177 research outputs found
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
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
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
Critical entropies for magnetic ordering in bosonic mixtures on a lattice
We perform a numeric study (worm algorithm Monte Carlo simulations) of
ultracold two-component bosons in two- and three-dimensional optical lattices.
At strong enough interactions and low enough temperatures the system features
magnetic ordering. We compute critical temperatures and entropies for the
disappearance of the Ising antiferromagnetic and the xy-ferromagnetic order and
find that the largest possible entropies per particle are ~0.5kB. We also
estimate (optimistically) the experimental hold times required to reach
equilibrium magnetic states to be on a scale of seconds. Low critical entropies
and long hold times render the experimental observations of magnetic phases
challenging and call for increased control over heating sources.Comment: 6 pages, 6 figure
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
Superfluid-Insulator and Roughening Transitions in Domain Walls
We have performed quantum Monte Carlo simulations to investigate the
superfluid behavior of one- and two-dimensional interfaces separating
checkerboard solid domains. The system is described by the hard-core
Bose-Hubbard Hamiltonian with nearest-neighbor interaction. In accordance with
Ref.1, we find that (i) the interface remains superfluid in a wide range of
interaction strength before it undergoes a superfluid-insulator transition;
(ii) in one dimension, the transition is of the Kosterlitz-Thouless type and is
accompanied by the roughening transition, driven by proliferation of charge 1/2
quasiparticles; (iii) in two dimensions, the transition belongs to the 3D U(1)
universality class and the interface remains smooth. Similar phenomena are
expected for domain walls in quantum antiferromagnets.Comment: 6 pages, 7 figures; references added, typo corrected in fig
Superconducting transition temperature of the Bose one-component plasma
We present results of first principle numerical simulations of the Bose
one-component plasma, i.e., a Bose gas with pairwise Coulomb interactions among
particles and a uniform neutralizing background. We compute the superconducting
transition temperature for a wide range of densities, in two and three
dimensions, for both continuous and lattice versions of the model. Our results
are of direct relevance to quantitative studies of bipolaron mechanisms of
(high-temperature) superconductivity.Comment: 5 pages, 5 figure
Quantum magnetism and counterflow supersolidity of up-down bosonic dipoles
We study a gas of dipolar Bosons confined in a two-dimensional optical
lattice. Dipoles are considered to point freely in both up and down directions
perpendicular to the lattice plane. This results in a nearest neighbor
repulsive (attractive) interaction for aligned (anti-aligned) dipoles. We find
regions of parameters where the ground state of the system exhibits insulating
phases with ferromagnetic or anti-ferromagnetic ordering, as well as with
rational values of the average magnetization. Evidence for the existence of a
novel counterflow supersolid quantum phase is also presented.Comment: 8 pages, 6 figure
Multiworm algorithm quantum Monte Carlo
We review the path-integral quantum Monte Carlo method and discuss its
implementation by multiworm algorithms. We analyze in details the features of
the algorithms, and focus our attention on the computation of the -body
density matrix to study N-body correlations. Finally, we demonstrate the
validity of the algorithms on a system of dipolar bosons trapped in a stack of
one-dimensional layers in the case of zero and finite inter-layer hopping.Comment: 20 pages, 10 figure
Novel Mechanism of Supersolid of Ultracold Polar Molecules in Optical Lattices
We study the checkerboard supersolid of the hard-core Bose-Hubbard model with
the dipole-dipole interaction. This supersolid is different from all other
supersolids found in lattice models in the sense that superflow paths through
which interstitials or vacancies can hop freely are absent in the crystal. By
focusing on repulsive interactions between interstitials, we reveal that the
long-range tail of the dipole-dipole interaction have the role of increasing
the energy cost of domain wall formations. This effect produces the supersolid
by the second-order hopping process of defects. We also perform exact quantum
Monte Carlo simulations and observe a novel double peak structure in the
momentum distribution of bosons, which is a clear evidence for supersolid. This
can be measured by the time-of-flight experiment in optical lattice systems
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