640 research outputs found
Interacting spin-1 bosons in a two-dimensional optical lattice
We study, using quantum Monte Carlo (QMC) simulations, the ground state
properties of spin-1 bosons trapped in a square optical lattice. The phase
diagram is characterized by the mobility of the particles (Mott insulating or
superfluid phase) and by their magnetic properties. For ferromagnetic on-site
interactions, the whole phase diagram is ferromagnetic and the Mott
insulators-superfluid phase transitions are second order. For antiferromagnetic
on-site interactions, spin nematic order is found in the odd Mott lobes and in
the superfluid phase. Furthermore, the superfluid-insulator phase transition is
first or second order depending on whether the density in the Mott is even or
odd. Inside the even Mott lobes, we observe a singlet-to-nematic transition for
certain values of the interactions. This transition appears to be first order
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
Two-photon Rabi-Hubbard and Jaynes-Cummings-Hubbard models: photon pair superradiance, Mott insulator and normal phases
We study the ground state phase diagrams of two-photon Dicke, the
one-dimensional Jaynes-Cummings-Hubbard (JCH), and Rabi-Hubbard (RH) models
using mean field, perturbation, quantum Monte Carlo (QMC), and density matrix
renormalization group (DMRG) methods. We first compare mean field predictions
for the phase diagram of the Dicke model with exact QMC results and find
excellent agreement. The phase diagram of the JCH model is then shown to
exhibit a single Mott insulator lobe with two excitons per site, a superfluid
(SF, superradiant) phase and a large region of instability where the
Hamiltonian becomes unbounded. Unlike the one-photon model, there are no higher
Mott lobes. Also unlike the one-photon case, the SF phases above and below the
Mott are surprisingly different: Below the Mott, the SF is that of photon {\it
pairs} as opposed to above the Mott where it is SF of simple photons. The mean
field phase diagram of the RH model predicts a transition from a normal to a
superradiant phase but none is found with QMC.Comment: 14 pages, 14 figure
Transport and magnetic properties in YBaCo2O5.45: Focus on the high-temperature transition
The electronic transport properties and the magnetic susceptibility were
measured in detail in . Close to the so-called metal-insulator
transition, strong effects of resistance relaxation, a clear thermal hysteresis
and a sudden increase of the resistance noise are observed. This is likely due
to the first order character of the transition and to the underlying phases
coexistence. Despite these out of equilibrium features, a positive and linear
magneto-resistance is also observed, possibly linked to the heterogeneity of
the state. From a magnetic point of view, the paramagnetic to ordered magnetic
state transition is observed using non linear susceptibilty. This transition
shows the characteristics of a continuous transition, and time dependent
effects can be linked with the dynamics of magnetic domains in presence of
disorder. Thus, when focusing on the order of the transitions, the electronic
one and the magnetic one can not be directly associated.Comment: accepted for publication in PR
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
Rhodium Doped Manganites : Ferromagnetism and Metallicity
The possibility to induce ferromagnetism and insulator to metal transitions
in small A site cation manganites Ln_{1-x}Ca_xMnO_3 by rhodium doping is shown
for the first time. Colossal magnetoresistance (CMR) properties are evidenced
for a large compositional range (0.35 \leq x < 0.60). The ability of rhodium to
induce such properties is compared to the results obtained by chromium and
ruthenium doping. Models are proposed to explain this behavior.Comment: 11 pages, 8 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
Field-Induced Magnetization Steps in Intermetallic Compounds and Manganese Oxides: The Martensitic Scenario
Field-induced magnetization jumps with similar characteristics are observed
at low temperature for the intermetallic germanide Gd5Ge4and the mixed-valent
manganite Pr0.6Ca0.4Mn0.96Ga0.04O3. We report that the field location -and even
the existence- of these jumps depends critically on the magnetic field sweep
rate used to record the data. It is proposed that, for both compounds, the
martensitic character of their antiferromagnetic-to-ferromagnetic transitions
is at the origin of the magnetization steps.Comment: 4 pages,4 figure
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