31,678 research outputs found
Disordered two-dimensional superconductors: roles of temperature and interaction strength
We have considered the half-filled disordered attractive Hubbard model on a
square lattice, in which the on-site attraction is switched off on a fraction
of sites, while keeping a finite on the remaining ones. Through Quantum
Monte Carlo (QMC) simulations for several values of and , and for system
sizes ranging from to , we have calculated the
configurational averages of the equal-time pair structure factor , and,
for a more restricted set of variables, the helicity modulus, , as
functions of temperature. Two finite-size scaling {\it ansatze} for have
been used, one for zero-temperature and the other for finite temperatures. We
have found that the system sustains superconductivity in the ground state up to
a critical impurity concentration, , which increases with , at least up
to U=4 (in units of the hopping energy). Also, the normalized zero-temperature
gap as a function of shows a maximum near , for . Analyses of the helicity modulus and of the pair structure factor
led to the determination of the critical temperature as a function of , for
4 and 6: they also show maxima near , with the highest
increasing with in this range. We argue that, overall, the observed
behavior results from both the breakdown of CDW-superconductivity degeneracy
and the fact that free sites tend to "push" electrons towards attractive sites,
the latter effect being more drastic at weak couplings.Comment: 9 two-column pages, 14 figures, RevTe
Size and shape of Mott regions for fermionic atoms in a two-dimensional optical lattice
We investigate the harmonic-trap control of size and shape of Mott regions in
the Fermi Hubbard model on a square optical lattice. The use of Lanczos
diagonalization on clusters with twisted boundary conditions, followed by an
average over 50-80 samples, drastically reduce finite-size effects in some
ground state properties; calculations in the grand canonical ensemble together
with a local-density approximation (LDA) allow us to simulate the radial
density distribution. We have found that as the trap closes, the atomic cloud
goes from a metallic state, to a Mott core, and to a Mott ring; the coverage of
Mott atoms reaches a maximum at the core-ring transition. A `phase diagram' in
terms of an effective density and the on-site repulsion is proposed, as a guide
to maximize the Mott coverage. We also predict that the usual experimentally
accessible quantities, the global compressibility and the average double
occupancy (rather, its density derivative) display detectable signatures of the
core-ring transition. Some spin correlation functions are also calculated, and
predict the existence N\'eel ordering within Mott cores and rings.Comment: 5 pages, 6 figure
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