41 research outputs found
Phase Diagram of -(BEDT-TTF)ICl under High Pressure Based on the First-Principles Electronic Structure
We present a theoretical study on the superconductivity of
-(BEDT-TTF)ICl at 14.2 K under a high hydrostatic
pressure recently found, which is the highest among organic superconductors. In
the present work, we study an effective model using the fluctuation-exchange
(FLEX) approximation based on the results of first-principles calculation. In
the obtained phase diagram, the superconductivity with -like symmetry
is realized next to the antiferromagnetic phase, as a result of the
one-dimensional to two-dimensional crossover driven by the pressure.Comment: 4 pages, 3 figures. accepted for publication in J. Phys. Soc. Jpn.
errors correcte
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
Antiferromagnetic Phases of One-Dimensional Quarter-Filled Organic Conductors
The magnetic structure of antiferromagnetically ordered phases of
quasi-one-dimensional organic conductors is studied theoretically at absolute
zero based on the mean field approximation to the quarter-filled band with
on-site and nearest-neighbor Coulomb interaction. The differences in magnetic
properties between the antiferromagnetic phase of (TMTTF)X and the spin
density wave phase in (TMTSF)X are seen to be due to a varying degrees of
roles played by the on-site Coulomb interaction. The nearest-neighbor Coulomb
interaction introduces charge disproportionation, which has the same spatial
periodicity as the Wigner crystal, accompanied by a modified antiferromagnetic
phase. This is in accordance with the results of experiments on (TMTTF)Br
and (TMTTF)SCN. Moreover, the antiferromagnetic phase of (DI-DCNQI)Ag
is predicted to have a similar antiferromagnetic spin structure.Comment: 8 pages, LaTeX, 4 figures, uses jpsj.sty, to be published in J. Phys.
Soc. Jpn. 66 No. 5 (1997
Effects of Spin Fluctuations in Quasi-One-Dimensional Organic Superconductors
We study the electronic states of quasi-one-dimensional organic conductors
using the single band Hubbard model at half-filling. We treat the effects of
the on-site Coulomb interaction by the fluctuation-exchange (FLEX) method, and
calculate the phase diagram and physical properties. The calculated pressure
dependence of the Neel temperature coincides well with the experimental one. We
also show that a pseudogap is formed in the density of states near the chemical
potential and that d-wave superconductivity appears next to the
antiferromagnetic state. Moreover the NMR relaxation rate increases on cooling
in the low-temperature region.Comment: 4 pages, 5 figures, to apprear in J. Phys. Soc. Jp
Spin-Triplet Superconductivity Mediated by Phonons in Quasi-One-Dimensional Systems
We investigate the spin-triplet superconductivity mediated by phonons in
quasi-one-dimensional (Q1D) systems with open Fermi surfaces. We obtain the
ground state phase diagrams. It is found that spin-triplet superconductivity
occurs for weak screening and strong on-site Coulomb interaction, even in the
absence of any additional nonphonon pairing interactions. We find that the
nodeless spin-triplet state is more favorable than the spin-triplet state with
line nodes, for the parameter values of the Q1D superconductors (TMTSF)_2X. We
also find that Q1D open Fermi surface, which is the specific feature of this
system, plays an essential role in the pairing symmetry. We discuss the
compatibility of the present results with the experimental results in these
compounds.Comment: 8 pages, 15 figures, with jpsj2.cl
Theoretical Aspects of Charge Ordering in Molecular Conductors
Theoretical studies on charge ordering phenomena in quarter-filled molecular
(organic) conductors are reviewed. Extended Hubbard models including not only
the on-site but also the inter-site Coulomb repulsion are constructed in a
straightforward way from the crystal structures, which serve for individual
study on each material as well as for their systematic understandings. In
general the inter-site Coulomb interaction stabilizes Wigner crystal-type
charge ordered states, where the charge localizes in an arranged manner
avoiding each other, and can drive the system insulating. The variety in the
lattice structures, represented by anisotropic networks in not only the
electron hopping but also in the inter-site Coulomb repulsion, brings about
diverse problems in low-dimensional strongly correlated systems. Competitions
and/or co-existences between the charge ordered state and other states are
discussed, such as metal, superconductor, and the dimer-type Mott insulating
state which is another typical insulating state in molecular conductors.
Interplay with magnetism, e.g., antiferromagnetic state and spin gapped state
for example due to the spin-Peierls transition, is considered as well. Distinct
situations are pointed out: influences of the coupling to the lattice degree of
freedom and effects of geometrical frustration which exists in many molecular
crystals. Some related topics, such as charge order in transition metal oxides
and its role in new molecular conductors, are briefly remarked.Comment: 21 pages, 19 figures, to be published in J. Phys. Soc. Jpn. special
issue on "Organic Conductors"; figs. 4 and 11 replaced with smaller sized
fil
Strong electronic correlations in superconducting organic charge transfer salts
We review the role of strong electronic correlations in
quasi--two-dimensional organic charge transfer salts such as (BEDT-TTF),
(BETS) and -[Pd(dmit)]. We begin by defining minimal
models for these materials. It is necessary to identify two classes of
material: the first class is strongly dimerised and is described by a
half-filled Hubbard model; the second class is not strongly dimerised and is
described by a quarter filled extended Hubbard model. We argue that these
models capture the essential physics of these materials. We explore the phase
diagram of the half-filled quasi--two-dimensional organic charge transfer
salts, focusing on the metallic and superconducting phases. We review work
showing that the metallic phase, which has both Fermi liquid and `bad metal'
regimes, is described both quantitatively and qualitatively by dynamical mean
field theory (DMFT). The phenomenology of the superconducting state is still a
matter of contention. We critically review the experimental situation, focusing
on the key experimental results that may distinguish between rival theories of
superconductivity, particularly probes of the pairing symmetry and measurements
of the superfluid stiffness. We then discuss some strongly correlated theories
of superconductivity, in particular, the resonating valence bond (RVB) theory
of superconductivity. We conclude by discussing some of the major challenges
currently facing the field.Comment: A review: 52 pages; 10 fig
Pairing Symmetry Competition in Organic Superconductors
A review is given on theoretical studies concerning the pairing symmetry in
organic superconductors. In particular, we focus on (TMTSF)X and
-(BEDT-TTF)X, in which the pairing symmetry has been extensively
studied both experimentally and theoretically. Possibilities of various pairing
symmetry candidates and their possible microscopic origin are discussed. Also
some tests for determining the actual pairing symmtery are surveyed.Comment: 16 pages, 8 figures, to be published in J. Phys. Soc. Jpn., special
issue on "Organic Conductors
Pairing competition in a quasi-one-dimensional model of organic superconductors (TMTSF) in magnetic field
We microscopically study the effect of the magnetic field (Zeeman splitting)
on the superconducting state in a model for quasi-one-dimensional organic
superconductors (TMTSF). We investigate the competition between spin
singlet and spin triplet pairings and the
Fulde-Ferrell-Larkin-Ovchinnikov(FFLO) state by random phase approximation.
While we studied the competition by comparison with the eigenvalue of the gap
equation at a fixed temperature in our previous study (Phys. Rev. Lett.
\textbf{102} (2009) 016403), here we obtain both the for each pairing
state and a phase diagram in the (temperature)-(field)-(strength
of the charge fluctuation) space. The phase diagram shows that consecutive
transitions from singlet pairing to the FFLO state and further to
triplet pairing can occur upon increasing the magnetic field when
charge fluctuations coexist with spin fluctuations. In the FFLO state,
the singlet d-wave and triplet -wave components are strongly mixed
especially when the charge fluctuations are strong.Comment: 11 pages, 9 figure