507 research outputs found

### Similarities between organic and cuprate superconductors

This ``Perspective'' briefly reviews recent work showing that a class of
organic superconductors have important similarities to the cuprate
superconductors: (i) There is competition betweeen superconductivity and
antiferromagnetism. (ii) Uncoventional metallic behavior is observed near the
metal-insulator transition. A more detailed review and discussion of the
appropriate strongly correlated electron model can be found in
cond-mat/9802198.Comment: 2 pages, RevTeX + epsf, 1 figure

### A two-state model of twisted intramolecular chargetransfer in monomethine dyes

A two-state model Hamiltonian is proposed to model the coupling of twisting
displacements to charge-transfer behavior in the ground and excited states of a
general monomethine dye molecule. This coupling may be relevant to the
molecular mechanism of environment-dependent fluorescence yield enhancement.
The model is parameterized against quantum chemical calculations on different
protonation states of the green fluorescent protein chromophore (GFP), which
are chosen to sample different regimes of detuning from the cyanine (resonant)
limit. The model provides a simple yet realistic description of the charge
transfer character along two possible excited state twisting channels
associated with the methine bridge. It describes qualitatively different
behavior in three regions that can be classified by their relationship to the
resonant (cyanine) limit. The regimes differ by the presence or absence of
twist-dependent polarization reversal and the occurrence of conical
intersections. We find that selective biasing of one twisting channel over
another by an applied diabatic biasing potential can only be achieved in a
finite range of parameters near the cyanine limit.Comment: 45 pages, 9 Figures (incl. 2 chemical schemes). Accepted for
publication by the Journal of Chemical Physics. Changes include 2 additional
figures to and expanded discussion of key points felt to be important, and
condensed discussion of some points felt to be less importan

### Magnetoresistance in quasi-one-dimensional metals due to Fermi surface cold spots

In a number of quasi-one-dimensional organic metals the dependence of the
magnetoresistance on the direction of the magnetic field is quite different
from the predictions of Boltzmann transport theory for a Fermi liquid with a
scattering rate that is independent of momentum. We consider a model in which
there are large variations in the scattering rate over the Fermi surface. The
model is the quasi-one-dimensional version of the ``cold spots'' model
introduced by Ioffe and Millis to explain anomalous transport properties of the
metallic phase of the cuprate superconductors. The dependence of the
resistance, in the most and least conducting directions, on the direction and
magnitude of the magnetic field are calculated. The calculated
magnetoresistance has a number of properties that are quite distinct from
conventional transport theory such as magic angle effects a significant
magnetoresistance when the field and current are both in the least conducting
direction. However, the model cannot give a complete description of the unusual
properties of (TMTSF)2PF6 at pressures of 8-11 kbar.Comment: 16 pages, RevTex + epsf, 6 figure

### Shear viscosity of strongly interacting fermionic quantum fluids

Eighty years ago Eyring proposed that the shear viscosity of a liquid,
$\eta$, has a quantum limit $\eta \gtrsim n\hbar$ where $n$ is the density of
the fluid. Using holographic duality and the AdS/CFT correspondence in string
theory Kovtun, Son, and Starinets (KSS) conjectured a universal bound
$\frac{\eta}{s}\geq \frac{\hbar}{4\pi k_{B}}$ for the ratio between the shear
viscosity and the entropy density, $s$. Using Dynamical Mean-Field Theory
(DMFT) we calculate the shear viscosity and entropy density for a fermionic
fluid described by a single band Hubbard model at half filling. Our calculated
shear viscosity as a function of temperature is compared with experimental data
for liquid $^{3}$He. At low temperature the shear viscosity is found to be well
above the quantum limit and is proportional to the characteristic Fermi liquid
$1/T^{2}$ dependence, where $T$ is the temperature. With increasing temperature
and interaction strength $U$ there is significant deviation from the Fermi
liquid form. Also, the shear viscosity violates the quantum limit near the
crossover from coherent quasi-particle based transport to incoherent transport
(the bad metal regime). Finally, the ratio of the shear viscosity to the
entropy density is found to be comparable to the KSS bound for parameters
appropriate to liquid $^{3}$He. However, this bound is found to be strongly
violated in the bad metal regime for parameters appropriate to lattice
electronic systems such as organic charge transfer salts.Comment: Revised manuscript with added references, 14 pages 5 figure

### Enhancement of the thermal expansion of organic charge transfer salts by strong electronic correlations

Organic charge transfer salts exhibit thermal expansion anomalies similar to
those found in other strongly correlated electron systems. The thermal
expansion can be anisotropic and have a non-monotonic temperature dependence.
We show how these anomalies can arise from electronic effects and be
significantly enhanced, particularly at temperatures below 100 K, by strong
electronic correlations. For the relevant Hubbard model the thermal expansion
is related to the dependence of the entropy on the parameters ($t$, $t'$, and
$U$) in the Hamiltonian or the temperature dependence of bond orders and double
occupancy. The latter are calculated on finite lattices with the Finite
Temperature Lanczos Method. Although many features seen in experimental data,
in both the metallic and Mott insulating phase, are described qualitatively,
the calculated magnitude of the thermal expansion is smaller than that observed
experimentally.Comment: 9 pages, 6 figure

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