1,091 research outputs found
Competition between Phase Separation and Spin Density Wave or Charge Density Wave Order: Role of Long-Range Interactions
Recent studies of pairing and charge order in materials such as FeSe,
SrTiO, and 2H-NbSe have suggested that momentum dependence of the
electron-phonon coupling plays an important role in their properties. Initial
attempts to study Hamiltonians which either do not include or else truncate the
range of Coulomb repulsion have noted that the resulting spatial non-locality
of the electron-phonon interaction leads to a dominant tendency to phase
separation. Here we present Quantum Monte Carlo results for such models in
which we incorporate both on-site and intersite electron-electron interactions.
We show that these can stabilize phases in which the density is homogeneous and
determine the associated phase boundaries. As a consequence, the physics of
momentum dependent electron-phonon coupling can be determined outside of the
trivial phase separated regime.Comment: 9 pages, 7 figure
Mott insulators and correlated superfluids in ultracold Bose-Fermi mixtures
We study the effects of interaction between bosons and fermions in a
Bose-Fermi mixtures loaded in an optical lattice. We concentrate on the
destruction of a bosonic Mott phase driven by repulsive interaction between
bosons and fermions. Once the Mott phase is destroyed, the system enters a
superfluid phase where the movements of bosons and fermions are correlated. We
show that this phase has simultaneously correlations reminiscent of a
conventional superfluid and of a pseudo-spin density wave order
Metallic phase in the two-dimensional ionic Hubbard model
We investigate the phases of the ionic Hubbard model in a two-dimensional
square lattice using determinant quantum Monte Carlo (DQMC). At half-filling,
when the interaction strength or the staggered potential dominate we find Mott
and band insulators, respectively. When these two energies are of the same
order we find a metallic region. Charge and magnetic structure factors
demonstrate the presence of antiferromagnetism only in the Mott region,
although the externally imposed density modulation is present everywhere in the
phase diagram. Away from half-filling, other insulating phases are found.
Kinetic energy correlations do not give clear signals for the existence of a
bond-ordered phase
Supersolid phases in the one dimensional extended soft core Bosonic Hubbard model
We present results of Quantum Monte Carlo simulations for the soft core
extended bosonic Hubbard model in one dimension exhibiting the presence of
supersolid phases similar to those recently found in two dimensions. We find
that in one and two dimensions, the insulator-supersolid transition has dynamic
critical exponent z=2 whereas the first order insulator-superfluid transition
in two dimensions is replaced by a continuous transition with z=1 in one
dimension. We present evidence that this transition is in the
Kosterlitz-Thouless universality class and discuss the mechanism behind this
difference. The simultaneous presence of two types of quasi long range order
results in two soliton-like dips in the excitation spectrum.Comment: 4 pages, 5 figure
Superconducting Transitions in Flat Band Systems
The physics of strongly correlated quantum particles within a flat band was
originally explored as a route to itinerant ferromagnetism and, indeed, a
celebrated theorem by Lieb rigorously establishes that the ground state of the
repulsive Hubbard model on a bipartite lattice with unequal number of sites in
each sublattice must have nonzero spin S at half-filling. Recently, there has
been interest in Lieb geometries due to the possibility of novel topological
insulator, nematic, and Bose-Einstein condensed (BEC) phases. In this paper, we
extend the understanding of the attractive Hubbard model on the Lieb lattice by
using Determinant Quantum Monte Carlo to study real space charge and pair
correlation functions not addressed by the Lieb theorems
Thermal phase diagrams of columnar liquid crystals
In order to understand the possible sequence of transitions from the
disordered columnar phase to the helical phase in hexa(hexylthio)triphenylene
(HHTT), we study a three-dimensional planar model with octupolar interactions
inscribed on a triangular lattice of columns. We obtain thermal phase diagrams
using a mean-field approximation and Monte Carlo simulations. These two
approaches give similar results, namely, in the quasi one-dimensional regime,
as the temperature is lowered, the columns order with a linear polarization,
whereas helical phases develop at lower temperatures. The helicity patterns of
the helical phases are determined by the exact nature of the frustration in the
system, itself related to the octupolar nature of the molecules.Comment: 12 pages, 9 figures, ReVTe
Exotic phases of interacting p-band bosons
We study a model of interacting bosons that occupy the first excited p-band
states of a two-dimensional optical lattice. In contrast to the much studied
single band Bose-Hubbard Hamiltonian, this more complex model allows for
non-trivial superfluid phases associated with condensation at non-zero momentum
and staggered order of the orbital angular momentum in addition to the
superfluid-Mott insulator transition. More specifically, we observe staggered
orbital angular momentum order in the Mott phase at commensurate filling and
superfluidity at all densities. We also observe a transition between the
staggered angular momentum superfluid phase and a striped superfluid, with an
alternation of the phase of the superfluid along one direction. The transition
between these two phases was observed in a recent experiment, which is then
qualitatively well described by our model.Comment: 8 pages, 12 figure
Monte Carlo Simulations of an Extended Feynman-Kikuchi Model
We present Quantum Monte Carlo simulations of a generalization of the
Feynman-Kikuchi model which includes the possibility of vacancies and
interactions between the particles undergoing exchange. By measuring the
winding number (superfluid density) and density structure factor, we determine
the phase diagram, and show that it exhibits regions which possess both
superfluid and charge ordering.Comment: 10 pages, 15 figure
Exact Study of the 1D Boson Hubbard Model with a Superlattice Potential
We use Quantum Monte Carlo simulations and exact diagonalization to explore
the phase diagram of the Bose-Hubbard model with an additional superlattice
potential. We first analyze the properties of superfluid and insulating phases
present in the hard-core limit where an exact analytic treatment is possible
via the Jordan-Wigner transformation. The extension to finite on-site
interaction is achieved by means of quantum Monte Carlo simulations. We
determine insulator/superfluid phase diagrams as functions of the on-site
repulsive interaction, superlattice potential strength, and filling, finding
that insulators with fractional occupation numbers, which are present in the
hard-core case, extend deep into the soft-core region. Furthermore, at integer
fillings, we find that the competition between the on-site repulsion and the
superlattice potential can produce a phase transition between a Mott insulator
and a charge density wave insulator, with an intermediate superfluid phase. Our
results are relevant to the behavior of ultracold atoms in optical
superlattices which are beginning to be studied experimentally.Comment: 13 pages, 23 figure
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