9,924 research outputs found

### Competing Glauber and Kawasaki Dynamics

Using a quantum formulation of the master equation we study a kinetic Ising
model with competing stochastic processes: the Glauber dynamics with
probability $p$ and the Kawasaki dynamics with probability $1 - p$. Introducing
explicitely the coupling to a heat bath and the mutual static interaction of
the spins the model can be traced back exactly to a Ginzburg Landau functional
when the interaction is of long range order. The dependence of the correlation
length on the temperature and on the probability $p$ is calculated. In case
that the spins are subject to flip processes the correlation length disappears
for each finite temperature. In the exchange dominated case the system is
strongly correlated for each temperature.Comment: 9 pages, Revte

### Non-equilibrium dynamics of the Bose-Hubbard model: A projection operator approach

We study the phase diagram and non-equilibrium dynamics, both subsequent to a
sudden quench of the hopping amplitude $J$ and during a ramp $J(t)=Jt/\tau$
with ramp time $\tau$, of the Bose-Hubbard model at zero temperature using a
projection operator formalism which allows us to incorporate the effects of
quantum fluctuations beyond mean-field approximations in the strong coupling
regime. Our formalism yields a phase diagram which provides a near exact match
with quantum Monte Carlo results in three dimensions. We also compute the
residual energy $Q$, the superfluid order parameter $\Delta(t)$, the equal-time
order parameter correlation function $C(t)$, and the wavefunction overlap $F$
which yields the defect formation probability $P$ during non-equilibrium
dynamics of the model. We find that $Q$, $F$, and $P$ do not exhibit the
expected universal scaling. We explain this absence of universality and show
that our results compare well with recent experiments.Comment: Replaced with the accepted version, added one figure. 4 pages, 4
figures, to appear in Phys. Rev. Let

### Broad Line Radio Galaxies: Jet Contribution to the nuclear X-Ray Continuum

It is shown that, for Broad Line Radio Galaxies the strength of the
non-thermal beamed radiation, when present, is always smaller than the
accretion flow by a factor < 0.7 in the 2-10 keV band. The result has been
obtained using the procedure adopted for disentangling the Flat Spectrum Radio
Quasar 3C 273 (Grandi & Palumbo 2004). Although this implies a significantly
smaller non-thermal flux in Radio Galaxies when compared to Blazars, the jet
component, if present, could be important at very high energies and thus easily
detectable with GLAST.Comment: 12 pages including 2 figures (4 files), ApJ accepte

### A projection operator approach to the Bose-Hubbard model

We develop a projection operator formalism for studying both the zero
temperature equilibrium phase diagram and the non-equilibrium dynamics of the
Bose-Hubbard model. Our work, which constitutes an extension of Phys. Rev.
Lett. {\bf 106}, 095702 (2011), shows that the method provides an accurate
description of the equilibrium zero temperature phase diagram of the
Bose-Hubbard model for several lattices in two- and three-dimensions (2D and
3D). We show that the accuracy of this method increases with the coordination
number $z_0$ of the lattice and reaches to within 0.5% of quantum Monte Carlo
data for lattices with $z_0=6$. We compute the excitation spectra of the bosons
using this method in the Mott and the superfluid phases and compare our results
with mean-field theory. We also show that the same method may be used to
analyze the non-equilibrium dynamics of the model both in the Mott phase and
near the superfluid-insulator quantum critical point where the hopping
amplitude $J$ and the on-site interaction $U$ satisfy $z_0J/U \ll 1$. In
particular, we study the non-equilibrium dynamics of the model both subsequent
to a sudden quench of the hopping amplitude $J$ and during a ramp from $J_i$ to
$J_f$ characterized by a ramp time $\tau$ and exponent $\alpha$: $J(t)=J_i
+(J_f-J_i) (t/\tau)^{\alpha}$. We compute the wavefunction overlap $F$, the
residual energy $Q$, the superfluid order parameter $\Delta(t)$, the equal-time
order parameter correlation function $C(t)$, and the defect formation
probability $P$ for the above-mentioned protocols and provide a comparison of
our results to their mean-field counterparts. We find that $Q$, $F$, and $P$ do
not exhibit the expected universal scaling. We explain this absence of
universality and show that our results for linear ramps compare well with the
recent experimental observations.Comment: v2; new references and new sections adde

### Phenomenological Renormalization Group Methods

Some renormalization group approaches have been proposed during the last few
years which are close in spirit to the Nightingale phenomenological procedure.
In essence, by exploiting the finite size scaling hypothesis, the approximate
critical behavior of the model on infinite lattice is obtained through the
exact computation of some thermal quantities of the model on finite clusters.
In this work some of these methods are reviewed, namely the mean field
renormalization group, the effective field renormalization group and the finite
size scaling renormalization group procedures. Although special emphasis is
given to the mean field renormalization group (since it has been, up to now,
much more applied an extended to study a wide variety of different systems) a
discussion of their potentialities and interrelations to other methods is also
addressed.Comment: Review Articl

### Topological properties of the bond-modulated honeycomb lattice

We study the combined effects of lattice deformation, e-e interaction and
spin-orbit coupling in a two-dimensional (2D) honeycomb lattice. We adopt
different kinds of hopping modulation--generalized dimerization and a Kekule
distortion--and calculate topological invariants for the non-interacting system
and for the interacting system. We identify the parameter range (Hubbard U,
hopping modulation, spin-orbit coupling) where the 2D system behaves as a
trivial insulator or Quantum Spin Hall Insulator.Comment: 8 pages, 4 figures: discussion improved, typos corrected, references
updated. Matches version published in PR

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