778 research outputs found

### Phase Diagram for Ultracold Bosons in Optical Lattices and Superlattices

We present an analytic description of the finite-temperature phase diagram of
the Bose-Hubbard model, successfully describing the physics of cold bosonic
atoms trapped in optical lattices and superlattices. Based on a standard
statistical mechanics approach, we provide the exact expression for the
boundary between the superfluid and the normal fluid by solving the
self-consistency equations involved in the mean-field approximation to the
Bose-Hubbard model. The zero-temperature limit of such result supplies an
analytic expression for the Mott lobes of superlattices, characterized by a
critical fractional filling.Comment: 8 pages, 6 figures, submitted to Phys. Rev.

### Influence of copper on the electronic properties of amorphous chalcogenides

We have studied the influence of alloying copper with amorphous arsenic
sulfide on the electronic properties of this material. In our
computer-generated models, copper is found in two-fold near-linear and
four-fold square-planar configurations, which apparently correspond to Cu(I)
and Cu(II) oxidation states. The number of overcoordinated atoms, both arsenic
and sulfur, grows with increasing concentration of copper. Overcoordinated
sulfur is found in trigonal planar configuration, and overcoordinated
(four-fold) arsenic is in tetrahedral configuration. Addition of copper
suppresses the localization of lone-pair electrons on chalcogen atoms, and
localized states at the top of the valence band are due to Cu 3d orbitals.
Evidently, these additional Cu states, which are positioned at the same
energies as the states due to ([As4]-)-([S_3]+) pairs, are responsible for
masking photodarkening in Cu chalcogenides

### Heisenberg antiferromagnet on the square lattice for S>=1

Theoretical predictions of a semiclassical method - the pure-quantum
self-consistent harmonic approximation - for the correlation length and
staggered susceptibility of the Heisenberg antiferromagnet on the square
lattice (2DQHAF) agree very well with recent quantum Monte Carlo data for S=1,
as well as with experimental data for the S=5/2 compounds Rb2MnF4 and KFeF4.
The theory is parameter-free and can be used to estimate the exchange coupling:
for KFeF4 we find J=2.33 +- 0.33 meV, matching with previous determinations. On
this basis, the adequacy of the quantum nonlinear sigma model approach in
describing the 2DQHAF when S>=1 is discussed.Comment: 4 pages RevTeX file with 5 figures included by psfi

### Finite temperature strong-coupling expansions for the Kondo lattice model

Strong-coupling expansions, to order $(t/J)^8$, are derived for the Kondo
lattice model of strongly correlated electrons, in 1-, 2- and 3- dimensions at
arbitrary temperature. Results are presented for the specific heat, and spin
and charge susceptibilities.Comment: revtex

### Spin correlations in an isotropic spin-5/2 two-dimensional antiferromagnet

We report a neutron scattering study of the spin correlations for the spin
5/2, two-dimensional antiferromagnet Rb_2MnF_4 in an external magnetic field.
Choosing fields near the system's bicritical point, we tune the effective
anisotropy in the spin interaction to zero, constructing an ideal S=5/2
Heisenberg system. The correlation length and structure factor amplitude are
closely described by the semiclassical theory of Cuccoli et al. over a broad
temperature range but show no indication of approaching the low-temperature
renormalized classical regime of the quantum non-linear sigma model.Comment: 4 pages, 3 EPS figure

### Quantum phase transitions in the Triangular-lattice Bilayer Heisenberg Model

We study the triangular lattice bilayer Heisenberg model with
antiferromagnetic interplane coupling $J_\perp$ and nearest neighbour
intraplane coupling $J= \lambda J_\perp$, which can be ferro- or
antiferromagnetic, by expansions in $\lambda$. For negative $\lambda$ a phase
transition is found to an ordered phase at a critical $\lambda_c=-0.2636 \pm
0.0001$ which is in the 3D classical Heisenberg universality class. For
$\lambda>0$, we find a transition at a rather large $\lambda_c\approx 1.2$. The
universality class of the transition is consistent with that of Kawamura's 3D
antiferromagnetic stacked triangular lattice. The spectral weight for the
triplet excitations, at the ordering wavevector, remains finite at the
transition, suggesting that a phase with free spinons does not exist in this
model.Comment: revtex, 4 pages, 3 figure

### Ground State and Elementary Excitations of the S=1 Kagome Heisenberg Antiferromagnet

Low energy spectrum of the S=1 kagom\'e Heisenberg antiferromagnet (KHAF) is
studied by means of exact diagonalization and the cluster expansion. The
magnitude of the energy gap of the magnetic excitation is consistent with the
recent experimental observation for \mpynn. In contrast to the $S=1/2$ KHAF,
the non-magnetic excitations have finite energy gap comparable to the magnetic
excitation. As a physical picture of the ground state, the hexagon singlet
solid state is proposed and verified by variational analysis.Comment: 5 pages, 7 eps figures, 2 tables, Fig. 4 correcte

### Various series expansions for a Heisenberg antiferromagnet model for SrCu$_2$(BO$_3$)$_2$

We use a variety of series expansion methods at both zero and finite
temperature to study an antiferromagnetic Heisenberg spin model proposed
recently by Miyahara and Ueda for the quasi two-dimensional material
SrCu$_2$(BO$_3$)$_2$. We confirm that this model exhibits a first-order quantum
phase transition at T=0 between a gapped dimer phase and a gapless N\'eel phase
when the ratio $x=J'/J$ of nearest and next-nearest neighbour interactions is
varied, and locate the transition at $x_c=0.691(6)$. Using longer series we are
able to give more accurate estimates of the model parameters by fitting to the
high temperature susceptibility data.Comment: RevTeX, 13 figure

### Spin-1/2 Heisenberg-Antiferromagnet on the Kagome Lattice: High Temperature Expansion and Exact Diagonalisation Studies

For the spin-$\frac{1}{2}$ Heisenberg antiferromagnet on the Kagom\'e lattice
we calculate the high temperature series for the specific heat and the
structure factor. A comparison of the series with exact diagonalisation studies
shows that the specific heat has further structure at lower temperature in
addition to a high temperature peak at $T\approx 2/3$. At $T=0.25$ the
structure factor agrees quite well with results for the ground state of a
finite cluster with 36 sites. At this temperature the structure factor is less
than two times its $T=\infty$ value and depends only weakly on the wavevector
$\bf q$, indicating the absence of magnetic order and a correlation length of
less than one lattice spacing. The uniform susceptibility has a maximum at
$T\approx 1/6$ and vanishes exponentially for lower temperatures.Comment: 15 pages + 5 figures, revtex, 26.04.9

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