102 research outputs found
Mott transition in bosonic systems: Insights from the variational approach
We study the Mott transition occurring for bosonic Hubbard models in one,
two, and three spatial dimensions, by means of a variational wave function
benchmarked by Green's function Monte Carlo calculations. We show that a very
accurate variational wave function, constructed by applying a long-range
Jastrow factor to the non-interacting boson ground state, can describe the
superfluid-insulator transition in any dimensionality. Moreover, by mapping the
quantum averages over such a wave function into the the partition function of a
classical model, important insights into the insulating phase are uncovered.
Finally, the evidence in favor of anomalous scenarios for the Mott transition
in two dimensions are reported whenever additional long-range repulsive
interactions are added to the Hamiltonian.Comment: 12 pages and 19 figures. Related to arXiv:0705.268
Many-body calculation of the spatial extent of the wave-function of a non-magnetic impurity in a d-wave high-temperature superconductor using the t--J model
Scanning tunneling microscopy (STM) by providing images of the effects of
individual zinc-impurities in cuprate superconductors with unprecedented
atomic-resolution offers a stringent test to models of correlated fermions for
high-temperature superconductors. Using a t-J model supplemented by Variational
Monte Carlo many-body techniques, the spatial dependence of the hole density
and of the valence bond and superconducting pairing amplitudes around the
impurity are computed. A cross-shaped four-fold symmetric structure very
similar to the observed STM observation is found, giving strong credit to the
model.Comment: 4 figures, 1 table, 4 pages, important changes (not affecting
results
Stability of RVB hole stripes in high-temperature superconductors
Indications of density-wave states in underdoped cuprates, coming from recent
STM (scanning tunneling microscopy) and Hall-resistance measurements, have
raised new concerns whether stripes could be stabilized in the superconducting
phase of cuprate materials, even in the absence of antiferromagnetism. Here, we
investigate this issue using state-of-the-art quantum Monte Carlo calculations
of a model. In particular we consider the stability of unidirectional
hole domains in a modulated superconducting background, by taking into account
the effect of tetragonal-lattice distortions, next-nearest neighbor hopping and
long-range Coulomb repulsion.Comment: 8 pages, 9 figures. Enlarged versio
Superfluid to Mott-insulator transition in Bose-Hubbard models
We study the superfluid-insulator transition in Bose-Hubbard models in one-,
two-, and three-dimensional cubic lattices by means of a recently proposed
variational wave function. In one dimension, the variational results agree with
the expected Berezinskii-Kosterlitz-Thouless scenario of the interaction-driven
Mott transition. In two and three dimensions, we find evidences that, across
the transition,most of the spectral weight is concentrated at high energies,
suggestive of pre-formed Mott-Hubbard side-bands. This result is compatible
with the experimental data by Stoferle et al.
[Phys. Rev. Lett. 92, 130403 (2004)].Comment: 4 pages, 4 figures, based on cond-mat/061130
Unidirectional d-wave superconducting domains in the two-dimensional t-J model
Motivated by the recently observed pattern of unidirectional domains in
high-T_c superconductors [Y. Kohsaka et al., Science 315, 1380 (2007)], we
investigate the emergence of spontaneous modulations in the d-wave
superconducting resonating valence bond phase using the t-J model at x=1/8
doping. Half-filled charge domains separated by four lattice spacings are found
to form along one of the crystal axis leading to modulated superconductivity
with out-of-phase d-wave order parameters in neighboring domains. Both
renormalized mean-field theory and variational Monte Carlo calculations yield
that the energies of modulated and uniform phases are very close to each other.Comment: 4 pages, 5 figures, minor revisions, and added reference
Quantifying the interplay between environmental and social effects on aggregated-fish dynamics
Demonstrating and quantifying the respective roles of social interactions and
external stimuli governing fish dynamics is key to understanding fish spatial
distribution. If seminal studies have contributed to our understanding of fish
spatial organization in schools, little experimental information is available
on fish in their natural environment, where aggregations often occur in the
presence of spatial heterogeneities. Here, we applied novel modeling approaches
coupled to accurate acoustic tracking for studying the dynamics of a group of
gregarious fish in a heterogeneous environment. To this purpose, we
acoustically tracked with submeter resolution the positions of twelve small
pelagic fish (Selar crumenophthalmus) in the presence of an anchored floating
object, constituting a point of attraction for several fish species. We
constructed a field-based model for aggregated-fish dynamics, deriving
effective interactions for both social and external stimuli from experiments.
We tuned the model parameters that best fit the experimental data and
quantified the importance of social interactions in the aggregation, providing
an explanation for the spatial structure of fish aggregations found around
floating objects. Our results can be generalized to other gregarious species
and contexts as long as it is possible to observe the fine-scale movements of a
subset of individuals.Comment: 10 pages, 5 figures and 4 supplementary figure
- âŠ