5,537 research outputs found
Towards first-principles understanding of the metal-insulator transition in fluid alkali metals
By treating the electron-ion interaction as perturbation in the
first-principles Hamiltonian, we have calculated the density response functions
of a fluid alkali metal to find an interesting charge instability due to
anomalous electronic density fluctuations occurring at some finite wave vector
{\bi Q} in a dilute fluid phase above the liquid-gas critical point. Since
|{\bi Q}| is smaller than the diameter of the Fermi surface, this instability
necessarily impedes the electric conduction, implying its close relevance to
the metal-insulator transition in fluid alkali metals.Comment: 11 pages, 5 figure
Superfluid to Mott-insulator transition in Bose-Hubbard models
We study the superfluid-insulator transition in Bose-Hubbard models in one-,
two-, and three-dimensional cubic lattices by means of a recently proposed
variational wave function. In one dimension, the variational results agree with
the expected Berezinskii-Kosterlitz-Thouless scenario of the interaction-driven
Mott transition. In two and three dimensions, we find evidences that, across
the transition,most of the spectral weight is concentrated at high energies,
suggestive of pre-formed Mott-Hubbard side-bands. This result is compatible
with the experimental data by Stoferle et al.
[Phys. Rev. Lett. 92, 130403 (2004)].Comment: 4 pages, 4 figures, based on cond-mat/061130
A Historical Perspective of Catfish Production in the Southeast in Relation to Avian Predation
Production of aquaculture species, especially catfish (Ictalurus punctatus) in the Mississippi Delta, is a relatively new and expanding industry. Catfish production represents the largest dollar value of the aquaculture industry, accounting for approximately 50% of the entire industry. Mississippi is responsible for 82% of the total U.S. catfish production. Fish-eating bird populations have capitalized on this new food source. Double-crested cormorants (Phalacrocorax auritus), great blue herons (Ardea herodias), and great egrets (Casmerodius albus) are the primary predators on catfish. Cormorant caused losses in excess of $2 million per year have been reported in Mississippi. U.S. Department of Agriculture research and operational assistance programs have been established in the southeast to determine the economic impact that birds have on the aquaculture industry, and to develop and implement technology that can be used in integrated strategies to solve bird depredation problems
Doping Dependence of Polaron Hopping Energies in La(1-x)Ca(x)MnO(3) (0<= x<= 0.15)
Measurements of the low-frequency (f<= 100 kHz) permittivity at T<= 160 K and
dc resistivity (T<= 430 K) are reported for La(1-x)Ca(x)MnO(3) (0<= x<= 0.15).
Static dielectric constants are determined from the low-T limiting behavior of
the permittivity. The estimated polarizability for bound holes ~ 10^{-22}
cm^{-3} implies a radius comparable to the interatomic spacing, consistent with
the small polaron picture established from prior transport studies near room
temperature and above on nearby compositions. Relaxation peaks in the
dielectric loss associated with charge-carrier hopping yield activation
energies in good agreement with low-T hopping energies determined from
variable-range hopping fits of the dc resistivity. The doping dependence of
these energies suggests that the orthorhombic, canted antiferromagnetic ground
state tends toward an insulator-metal transition that is not realized due to
the formation of the ferromagnetic insulating state near Mn(4+) concentration ~
0.13.Comment: PRB in press, 5 pages, 6 figure
Quantum-defect theory of resonant charge exchange
We apply the quantum-defect theory for potential to study the
resonant charge exchange process. We show that by taking advantage of the
partial-wave-insensitive nature of the formulation, resonant charge exchange of
the type of S+S can be accurately described over a wide range of
energies using only three parameters, such as the \textit{gerade} and the
\textit{ungerade} wave scattering lengths, and the atomic polarizability,
even at energies where many partial waves contribute to the cross sections. The
parameters can be determined experimentally, without having to rely on accurate
potential energy surfaces, of which few exist for ion-atom systems. The theory
further relates ultracold interactions to interactions at much higher
temperatures.Comment: 8 pages, 7 figure
Anderson-Hubbard model with box disorder: Statistical dynamical mean-field theory investigation
Strongly correlated electrons with box disorder in high-dimensional lattices
are investigated. We apply the statistical dynamical mean-field theory, which
treats local correlations non-perturbatively. The incorporation of a finite
lattice connectivity allows for the detection of disorder-induced localization
via the probability distribution function of the local density of states. We
obtain a complete paramagnetic ground state phase diagram and find
correlation-induced as well as disorder-induced metal-insulator transitions.
Our results qualitatively confirm predictions obtained by typical medium
theory. Moreover, we find that the probability distribution function of the
local density of states in the metallic phase strongly deviates from a
log-normal distribution as found for the non-interacting case.Comment: 13 pages, 15 figures, published versio
Electronic transport coefficients from ab initio simulations and application to dense liquid hydrogen
Using Kubo's linear response theory, we derive expressions for the
frequency-dependent electrical conductivity (Kubo-Greenwood formula),
thermopower, and thermal conductivity in a strongly correlated electron system.
These are evaluated within ab initio molecular dynamics simulations in order to
study the thermoelectric transport coefficients in dense liquid hydrogen,
especially near the nonmetal-to-metal transition region. We also observe
significant deviations from the widely used Wiedemann-Franz law which is
strictly valid only for degenerate systems and give an estimate for its valid
scope of application towards lower densities
Theory for Gossamer and Resonating Valence Bond Superconductivity
We use an effective Hamiltonian for two-dimensional Hubbard model including
an antiferromagnetic spin-spin coupling term to study recently proposed
gossamer superconductivity. We formulate a renormalized mean field theory to
approximately take into account the strong correlation effect in the partially
projected Gutzwiller wavefucntions. At the half filled, there is a first order
phase transition to separate a Mott insulator at large Coulomb repulsion U from
a gossamer superconductor at small U. Away from the half filled,the Mott
insulator is evolved into an resonating valence bond state, which is
adiabatically connected to the gossamer superconductor.Comment: 10 pages, 13 figure
Analytical calculation of the Green's function and Drude weight for a correlated fermion-boson system
In classical Drude theory the conductivity is determined by the mass of the
propagating particles and the mean free path between two scattering events. For
a quantum particle this simple picture of diffusive transport loses relevance
if strong correlations dominate the particle motion. We study a situation where
the propagation of a fermionic particle is possible only through creation and
annihilation of local bosonic excitations. This correlated quantum transport
process is outside the Drude picture, since one cannot distinguish between free
propagation and intermittent scattering. The characterization of transport is
possible using the Drude weight obtained from the f-sum rule, although its
interpretation in terms of free mass and mean free path breaks down. For the
situation studied we calculate the Green's function and Drude weight using a
Green's functions expansion technique, and discuss their physical meaning.Comment: final version, minor correction
Model solution for volume reflection of relativistic particles in a bent crystal
For volume reflection process in a bent crystal, exact analytic expressions
for positively- and negatively-charged particle trajectories are obtained
within a model of parabolic continuous potential in each interplanar interval,
with the neglect of incoherent multiple scattering. In the limit of the crystal
bending radius greatly exceeding the critical value, asymptotic formulas are
obtained for the particle mean deflection angle in units of Lindhard's critical
angle, and for the final beam profile. Volume reflection of negatively charged
particles is shown to contain effects of rainbow scattering and orbiting,
whereas with positively charged particles none of these effects arise within
the given model. The model predictions are compared with experimental results
and numerical simulations. Estimates of the volume reflection mean angle and
the final beam profile robustness under multiple scattering are performed.Comment: 21 pages, 11 figure
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