4,602 research outputs found
ALKALI-RICH FRAGMENTS IN LL-CHONDRITIC BRECCIAS.
第2回極域科学シンポジウム/第34回南極隕石シンポジウム 11月17日(木) 国立国語研究所 2階講
Phase Diagram and Pairing Symmetry of the Two-Dimensional t-J Model by a Variation Theory
Two-dimensional t-J model is studied by a variational Monte Carlo method,
using Gutzwiller-Jastrow-type wave functions. Various kinds of superconducting
pairing symmetries are compared in order to determine the phase diagram of the
ground state in the full J/t-n plane. Near the half filling where the high
temperature superconductivity is expected, the d_{x^2-y^2} wave pairing state
is always the most stable among various symmetries. The three-site term hardly
changes the phase diagram in this regime. In the low electron density, the
extended s-type wave becomes a quantitatively good state for large J/t,
although the energy gain is small. The Gutzwiller wave function is shown to be
the exact ground state in the low-electron-density limit for the supersymmetric
case (J/t=2).Comment: 13 pages, LaTeX with jpsj.sty etc. Hard copies of 22 figures
available on request. Submitted to J.Phys.Soc.Jp
Identification of K-rich fragments in chondritic breccias using Imaging Plate (IP): an application to the planetary materials.
第2回極域科学シンポジウム/第34回南極隕石シンポジウム 11月17日(木) 国立国語研究所 2階講
Superconductivity and a Mott Transition in a Hubbard Model on an Anisotropic Triangular Lattice
A half-filled-band Hubbard model on an anisotropic triangular lattice (t in
two bond directions and t' in the other) is studied using an optimization
variational Monte Carlo method, to consider the Mott transition and
superconductivity arising in \kappa-BEDT-TTF_2X. Adopting wave functions with
doublon-holon binding factors, we reveal that a first-order Mott
(conductor-to-nonmagnetic insulator) transition takes place at U=U_c
approximately of the band width, for a wide range of t'/t. This transition is
not directly connected to magnetism. Robust d-wave superconductivity appears in
a restricted parameter range: immediately below U_c and moderate strength of
frustration (0.4\lsim t'/t\lsim 0.7), where short-range antiferromagnetic
correlation sufficiently develops but does not come to a long-range order. The
relevance to experiments is also discussed.Comment: 15 pages, 17 figures, submitted to J. Phys. Soc. Jp
Divergence of the Chaotic Layer Width and Strong Acceleration of the Spatial Chaotic Transport in Periodic Systems Driven by an Adiabatic ac Force
We show for the first time that a {\it weak} perturbation in a Hamiltonian
system may lead to an arbitrarily {\it wide} chaotic layer and {\it fast}
chaotic transport. This {\it generic} effect occurs in any spatially periodic
Hamiltonian system subject to a sufficiently slow ac force. We explain it and
develop an explicit theory for the layer width, verified in simulations.
Chaotic spatial transport as well as applications to the diffusion of particles
on surfaces, threshold devices and others are discussed.Comment: 4 pages including 3 EPS figures, this is an improved version of the
paper (accepted to PRL, 2005
Novel mechanism of photoinduced reversible phase transitions in molecule-based magnets
A novel microscopic mechanism of bi-directional structural changes is
proposed for the photo-induced magnetic phase transition in Co-Fe Prussian blue
analogues on the basis of ab initio quantum chemical cluster calculations. It
is shown that the local potential energies of various spin states of Co are
sensitive to the number of nearest neighbor Fe vacancies. As a result, the
forward and backward structural changes are most readily initiated by
excitation of different local regions by different photons. This mechanism
suggests an effective strategy to realize photoinduced reversible phase
transitions in a general system consisting of two local components.Comment: 4 pages, LaTex, 3 figures, to appear in Phys. Rev. Let
Variational Monte Carlo Study of Electron Differentiation around Mott Transition
We study ground-state properties of the two-dimensional Hubbard model at half
filling by improving variational Monte Carlo method and by implementing
quantum-number projection and multi-variable optimization. The improved
variational wave function enables a highly accurate description of the Mott
transition and strong fluctuations in metals. We clarify how anomalous metals
appear near the first-order Mott transition. The double occupancy stays nearly
constant as a function of the on-site Coulomb interaction in the metallic phase
near the Mott transition in agreement with the previous unbiased results. This
unconventional metal at half filling is stabilized by a formation of
``electron-like pockets'' coexisting with an arc structure, which leads to a
prominent differentiation of electrons in momentum space. An abrupt collapse of
the ``pocket'' and ``arc'' drives the first-order Mott transition.Comment: 4 pages, 3 figure
Charge order and superconductivity in a two-dimensional triangular lattice at n=2/3
To investigate the possibility of charge order and superconductivity in a
doped two-dimensional triangular lattice, we study the extended Hubbard model
with variational Monte Carlo method. At n=2/3, a commensurate filling for a
triangular lattice, it is shown that the nearest-neighbor Coulomb interaction V
induces honeycomb-type charge order and antiferromagnetic spin order at U>10t.
We also discuss the possibility of superconductivity induced by charge
fluctuation and the relation to the superconductivity in
Na_{0.35}CoO_{2}1.3H_{2}O and theta-type organic condoctors.Comment: 4 pages, 5 figure
Rb-Sr Isotopic Systematics of Alkalai-Rich Fragments in Yamato-74442 and Bhola
We have undertaken Rb.Sr isotopic studies on alkali-rich fragments in Bhola and Y-74442 to precisely deter-mine their crystallization ages and isotopic signatures of their precursor material(s)
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