943 research outputs found
Solid phases and pairing in a mixture of polar molecules and atoms
We consider a mixture of hard core bosonic polar molecules, interacting via
repulsive dipole-dipole interaction, and one atomic bosonic species. The
mixture is confined on a two-dimensional square lattice and, at low enough
temperatures, can be described by the two-component Bose-Hubbard model. The
latter displays a extremely rich phase diagram including solid, superfluid,
supersolid phases. Here we mainly focus on the checkerboard molecular solid,
stabilized by the long range dipolar interaction, and study how the presence of
atoms affects its robustness both at zero and finite temperature. We find that,
due to atom-molecule interaction, solid phases can be stabilized at both,
(much) lower strengths of dipolar interaction and higher temperatures, than
when no atoms are present. As a byproduct, atoms also order in a solid phase
with same melting temperatures as for molecules. Finally, we find that for
large enough interaction between atoms and molecules a paired supersolid phase
can be stabilized.Comment: 5 pages, 4 figure
Analysis and resolution of the ground-state degeneracy of the two-component Bose-Hubbard model
We study the degeneracy of the ground-state energy of the two-component
Bose-Hubbard model and of the perturbative correction . We show that the
degeneracy properties of and are closely related to the connectivity
properties of the lattice. We determine general conditions under which is
nondegenerate. This analysis is then extended to investigate the degeneracy of
. In this case, in addition to the lattice structure, the degeneracy also
depends on the number of particles present in the system. After identifying the
cases in which is degenerate and observing that the standard (degenerate)
perturbation theory is not applicable, we develop a method to determine the
zeroth-order correction to the ground state by exploiting the symmetry
properties of the lattice. This method is used to implement the perturbative
approach to the two-component Bose-Hubbard model in the case of degenerate
and is expected to be a valid tool to perturbatively study the asymmetric
character of the Mott-insulator to superfluid transition between the particle
and hole side
Inter-species entanglement of Bose-Bose mixtures trapped in optical lattices
In the present work we discuss inter-species entanglement in Bose-Bose
mixtures trapped in optical lattices. This work is motivated by the observation
that, in the presence of a second component, the Mott-insulator lobe shifts
{\em{differently}} on the hole- and particle-side with respect to the Mott lobe
of the single species system (Phys. Rev. A 82, 021601, Laser Phys. 21, 1443).
We use perturbation theory, formulated in a Hilbert space decomposed by means
of lattice symmetries, in order to show that the nonuniform shift of the Mott
lobe is a consequence of an inter-species entanglement which differs in the
lowest excited states to remove and add a particle. Our results indicate that
inter-species entanglement in mixtures can provide a new perspective in
understanding quantum phase transitions. To validate our approach, we compare
our results from perturbation theory with quantum Monte Carlo simulations
Multi-analytical study of historical semiconductor pigments
This work is focused on the study of semiconductor-based pigments, which substituted traditional pigments in the second half of the 19th century. Syn- thetic semiconductor pigments may be chemically unstable due to the presence of many impurities unintentionally introduced during manufacturing. The aim of this work is to provide an insight on the application of X-ray Fluorescence (XRF) for the analysis of these painting materials, including both Cd- and Zn-based pigments. Three different approaches have been followed: the semi-quantitative analysis of samples with similar elemental composition, the complementary use of XRF and Raman spectroscopy for the analysis of elemental and molecular composition and the synchrotron-based XRF and XANES for the detection of impurities. The syn- ergetic combination of different techniques provides information useful for the defi- nition of specific markers for future analysis of paint-samples with implications for the conservation and treatment of late 19th and early 20th century paintings
Equilibrium phases of dipolar lattice bosons in the presence of random diagonal disorder
Ultracold gases offer an unprecedented opportunity to engineer disorder and
interactions in a controlled manner. In an effort to understand the interplay
between disorder, dipolar interaction and quantum degeneracy, we study
two-dimensional hard-core dipolar lattice bosons in the presence of on-site
bound disorder. Our results are based on large-scale path-integral quantum
Monte Carlo simulations by the Worm algorithm. We study the ground state phase
diagram at fixed half-integer filling factor for which the clean system is
either a superfluid at lower dipolar interaction strength or a checkerboard
solid at larger dipolar interaction strength. We find that, even for weak
dipolar interaction, superfluidity is destroyed in favor of a Bose glass at
relatively low disorder strength. Interestingly, in the presence of disorder,
superfluidity persists for values of dipolar interaction strength for which the
clean system is a checkerboard solid. At fixed disorder strength, as the
dipolar interaction is increased, superfluidity is destroyed in favor of a Bose
glass. As the interaction is further increased, the system eventually develops
extended checkerboard patterns in the density distribution. Due to the presence
of disorder, though, grain boundaries and defects, responsible for a finite
residual compressibility, are present in the density distribution. Finally, we
study the robustness of the superfluid phase against thermal fluctuations
Thermometry of bosonic mixtures in Optical Lattices via Demixing
Motivated by recent experiments and theoretical investigations on binary
mixtures, we investigate the miscible-immiscible transition at finite
temperature by means of Quantum Monte Carlo. Based on the observation that the
segregated phase is strongly affected by temperature, we propose to use the
degree of demixing for thermometry of a binary bosonic mixture trapped in an
optical lattice. We show that the proposed method is especially sensitive at
low temperatures, of the order of the tunnelling amplitude, and therefore is
particularly suitable in the regime where quantum magnetism is expected.Comment: 10 pages, 6 figures, Supplemental Materia
Phase diagram and thermodynamics of the three-dimensional Bose-Hubbard model
We report results of quantum Monte Carlo simulations of the Bose-Hubbard
model in three dimensions. Critical parameters for the
superfluid-to-Mott-insulator transition are determined with significantly
higher accuracy than it has been done in the past. In particular, the position
of the critical point at filling factor n=1 is found to be at (U/t)_c =
29.34(2), and the insulating gap Delta is measured with accuracy of a few
percent of the hopping amplitude t. We obtain the effective mass of particle
and hole excitations in the insulating state--with explicit demonstration of
the emerging particle-hole symmetry and relativistic dispersion law at the
transition tip--along with the sound velocity in the strongly correlated
superfluid phase. These parameters are the necessary ingredients to perform
analytic estimates of the low temperature (T << Delta) thermodynamics in
macroscopic samples. We present accurate thermodynamic curves, including these
for specific heat and entropy, for typical insulating (U/t=40) and superfluid
(t/U=0.0385) phases. Our data can serve as a basis for accurate experimental
thermometry, and a guide for appropriate initial conditions if one attempts to
use interacting bosons in quantum information processing.Comment: 11 pages, 13 figure
A topological signature of multipartite entanglement
In this manuscript, we present a proposal to relate topological structure of
worldline configurations to multipartite entanglement. Configurations result
from the path-integral formulation of the density matrix in the limit of zero
temperature. We consider hard-core bosons for which configurations, i.e.
collections of particle paths, can be seen as geometric braids with a certain
topological structure. We propose that properties of worldline configurations
may realize a comprehensive deciphering of multipartite entanglement. By means
of Monte Carlo calculations, we study checkerboard, stripe, valence-bond
solids, topologically ordered spin liquid, and superfluid phase.
We find that each ground-state is characterized by a certain `topological
spectrum' which can be used to differentiate among different ground-states.Comment: 6 pages, 3 figures and supplemental material. Version 2 focuses on
the discussion of topological signatures of multipartite entanglement in the
ground-state expansion, with added results. The study of phase transitions
and permutation cycles in version 1 has been moved to a new manuscript:
arXiv:1912.00080. Version 3 corrected some typos and reference
Mott Insulator to Superfluid transition in Bose-Bose mixtures in a two-dimensional lattice
We perform a numeric study (Worm algorithm Monte Carlo simulations) of
ultracold two-component bosons in two-dimensional optical lattices. We study
how the Mott insulator to superfluid transition is affected by the presence of
a second superfluid bosonic species. We find that, at fixed interspecies
interaction, the upper and lower boundaries of the Mott lobe are differently
modified. The lower boundary is strongly renormalized even for relatively low
filling factor of the second component and moderate (interspecies) interaction.
The upper boundary, instead, is affected only for large enough filling of the
second component. Whereas boundaries are renormalized we find evidence of
polaron-like excitations. Our results are of interest for current experimental
setups.Comment: 4 pages, 3 figures, accepted as PRA Rapid Communicatio
Quasi-molecular bosonic complexes -- a pathway to atomic analog of SQUID with controlled sensitivity
Recent experimental advances in realizing degenerate quantum dipolar gases in
optical lattices and the flexibility of experimental setups in attaining
various geometries offer the opportunity to explore exotic quantum many-body
phases stabilized by anisotropic, long-range dipolar interaction. Moreover, the
unprecedented control over the various physical properties of these systems,
ranging from the quantum statistics of the particles, to the inter-particle
interactions, allow one to engineer novel devices. In this paper, we consider
dipolar bosons trapped in a stack of one-dimensional optical lattice layers,
previously studied in [1]. Building on our prior results, we provide a
description of the quantum phases stabilized in this system which include
composite superfluids, solids, and supercounterfluids, most of which are found
to be threshold- less with respect to the dipolar interaction strength. We also
demonstrate the effect of enhanced sensitivity to rotations of a SQUID-type
device made of two composite superfluids trapped in a ring-shaped optical
lattice layer with weak links.Comment: Special Issue Articl
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