1,259 research outputs found
Using off-diagonal confinement as a cooling method
In a recent letter [Phys. Rev. Lett. 104, 167201 (2010)] we proposed a new
confining method for ultracold atoms on optical lattices, based on off-diagonal
confinement (ODC). This method was shown to have distinct advantages over the
conventional diagonal confinement (DC) that makes use of a trapping potential,
including the existence of pure Mott phases and highly populated condensates.
In this paper we show that the ODC method can also lead to temperatures that
are smaller than with the conventional DC method, depending on the control
parameters. We determine these parameters using exact diagonalizations for the
hard-core case, then we extend our results to the soft-core case by performing
quantum Monte Carlo (QMC) simulations for both DC and ODC systems at fixed
temperatures, and analysing the corresponding entropies. We also propose a
method for measuring the entropy in QMC simulations.Comment: 6 pages, 6 figure
Complex phases in the doped two-species bosonic Hubbard Model
We study a two-dimensional bosonic Hubbard model with two hard-core species
away from half filling using Quantum Monte Carlo simulations. The model
includes a repulsive interspecies interaction and different nearest-neighbor
hopping terms for the two species. By varying the filling we find a total of
five distinct phases, including a normal liquid phase at higher temperature,
and four different phases at lower temperature. We find an
anti-ferromagnetically ordered Mott insulator and a region of coexistent
anti-ferromagnetic and superfluid phases near half filling. Further away from
half filling the phase diagram displays a superfluid phase and a novel phase
inside the superfluid region at even lower temperatures. In this novel phase
separated region, the heavy species has a Mott behavior with integer filling,
while the lighter species shows phase separated Mott and superfluid behaviors.Comment: 5 pages, 4 figure
Phase diagram of the Bose-Hubbard model on a ring-shaped lattice with tunable weak links
Motivated by recent experiments on toroidal Bose-Einstein condensates in
all-optical traps with tunable weak links, we study the one-dimensional
Bose-Hubbard model on a ring-shaped lattice with a small region of weak hopping
integrals using quantum Monte Carlo simulations. Besides the usual Mott
insulating and superfluid phases, we find a phase which is compressible but non
superfluid with a local Mott region. This `local Mott' phase extends in a large
region of the phase diagram. These results suggest that the insulating and
conducting phases can be tuned by a local parameter which may provide a new
insight to the design of atomtronic devices.Comment: 5 pages, 5 figure
Re-examining the electronic structure of germanium: A first-principle study
We report results from an efficient, robust, ab-initio method for
self-consistent calculations of electronic and structural properties of Ge. Our
non-relativistic calculations employed a generalized gradient approximation
(GGA) potential and the linear combination of atomic orbitals (LCAO) formalism.
The distinctive feature of our computations stem from the use of
Bagayoko-Zhao-Williams-Ekuma-Franklin (BZW-EF) method. Our results are in
agreement with experimental ones where the latter are available. In particular,
our theoretical, indirect band gap of 0.65 eV, at the experimental lattice
constant of 5.66 \AA{}, is in excellent agreement with experiment. Our
predicted, equilibrium lattice constant is 5.63 \AA{}, with a corresponding
indirect band gap of 0.65 eV and a bulk modulus of 80 GPa. We also calculated
the effective masses in various directions with respect to the point.Comment: 10 Pages, 3 Figures, and 1 tabl
The Bose-Hubbard model on a triangular lattice with diamond ring-exchange
Ring-exchange interactions have been proposed as a possible mechanism for a
Bose-liquid phase at zero temperature, a phase that is compressible with no
superfluidity. Using the Stochastic Green Function algorithm (SGF), we study
the effect of these interactions for bosons on a two-dimensional triangular
lattice. We show that the supersolid phase, that is known to exist in the
ground state for a wide range of densities, is rapidly destroyed as the
ring-exchange interactions are turned on. We establish the ground-state phase
diagram of the system, which is characterized by the absence of the expected
Bose-liquid phase.Comment: 6 pages, 10 figure
Efficient calculation of the antiferromagnetic phase diagram of the 3D Hubbard model
The Dynamical Cluster Approximation with Betts clusters is used to calculate
the antiferromagnetic phase diagram of the 3D Hubbard model at half filling.
Betts clusters are a set of periodic clusters which best reflect the properties
of the lattice in the thermodynamic limit and provide an optimal finite-size
scaling as a function of cluster size. Using a systematic finite-size scaling
as a function of cluster space-time dimensions, we calculate the
antiferromagnetic phase diagram. Our results are qualitatively consistent with
the results of Staudt et al. [Eur. Phys. J. B 17 411 (2000)], but require the
use of much smaller clusters: 48 compared to 1000
Heavy-quarks in the QGP: study of medium effects through euclidean propagators and spectral functions
The heavy-quark spectral function in a hot plasma is reconstructed from the
corresponding euclidean propagator. The latter is evaluated through a
path-integral simulation. A weak-coupling calculation is also performed,
allowing to interpret the qualitative behavior of the spectral function in
terms of quite general physical processes.Comment: 4 pages, 3 figures - To appear in the conference proceedings for
Quark Matter 2009, March 30 - April 4, Knoxville, Tennesse
Phase Stability in the Two dimensional Anisotropic Boson Hubbard Hamiltonian
The two dimensional square lattice hard-core boson Hubbard model with near
neighbor interactions has a `checkerboard' charge density wave insulating phase
at half-filling and sufficiently large intersite repulsion. When doped, rather
than forming a supersolid phase in which long range charge density wave
correlations coexist with a condensation of superfluid defects, the system
instead phase separates. However, it is known that there are other lattice
geometries and interaction patterns for which such coexistence takes place. In
this paper we explore the possibility that anisotropic hopping or anisotropic
near neighbor repulsion might similarly stabilize the square lattice
supersolid. By considering the charge density wave structure factor and
superfluid density for different ratios of interaction strength and
hybridization in the and directions, we conclude that phase
separation still occurs.Comment: 8 pages, 11 figure
Local Density of the Bose Glass Phase
We study the Bose-Hubbard model in the presence of on-site disorder in the
canonical ensemble and conclude that the local density of the Bose glass phase
behaves differently at incommensurate filling than it does at commensurate one.
Scaling of the superfluid density at incommensurate filling of and
on-site interaction predicts a superfluid-Bose glass transition at
disorder strength of . At this filling the local density
distribution shows skew behavior with increasing disorder strength.
Multifractal analysis also suggests a multifractal behavior resembling that of
the Anderson localization. Percolation analysis points to a phase transition of
percolating non-integer filled sites around the same value of disorder. Our
findings support the scenario of percolating superfluid clusters enhancing
Anderson localization near the superfluid-Bose glass transition. On the other
hand, the behavior of the commensurate filled system is rather different. Close
to the tip of the Mott lobe () we find a Mott insulator-Bose
glass transition at disorder strength of . An analysis of
the local density distribution shows Gaussian like behavior for a wide range of
disorders above and below the transition.Comment: 12 pages, 14 figure
Physics of cuprates with the two-band Hubbard model - The validity of the one-band Hubbard model
We calculate the properties of the two-band Hubbard model using the Dynamical
Cluster Approximation. The phase diagram resembles the generic phase diagram of
the cuprates, showing a strong asymmetry with respect to electron and hole
doped regimes, in agreement with experiment. Asymmetric features are also seen
in one-particle spectral functions and in the charge, spin and d-wave pairing
susceptibility functions. We address the possible reduction of the two-band
model to a low-energy single-band one, as it was suggested by Zhang and Rice.
Comparing the two-band Hubbard model properties with the single-band Hubbard
model ones, we have found similar low-energy physics provided that the
next-nearest-neighbor hopping term t' has a significant value (). The parameter t' is the main culprit for the electron-hole asymmetry.
However, a significant value of t' cannot be provided in a strict Zhang and
Rice picture where the extra holes added into the system bind to the existing
Cu holes forming local singlets. We notice that by considering approximate
singlet states, such as plaquette ones, reasonable values of t', which capture
qualitatively the physics of the two-band model can be obtained. We conclude
that a single-band t-t'-U Hubbard model captures the basic physics of the
cuprates concerning superconductivity, antiferromagnetism, pseudogap and
electron-hole asymmetry, but is not suitable for a quantitative analysis or to
describe physical properties involving energy scales larger than about 0.5 eV.Comment: 14 pages, 16 figure
- …