic phase is particularly stable when the
intra- and inter-band Coulomb interactions are nearly equal.Comment: 3 pages, To appear in JPSJ Vol. 72, No. 5 200
Effective Crystalline Electric Field Potential in a j-j Coupling Scheme
We propose an effective model on the basis of a - coupling scheme to
describe local -electron states for realistic values of Coulomb interaction
and spin-orbit coupling , for future development of microscopic
theory of magnetism and superconductivity in -electron systems, where
is the number of local electrons. The effective model is systematically
constructed by including the effect of a crystalline electric field (CEF)
potential in the perturbation expansion in terms of . In this paper,
we collect all the terms up to the first order of . Solving the
effective model, we show the results of the CEF states for each case of
=25 with symmetry in comparison with those of the Stevens
Hamiltonian for the weak CEF. In particular, we carefully discuss the CEF
energy levels in an intermediate coupling region with in the order
of 0.1 corresponding to actual -electron materials between the and
- coupling schemes. Note that the relevant energy scale of is the
Hund's rule interaction. It is found that the CEF energy levels in the
intermediate coupling region can be quantitatively reproduced by our modified
- coupling scheme, when we correctly take into account the corrections in
the order of in addition to the CEF terms and Coulomb interactions
which remain in the limit of =. As an application of the
modified - coupling scheme, we discuss the CEF energy levels of filled
skutterudites with symmetry.Comment: 12 pages, 7 figures. Typeset with jpsj2.cl
Weak-Coupling Theory for Multiband Superconductivity Induced by Jahn-Teller Phonons
Emergence of superconductivity in a two-band system coupled with breathing
and Jahn-Teller phonons is discussed in a weak-coupling limit. With the use of
a standard quantum mechanical procedure, the phonon-mediated attraction is
derived. From the analysis of the model including such attraction, a BCS-like
formula for a superconducting transition temperature is obtained.
When only the breathing phonon is considered, is the same as that
of the one-band model. On the other hand, when Jahn-Teller phonons are active,
is significantly enhanced by the interband attraction even within
the weak-coupling limit. Relevance of the present result to actual materials
such as iron pnictides is briefly commented.Comment: 4 pages, 3 figures
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