747 research outputs found
Low temperature thermodynamics of charged bosons in a random potential and the specific heat of La_{2-x}Sr_{x}CuO_{4} below Tc
We propose a simple analytical form of the partition function for charged
bosons localised in a random potential and derive the consequent thermodynamics
below the superfluid transition temperature. In the low temperature limit, the
specific heat, C, depends on the localisation length exponent nu: C is linear
for nu1 we find C proportional to T^{1/nu}. This unusual
sub-linear temperature dependence of the specific heat has recently been
observed in La_{2-x}Sr_{x}CuO_{4} below Tc.Comment: Revtex, 6 pages, 4 postscript figure
D-wave Bose-Einstein condensation and the London penetration depth in superconducting cuprates
We show that bipolaron formation leads to a d-wave Bose-Einstein condensate
in cuprates. It is the bipolaron energy dispersion rather than a particular
pairing interaction which is responsible for the d-wave symmetry. The unusual
low-temperature dependence of the magnetic field penetration depth in cuprates
is explained by the localisation of bosons in the random potential. The
temperature dependence of the penetration depth is linear with positive or
negative slope depending on the random field profile.Comment: 4 pages (RevTeX), 4 figure
Disordered Hubbard Model with Attraction: Coupling Energy of Cooper Pairs in Small Clusters
We generalize the Cooper problem to the case of many interacting particles in
the vicinity of the Fermi level in the presence of disorder. On the basis of
this approach we study numerically the variation of the pair coupling energy in
small clusters as a function of disorder. We show that the Cooper pair energy
is strongly enhanced by disorder, which at the same time leads to the
localization of pairs.Comment: revtex, 5 pages, 6 figure
Critical Exponents for Three-Dimensional Superfluid--Bose-Glass Phase Transition
The critical phenomenon of the zero temperature superfluid--Bose-glass phase
transition for hard-core bosons on a three-dimensional disordered lattice is
studied using a quantum real-space renormalization-group method. The
correlation-length exponent and the dynamic exponent z are computed. The
critical exponent z is found to be 2.5 for compressible states and 1.3 for
incompressible states. The exponent is shown to be insensitive to z as
that in the two-dimensional case, and has value roughly equal to 1.Comment: 11 pages, REVTE
Disordered Boson Systems: A Perturbative Study
A hard-core disordered boson system is mapped onto a quantum spin 1/2
XY-model with transverse random fields. It is then generalized to a system of
spins with an arbitrary magnitude S and studied through a 1/S expansion. The
first order 1/S expansion corresponds to a spin-wave theory. The effect of weak
disorder is studied perturbatively within such a first order 1/S scheme. We
compute the reduction of the speed of sound and the life time of the Bloch
phonons in the regime of weak disorder. Generalizations of the present study to
the strong disordered regime are discussed.Comment: 27 pages, revte
Critical temperature for the two-dimensional attractive Hubbard Model
The critical temperature for the attractive Hubbard model on a square lattice
is determined from the analysis of two independent quantities, the helicity
modulus, , and the pairing correlation function, . These
quantities have been calculated through Quantum Monte Carlo simulations for
lattices up to , and for several densities, in the
intermediate-coupling regime. Imposing the universal-jump condition for an
accurately calculated , together with thorough finite-size scaling
analyses (in the spirit of the phenomenological renormalization group) of
, suggests that is considerably higher than hitherto assumed.Comment: 5 pages, 6 figures. Accepted for publication in Phys. Rev.
Magnetization process of the spin-1/2 XXZ models on square and cubic lattices
The magnetization process of the spin-1/2 antiferromagnetic XXZ model with
Ising-like anisotropy in the ground state is investigated. We show numerically
that the Ising-like XXZ models on square and cubic lattices show a first-order
phase transition at some critical magnetic field. We estimate the value of the
critical field and the magnetization jump on the basis of the Maxwell
construction. The magnetization jump in the Ising-limit is investigated by
means of perturbation theory. Based on our numerical results, we briefly
discuss the phase diagram of the extended Bose-Hubbard model in the hard-core
limit.Comment: 13 pages, RevTex, 7 PostScript figures, to appear in Phys.Rev.
Simultaneous Diagonal and Off Diagonal Order in the Bose--Hubbard Hamiltonian
The Bose-Hubbard model exhibits a rich phase diagram consisting both of
insulating regimes where diagonal long range (solid) order dominates as well as
conducting regimes where off diagonal long range order (superfluidity) is
present. In this paper we describe the results of Quantum Monte Carlo
calculations of the phase diagram, both for the hard and soft core cases, with
a particular focus on the possibility of simultaneous superfluid and solid
order. We also discuss the appearance of phase separation in the model. The
simulations are compared with analytic calculations of the phase diagram and
spin wave dispersion.Comment: 28 pages plus 24 figures, uuencoded Revtex+postscript file
Management recommendations for the northern goshawk in the southwestern United States
Present forest conditions--loss of a herbaceous and shrubby understory, reductions in the amount of older forests, and increased areas of dense tree regeneration--reflect the extent of human influence on these forests. These changes may also be affecting goshawk populations. Information on goshawk nesting habitat and foraging behavior, and the food and habitats of selected goshawk prey, was therefore synthesized to develop a set of management objectives, desired forest conditions, and management recommendations. Key objectives of the guidelines are to provide (1) nesting, post-fledging, and foraging areas for goshawks, and (2) habitat to support abundant populations of 14 primary goshawk prey. Thinning trees in the understory, creating small openings in the forest, and prescribed fires should help produce and maintain the desired forest conditions. Other habitat elements critical for maintaining both goshawk and prey populations include abundant snags and large downed logs, woody debris, interspersion of different tree sizes across the landscape, and the majority of a goshawk's home range in older-aged forests. These guidelines should also benefit forest health, soil productivity, and the habitats of other old-growth dependent plants and animals
Phase Transitions in One-Dimensional Truncated Bosonic Hubbard Model and Its Spin-1 Analog
We study one-dimensional truncated (no more than 2 particles on a site)
bosonic Hubbard model in both repulsive and attractive regimes by exact
diagonalization and exact worldline Monte Carlo simulation. In the commensurate
case (one particle per site) we demonstrate that the point of Mott-insulator --
superfluid transition, , is remarkably far from that of
the full model. In the attractive region we observe the phase transition from
one-particle superfluid to two-particle one. The paring gap demonstrates a
linear behavior in the vicinity of the critical point. The critical state
features marginal response to the gauge phase. We argue that the two-particle
superfluid is a macroscopic analog of a peculiar phase observed earlier in a
spin-1 model with axial anisotropy.Comment: Revtex, 5 pages, 9 figures. Submitted to Phys. Rev.
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