11,145 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
Self-selection patterns in Mexico-U.S. migration: the role of migration networks
This paper examines the role of migration networks in determining self-selection
patterns of Mexico-U.S. migration. We first present a simple theoretical framework
showing how such networks impact on migration incentives at different education
levels and, consequently, how they are likely to affect the expected skill composition
of migration. Using survey data from Mexico, we then show that the probability of
migration is increasing with education in communities with low migrant networks,
but decreasing with education in communities with high migrant networks. This is
consistent with positive self-selection of migrants being driven by high migration
costs, as advocated by Chiquiar and Hanson (2005), and with negative self-selection
of migrants being driven by lower returns to education in the U.S. than in Mexico, as
advocated by Borjas (1987)
Can migration reduce educational attainments? depressing evidence from Mexico
This paper examines the impact of migration on educational attainments in rural
Mexico. Using historical migration rates by state to instrument for current
migration, we find evidence of a significant negative effect of migration on
schooling attendance and attainments of 12 to 18 year-old boys and of 16 to 18
year-old girls. IV-Censored Ordered Probit results show that living in a migrant
household lowers the chances of boys completing junior high-school and of boys
and girls completing high-school. The negative effect of migration on schooling is
somewhat mitigated for younger girls with low educated mothers, which is
consistent with remittances relaxing credit constraints on education investment
for the very poor. However, for the majority of rural Mexican children, family
migration depresses educational attainment. Comparison of the marginal effects
of migration on school attendance and on participation to other activities shows
that the observed decrease in schooling of 16 to 18 year olds is accounted for by
current migration of boys and increases in housework for girls
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
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 (, , and
) 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
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,
, has a quantum limit where 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
for the ratio between the shear
viscosity and the entropy density, . 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 He. At low temperature the shear viscosity is found to be well
above the quantum limit and is proportional to the characteristic Fermi liquid
dependence, where is the temperature. With increasing temperature
and interaction strength 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 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
- …