1,574 research outputs found
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
Editors’ Introduction to [Algorithmic Learning Theory: 18th International Conference, ALT 2007, Sendai, Japan, October 1-4, 2007. Proceedings]
Learning theory is an active research area that incorporates ideas,
problems, and techniques from a wide range of disciplines including
statistics, artificial intelligence, information theory, pattern
recognition, and theoretical computer science. The research reported
at the 18th International Conference on Algorithmic Learning Theory
(ALT 2007) ranges over areas such as unsupervised learning,
inductive inference, complexity and learning, boosting and
reinforcement learning, query learning models, grammatical
inference, online learning and defensive forecasting, and kernel
methods. In this introduction we give an overview of the five
invited talks and the regular contributions of ALT 2007
Spin fluctuations probed by NMR in paramagnetic spinel LiVO: a self-consistent renormalization theory
Low frequency spin fluctuation dynamics in paramagnetic spinel LiVO,
a rare 3-electron heavy fermion system, is investigated. A parametrized
self-consistent renormalization (SCR) theory of the dominant AFM spin
fluctuations is developed and applied to describe temperature and pressure
dependences of the low- nuclear spin-lattice relaxation rate in this
material. The experimental data for available down to K are
well reproduced by the SCR theory, showing the development of AFM spin
fluctuations as the paramagnetic metal approaches a magnetic instability under
the applied pressure. The low- upturn of detected below 0.6 K under
the highest applied pressure of 4.74 GPa is explained as the nuclear spin
relaxation effect due to the spin freezing of magnetic defects unavoidably
present in the measured sample of LiVO.Comment: 11 pages, 2 figure
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
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
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
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
Theory of Nonlinear Meissner Effect in High-Tc Superconductors
We investigate the nonlinear Meissner effect microscopically. Previous
studies did not consider a certain type of interaction effect on the nonlinear
phenomena. The scattering amplitude barely appears without being renormalized
into the Fermi-liquid parameter. With this effect we can solve the outstanding
issues (the quantitative problem, the temperature and angle dependences). The
quantitative calculation is performed with use of the fluctuation-exchange
approximation on the Hubbard model. It is also shown that the perturbation
expansion on the supercurrent by the vector potential converges owing to the
nonlocal effect
- …