1,995 research outputs found
Theoretical Aspects of Charge Correlations in -(BEDT-TTF)
A review is given on the theoretical studies of charge correlations in
-(BEDT-TTF). Various studies show that within a purely electronic
model on the -type lattice with the on-site and the nearest
neighbor and interactions, the diagonal stripe, c-axis three-fold,
and the vertical stripe charge correlations are favored in the regime , , and , respectively. In the realistic parameter
regime of , there is a competition between c-axis three fold state
and the diagonal stripe state. Since these are different from the
experimentally observed a-axis three fold and the horizontal stripe charge
correlations, additional effects have to be included in order to understand the
experiments. The electron-lattice coupling, which tends to distort the lattice
into the -type, is found to favor the horizontal stripe state,
suggesting that the occurrence of this stripe ordering in the actual materials
may not be of purely electronic origin. On the other hand, distant
electron-electron interactions have to be considered in order to understand the
a-axis three fold correlation, whose wave vector is close to the nesting vector
of the Fermi surface. These studies seem to suggest that the minimal model to
understand the charge correlation in -(BEDT-TTF) may be more
complicated than expected. Future problems regarding the competition between
different types of charge correlations are discussed.Comment: 22 pages, 15 figures, to be published in Sci. Technol. Adv. Mater.,
Special Edition on Organic Conductor
A quantum Monte Carlo study on the superconducting Kosterlitz-Thouless transition of the attractive Hubbard model on a triangular lattice
We study the superconducting Kosterlitz-Thouless transition of the attractive
Hubbard model on a two-dimensional triangular lattice using auxiliary field
quantum Monte Carlo method for system sizes up to sites.
Combining three methods to analyze the numerical data, we find, for the
attractive interaction of , that the transition temperature stays almost
constant within the band filling range of , while it is found to
be much lower in the region.Comment: RevTeX 6 page
Dynamical Generation of Non-Abelian Gauge Group via the Improved Perturbation Theory
It was suggested that the massive Yang-Mills-Chern-Simons matrix model has
three phases and that in one of them a non-Abelian gauge symmetry is
dynamically generated. The analysis was at the one-loop level around a
classical solution of fuzzy sphere type. We obtain evidences that three phases
are indeed realized as nonperturbative vacua by using the improved perturbation
theory. It also gives a good example that even if we start from a trivial
vacuum, the improved perturbation theory around it enables us to observe
nontrivial vacua.Comment: 31 pages, published versio
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
Possible high superconductivity mediated by antiferromagnetic spin fluctuations in systems with Fermi surface pockets
We propose that if there are two small pocket-like Fermi surfaces, and the
spin susceptibility is pronounced around a wave vector {\bf Q} that bridges the
two pockets, the spin-singlet superconductivity mediated by spin fluctuations
may have a high transition temperature. Using the fluctuation exchange
approximation, this idea is confirmed for the Hubbard on a lattice with
alternating hopping integrals, for which is estimated to be almost an
order of magnitude larger than those for systems with a large connected Fermi
surface.Comment: 5 pages, uses RevTe
Superconductivity and spin correlation in organic conductors: a quantum Monte Carlo study
The d-wave pairing correlations along with spin correlation are calculated
with quantum Monte Carlo method for the two-dimensional Hubbard model on
lattice structures representing organic superconductors
-(BEDT-TTF)X and (TMTSF)X. In both cases the pairing
correlations for superconducting order parameters with nodes are found to be
enhanced. The symmetry and the enhancement of the pairing is systematically
correlated with the spin structure factor, suggesting a spin-fluctuation
mediated pairing. We have further found that, as we deform the Fermi surface to
make the system approach the half-filled square lattice, the coherence of the
pairing saturates while the local pairing amplitude continues to increase.Comment: 5 pages, RevTeX, uses epsf.sty and multicol.st
Competition between singlet and triplet pairings in Na_xCoO_2 yH_2O
We discuss the pairing symmetry of a cobaltate superconductor
NaCoO HO by adopting an effective single band model that
takes into account the hole pockets, as discussed in our previous paper
[to appear in Phys. Rev. Lett.] Here we consider the off-site repulsions in
addition to the on-site repulsion considered in our previous study. We show
that the spin-triplet f-wave pairing proposed in our previous study is robust
to some extent even in the presence of off-site repulsions. However, f-wave
pairing gives way to singlet pairings for sufficiently large values of off-site
repulsions. Among the singlet pairings, i-wave and extended s-wave pairings are
good candidates which do not break time reversal symmetry below in
agreement with the experiments.Comment: 12 page
Two-band Fluctuation Exchange Study on the Superconductivity of -(BEDT-TTF)ICl under High Pressure
We study the pressure dependence of the superconducting transition
temperature of an organic superconductor -(BEDT-TTF)ICl by
applying the fluctuation exchange method to the Hubbard model on the original
two-band lattice at 3/4-filling rather than the single band model in the strong
dimerization limit. Our study is motivated by the fact that hopping parameters
evaluated from a first-principles study suggest that the dimerization of the
BEDT-TTF molecules is not so strong especially at high pressure. Solving the
linearized Eliashberg's equation, a d-wave-like superconducting state
with realistic values of is obtained in a pressure regime somewhat higher
than the actual experimental result. These results are similar to those
obtained within the single band model in the previous study by Kino {\it et
al}. We conclude that the resemblance to the dimer limit is due to a
combination of a good Fermi surface nesting, a large density of states near the
Fermi level, and a moderate dimerization, which cooperatively enhance electron
correlation effects and also the superconducting .Comment: 6 pages, 8 figure
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