47 research outputs found
Model construction and superconductivity analysis of organic conductors \beta-(BDA-TTP)_2MF_6 (M=P, As, Sb, Ta) based on first principles band calculation
We perform a first principles band calculation for a group of
quasi-two-dimensional organic conductors \beta-(BDA-TTP)2MF6 (M=P, As, Sb, Ta).
The ab-initio calculation shows that the density of states (DOS) is correlated
with the band width of singly occupied (highest) molecular orbital (SOMO),
while it is not necessarily correlated with the unit cell volume. The direction
of the major axis of the cross section of the Fermi surface lies in the
\Gamma-B direction, which differs from that obtained by the extended Huckel
calculation. Then, we construct a tight-binding model which accurately
reproduces the ab-initio band structure. The obtained transfer energies give
smaller dimerization than in the extended Huckel band. As for the difference of
the anisotropy of the Fermi surface, the transfer energies along the
inter-stacking direction are smaller than those obtained in the extended Huckel
calculation. Assuming spin-fluctuation-mediated superconductivity, we apply
random phase approximation (RPA) to a two-band Hubbard model. This two-band
Hubbard model is composed of the tight-binding model derived from the first
principles band structure and an on-site (intra-molecule) repulsive interaction
taken as a variable parameter. The obtained superconducting gap changes sign
four times along the Fermi surface like in a d-wave gap, and the nodal
direction is different from that obtained in the extended Huckel model. Anion
dependence of Tc is qualitatively consistent with the experimental observation.Comment: 17 pages, 9 figure
Quantum Melting of the Charge Density Wave State in 1T-TiSe2
We report a Raman scattering study of low-temperature, pressure-induced
melting of the CDW phase of 1T-TiSe2. Our Raman scattering measurements reveal
that the collapse of the CDW state occurs in three stages: (i) For P<5 kbar,
the pressure dependence of the CDW amplitude mode energies and intensities are
indicative of a ``crystalline'' CDW regime; (ii) for 5 < P < 25 kbar, there is
a decrease in the CDW amplitude mode energies and intensities with increasing
pressure that suggests a regime in which the CDW softens, and may decouple from
the lattice; and (iii) for P>25 kbar, the absence of amplitude modes reveals a
melted CDW regime.Comment: 5 pages, 4 figure
Multi-Orbital Molecular Compound (TTM-TTP)I_3: Effective Model and Fragment Decomposition
The electronic structure of the molecular compound (TTM-TTP)I_3, which
exhibits a peculiar intra-molecular charge ordering, has been studied using
multi-configuration ab initio calculations. First we derive an effective
Hubbard-type model based on the molecular orbitals (MOs) of TTM-TTP; we set up
a two-orbital Hamiltonian for the two MOs near the Fermi energy and determine
its full parameters: the transfer integrals, the Coulomb and exchange
interactions. The tight-binding band structure obtained from these transfer
integrals is consistent with the result of the direct band calculation based on
density functional theory. Then, by decomposing the frontier MOs into two
parts, i.e., fragments, we find that the stacked TTM-TTP molecules can be
described by a two-leg ladder model, while the inter-fragment Coulomb energies
are scaled to the inverse of their distances. This result indicates that the
fragment picture that we proposed earlier [M.-L. Bonnet et al.: J. Chem. Phys.
132 (2010) 214705] successfully describes the low-energy properties of this
compound.Comment: 5 pages, 4 figures, published versio
Theory of the beta-type Organic Superconductivity under Uniaxial Compression
We study theoretically the shift of the superconducting transition
temperature (Tc) under uniaxial compression in beta-type organic
superconductors, beta-(BEDT-TTF)2I3 and beta-(BDA-TTP)2X[X=SbF6,AsF6], in order
to clarify the electron correlation, the spin frustration and the effect of
dimerization. The transfer integrals are calculated by the extended Huckel
method assuming the uniaxial strain and the superconducting state mediated by
the spin fluctuation is solved using Eliashberg's equation with the
fluctuation-exchange approximation. The calculation is carried out on both the
dimerized (one-band) and nondimerized (two-band) Hubbard models. We have found
that (i) the behavior of Tc in beta-(BEDT-TTF)2I3 with a stronger dimerization
is well reproduced by the dimer model, while that in weakly dimerized
beta-BDA-TTP salts is rather well reproduced by the two-band model, and (ii)
the competition between the spin frustration and the effect induced by the
fluctuation is important in these materials, which causes nonmonotonic shift of
Tc against uniaxial compression.Comment: 18 pages, 16 figures, 2 tabl
Organic Superconductors: when correlations and magnetism walk in
This survey provides a brief account for the start of organic
superconductivity motivated by the quest for high Tc superconductors and its
development since the eighties'. Besides superconductivity found in 1D organics
in 1980, progresses in this field of research have contributed to better
understand the physics of low dimensional conductors highlighted by the wealth
of new remarkable properties. Correlations conspire to govern the low
temperature properties of the metallic phase. The contribution of
antiferromagnetic fluctuations to the interchain Cooper pairing proposed by the
theory is borne out by experimental investigations and supports
supercondutivity emerging from a non Fermi liquid background. Quasi one
dimensional organic superconductors can therefore be considered as simple
prototype systems for the more complex high Tc materials.Comment: 41 pages, 21 figures to be published in Journal of Superconductivity
and Novel Magnetis
Pressure effects on the CDW transitions and magnetoresistances in NbSe3
Magnetoresistance (MR) of NbSe3 has been measured between 2 K alid 150 K under high pressure including critical pressures (Pc2 and Pc1) where the lower and upper CDW phase are totally suppressed by pressure respectively. At ambient pressure, a large MR (LMR) is observed just below Tc2 but no MR is detected above Tc2. Under high pressure, in addition to the LMR, we observe newly a MR above Tc2. Here we call il the pressure-induced MR (PIMR). The LMR always appears as long as the lower CDW phase exists but it rapidly disappears above Pc2. Similarly, the PIMR disappears above Pc1. Furthermore, no anomalies associated with the field-induced CDW are observed within our experimental limits. From these findings, we claim that the LMR and PIMR are due to normal carriers in small pockets created by a pressure-dependent imperfect nesting of the Fermi surface in the CDW transitions