26 research outputs found
Inequivalent routes across the Mott transition in V2O3 explored by X-ray absorption
The changes in the electronic structure of V2O3 across the metal-insulator
transition induced by temperature, doping and pressure are identified using
high resolution x-ray absorption spectroscopy at the V pre K-edge. Contrary to
what has been taken for granted so far, the metallic phase reached under
pressure is shown to differ from the one obtained by changing doping or
temperature. Using a novel computational scheme, we relate this effect to the
role and occupancy of the a1g orbitals. This finding unveils the inequivalence
of different routes across the Mott transition in V2O
1/T_1 nuclear relaxation time of \kappa-(BEDT-TTF)_ 2 Cu [N(CN)_2] Cl : effects of magnetic frustration
We study the role played by the magnetic frustration in the antiferromagnetic
phase of the organic salt \kappa-(BEDT-TTF)_ 2 Cu [N(CN)_2] Cl. Using the
spatially anisotropic triangular Heisenberg model we analyze previous and new
performed NMR experiments. We compute the 1/T_1 relaxation time by means of the
modified spin wave theory. The strong suppression of the nuclear relaxation
time observed experimentally under varying pressure and magnetic field is
qualitatively well reproduced by the model. Our results suggest the existence
of a close relation between the effects of pressure and magnetic frustration.Comment: 10 pages, 9 figures, to appear in Journal of Phys.: Condens Matte
Charge gap in the one--dimensional dimerized Hubbard model at quarter-filling
We propose a quantitative estimate of the charge gap that opens in the
one-dimensional dimerized Hubbard model at quarter-filling due to dimerization,
which makes the system effectively half--filled, and to repulsion, which
induces umklapp scattering processes. Our estimate is expected to be valid for
any value of the repulsion and of the parameter describing the dimerization. It
is based on analytical results obtained in various limits (weak coupling,
strong coupling, large dimerization) and on numerical results obtained by exact
diagonalization of small clusters. We consider two models of dimerization:
alternating hopping integrals and alternating on--site energies. The former
should be appropriate for the Bechgaard salts, the latter for compounds where
the stacks are made of alternating and molecules. % and ( denotes , , ...).Comment: 33 pages, RevTeX 3.0, figures on reques
Mechanism for the Singlet to Triplet Superconductivity Crossover in Quasi-One-Dimensional Organic Conductors
Superconductivity of quasi-one-dimensional organic conductors with a
quarter-filled band is investigated using the two-loop renormalization group
approach to the extended Hubbard model for which both the single electron
hopping t_{\perp} and the repulsive interaction V_{\perp} perpendicular to the
chains are included. For a four-patches Fermi surface with deviations to
perfect nesting, we calculate the response functions for the dominant
fluctuations and possible superconducting states. By increasing V_{\perp}, it
is shown that a d-wave (singlet) to f-wave (triplet) superconducting state
crossover occurs, and is followed by a vanishing spin gap. Furthermore, we
study the influence of a magnetic field through the Zeeman coupling, from which
a triplet superconducting state is found to emerge.Comment: 11 pages, 15 figures, published versio
Superconducting pairing and density-wave instabilities in quasi-one-dimensional conductors
Using a renormalization group approach, we determine the phase diagram of an
extended quasi-one-dimensional electron gas model that includes interchain
hopping, nesting deviations and both intrachain and interchain repulsive
interactions. d-wave superconductivity, which dominates over the
spin-density-wave (SDW) phase at large nesting deviations, becomes unstable to
the benefit of a triplet -wave phase for a weak repulsive interchain
backscattering term , despite the persistence of dominant SDW
correlations in the normal state. Antiferromagnetism becomes unstable against
the formation of a charge-density-wave state when exceeds some
critical value. While these features persist when both Umklapp processes and
interchain forward scattering () are taken into account, the effect
of alone is found to frustrate nearest-neighbor interchain - and
-wave pairing and instead favor next-nearest-neighbor interchain singlet or
triplet pairing. We argue that the close proximity of SDW and
charge-density-wave phases, singlet d-wave and triplet -wave superconducting
phases in the theoretical phase diagram provides a possible explanation for
recent puzzling experimental findings in the Bechgaard salts, including the
coexistence of SDW and charge-density-wave phases and the possibility of a
triplet pairing in the superconducting phase.Comment: 19 pages, 13 figure
Hall effect and conduction anisotropy in the organic conductor TMTSF2PF6
Long missing basic experiments in the normal phase of the anisotropic
electron system of TMTSF2PF6 were performed. Both the Hall effect and the
ab'-plane conduction anisotropy are directly addressing the unconventional
electrical properties of this Bechgaard salt. We found that the dramatic
reduction of the carrier density deduced from recent optical data is not
reflected in an enhanced Hall-resistance. The pressure- and temperature
dependence of the b'-direction resitivity reveal isotropic relaxation time and
do not require explanations beyond the Fermi liquid theory. Our results allow a
coherent-diffusive transition in the interchain carrier propagation, however
the possible crossover to Luttinger liquid behavior is placed to an energy
scale above room temperature.Comment: 5 pages, 5 figures, to be published in Phys. Rev. Let
Role of Interchain Hopping in the Magnetic Susceptibility of Quasi-One-Dimensional Electron Systems
The role of interchain hopping in quasi-one-dimensional (Q-1D) electron
systems is investigated by extending the Kadanoff-Wilson renormalization group
of one-dimensional (1D) systems to Q-1D systems. This scheme is applied to the
extended Hubbard model to calculate the temperature () dependence of the
magnetic susceptibility, . The calculation is performed by taking
into account not only the logarithmic Cooper and Peierls channels, but also the
non-logarithmic Landau and finite momentum Cooper channels, which give relevant
contributions to the uniform response at finite temperatures. It is shown that
the interchain hopping, , reduces at low temperatures,
while it enhances at high temperatures. This notable
dependence is ascribed to the fact that enhances the
antiferromagnetic spin fluctuation at low temperatures, while it suppresses the
1D fluctuation at high temperatures. The result is at variance with the
random-phase-approximation approach, which predicts an enhancement of by over the whole temperature range. The influence of both the
long-range repulsion and the nesting deviations on is further
investigated. We discuss the present results in connection with the data of
in the (TMTTF) and (TMTSF) series of Q-1D organic
conductors, and propose a theoretical prediction for the effect of pressure on
magnetic susceptibility.Comment: 17 pages, 19figure
Superconductivity and Antiferromagnetism in Quasi-one-dimensional Organic Conductors
We review the current understanding of superconductivity in the
quasi-one-dimensional organic conductors of the Bechgaard and Fabre salt
families. We discuss the interplay between superconductivity,
antiferromagnetism, and charge-density-wave fluctuations. The connection to
recent experimental observations supporting unconventional pairing and the
possibility of a triplet-spin order parameter for the superconducting phase is
also presented.Comment: (v1) 30 pages, 13 figures; Review article for the 20th anniversary of
high-Tc superconductivity, to appear in J. Low Temp. Phys. (v2) 1 Ref. adde
Properties of a Luttinger Liquid with Boundaries at Finite Temperature and Size
We use bosonization methods to calculate the exact finite-temperature
single-electron Green's function of a spinful Luttinger liquid confined by open
boundaries. The corresponding local spectral density is constructed and
analyzed in detail. The interplay between boundary, finite-size and thermal
effects are shown to dramatically influence the low-energy properties of the
system. In particular, the well-known zero-temperature critical behavior in the
bulk always crosses over to a boundary dominated regime in the vicinity of the
Fermi level. Thermal fluctuations cause an enhanced depletion of spectral
weight for small energies E, with the spectral density scaling as E^2 for E
much less than the temperature. Consequences for photoemission experiments are
discussed.Comment: 18 pages in revtex format including 5 embedded figures (using epsf).
The latest complete postscript file is available from
http://fy.chalmers.se/~eggert/papers/longlutt.ps or by request from
[email protected]. To appear in Phys. Rev. B (Dec. 1997
Bond and charge density waves in the isotropic interacting two-dimensional quarter-filled band and the insulating state proximate to organic superconductivity
We report two surprising results regarding the nature of the spatial broken
symmetries in the two-dimensional (2D), quarter-filled band with strong
electron-electron interactions. First, in direct contradiction to the
predictions of one-electron theory, we find a coexisting ``bond-order and
charge density wave'' (BCDW) insulating ground state in the 2D rectangular
lattice for all anisotropies, including the isotropic limit. Second, we find
that the BCDW further coexists with a spin-density wave (SDW) in the range of
large anisotropy. Further, in contrast to the interacting half-filled band, in
the interacting quarter-filled band there are two transitions: first, a similar
singlet-to-AFM/SDW transition for large anisotropy and second, an
AFM/SDW-to-singlet transition at smaller anisotropy. We discuss how these
theoretical results apply to the insulating states that are proximate to the
superconducting states of 2:1 cationic charge-transfer solids (CTS).
An important consequence of this work is the suggestion that organic
superconductivity is related to the proximate Coulomb-induced BCDW, with the
SDW that coexists for large anisotropies being also a consequence of the BCDW,
rather than the driver of superconductivity.Comment: 29 pages, 18 eps figures. Revised with new appendices; to appear in
Phys. Rev. B 62, Nov 15, 200