657 research outputs found
Half-Integral Spin-Singlet Quantum Hall Effect
We provide numerical evidence that the ground state of a short range
interaction model at is incompressible and spin-singlet for a wide
range of repulsive interactions. Furthermore it is accurately described by a
trial wave function studied earlier. For the Coulomb interaction we find that
this wave function provides a good description of the lowest lying spin-singlet
state, and propose that fractional quantum Hall effect would occur at
if this state became the global ground state.Comment: Latex 13 pages, 3 figures upon reques
From Cooper Pairs to Composite Bosons: A Generalized RPA Analysis of Collective Excitations
The evolution of the ground state and the excitation spectrum of the two and
three dimensional attractive Hubbard model is studied as the system evolves
from a Cooper pair regime for weak attraction to a composite boson regime for a
strong attraction.Comment: 20 pages RevTex, 7 figures on reques
Isolation of polymorphic microsatellites in the stemless thistle (Cirsium acaule) and their utility in other Cirsium species
The genus Cirsium includes species with both widespread and restricted geographical distributions, several of which are serious weeds. Nine polymorphic microsatellite loci were isolated from the stemless thistle Cirsium acaule. Eight were polymorphic in C. acaule, six in C. arvense and seven in C. heterophyllum. One locus monomorphic in C. acaule showed polymorphism in C. heterophyllum. The mean number of alleles per locus was 4.1 in C. acaule, 6.2 in C. arvense and 2.9 in C. heterophyllum. These nine loci were also amplified in C. eriophorum and C. vulgare, suggesting that these markers may be of use throughout the genus
Growth and linear optical properties of CuCl nanocrystals
Linear optical properties of CuCl nanocrystals in a NaCl matrix have been studied using optical absorption, cathodoluminescence and X-ray diffraction measurements. Our measurements showed that CuCl nanocrystals were really formed. Their average size was estimated to be 2.2 nm. Consistently with confinement effect resulting from the low size of the nanocrystals, a blue shift of excitonic levels is observed by comparison to the bulk crystal. Our analysis showed that the effect of thermal annealing depends on the temperature, the annealing time and the nature of cooling
Statistical Model of Superconductivity in a 2D Binary Boson-Fermion Mixture
A two-dimensional (2D) assembly of noninteracting, temperature-dependent,
composite-boson Cooper pairs (CPs) in chemical and thermal equilibrium with
unpaired fermions is examined in a binary boson-fermion statistical model as
the superconducting singularity temperature is approached from above. The model
is derived from {\it first principles} for the BCS model interfermion
interaction from three extrema of the system Helmholtz free energy (subject to
constant pairable-fermion number) with respect to: a) the pairable-fermion
distribution function; b) the number of excited (bosonic) CPs, i.e., with
nonzero total momenta--usually ignored in BCS theory--and with the appropriate
(linear, as opposed to quadratic) dispersion relation that arises from the
Fermi sea; and c) the number of CPs with zero total momenta. Compared with the
BCS theory condensate, higher singularity temperatures for the Bose-Einstein
condensate are obtained in the binary boson-fermion mixture model which are in
rough agreement with empirical critical temperatures for quasi-2D
superconductorsComment: 16 pages and 4 figures. This is a improved versio
Linear to quadratic crossover of Cooper pair dispersion relation
Cooper pairing is studied in three dimensions to determine its binding energy
for all coupling using a general separable interfermion interaction. Also
considered are Cooper pairs (CPs) with nonzero center-of-mass momentum (CMM). A
coupling-independent {\it linear} term in the CMM dominates the pair excitation
energy in weak coupling and/or high fermion density, while the more familiar
quadratic term prevails only in the extreme low-density (i.e., vacuum) limit
for any nonzero coupling. The linear-to-quadratic crossover of the CP
dispersion relation is analyzed numerically, and is expected to play a central
role in a model of superconductivity (and superfluidity) simultaneously
accommodating a BCS condensate as well as a Bose-Einstein condensate of CP
bosons.Comment: 13 pages plus 2 figure
Deviations from Fermi-liquid behavior above in 2D short coherence length superconductors
We show that there are qualitative differences between the temperature
dependence of the spin and charge correlations in the normal state of the 2D
attractive Hubbard model using quantum Monte Carlo simulations. The
one-particle density of states shows a pseudogap above \tc with a depleted
with decreasing . The susceptibility \cs and the low frequency spin
spectral weight track , which explains the spin-gap scaling: 1/T_1T \sim
\cs(T). However the charge channel is dominated by collective behavior and the
compressibility is -independent. This anomalous ``spin-charge
separation'' is shown to exist even at intermediate where the momentum
distribution n(\bk) gives evidence for degenerate Fermi system.Comment: 4 pages (twocolumn format), 5 Postscript figure
Possibility of p-wave pairing of composite fermions at
We find that for the pure Coulomb repulsion the composite Fermi sea at
is on the verge of an instability to triplet pairing of composite
fermions. It is argued that a transition into the paired state, described by a
Pfaffian wave function, may be induced if the short-range part of the
interaction is softened by increasing the thickness of the two-dimensional
electron system.Comment: 14 pages, 3 eps figures include
Cooper pair dispersion relation for weak to strong coupling
Cooper pairing in two dimensions is analyzed with a set of renormalized
equations to determine its binding energy for any fermion number density and
all coupling assuming a generic pairwise residual interfermion interaction. \
Also considered are Cooper pairs (CPs) with nonzero center-of-mass momentum
(CMM)--usually neglected in BCS theory--and their binding energy is expanded
analytically in powers of the CMM up to quadratic terms. A Fermi-sea-dependent
{\it linear} term in the CMM dominates the pair excitation energy in weak
coupling (also called the BCS regime) while the more familiar quadratic term
prevails in strong coupling (the Bose regime). The crossover, though strictly
unrelated to BCS theory {\it per se,} is studied numerically as it is expected
to play a central role in a model of superconductivity as a Bose-Einstein
condensation of CPs where the transition temperature vanishes for all
dimensionality for quadratic dispersion, but is {\it nonzero} for all
for linear dispersion.Comment: 11 pages plus 3 figures, revised version accepted in Physical Review
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