450 research outputs found
Superfluid-Superfluid Phase Transitions in Two-Component Bose System
Depending on the Hamiltonian parameters, two-component bosons in an optical
lattice can form at least three different superfluid phases in which both
components participate in the superflow: a (strongly interacting) mixture of
two miscible superfluids (2SF), a paired superfluid vacuum (PSF), and (at a
commensurate total filling factor) the super-counter-fluid state (SCF). We
study universal properties of the 2SF-PSF and 2SF-SCF quantum phase transitions
and show that (i) they can be mapped onto each other, and (ii) their
universality class is identical to the (d+1)-dimensional normal-superfluid
transition in a single-component liquid. Finite-temperature 2SF-PSF(SCF)
transitions and the topological properties of 2SF-PSF(SCF) interfaces are also
discussed.Comment: 4pages, 2 figures, REVTe
Revealing Superfluid--Mott-Insulator Transition in an Optical Lattice
We study (by an exact numerical scheme) the single-particle density matrix of
ultracold atoms in an optical lattice with a parabolic confining
potential. Our simulation is directly relevant to the interpretation and
further development of the recent pioneering experiment by Greiner et al. In
particular, we show that restructuring of the spatial distribution of the
superfluid component when a domain of Mott-insulator phase appears in the
system, results in a fine structure of the particle momentum distribution. This
feature may be used to locate the point of the superfluid--Mott-insulator
transition.Comment: 4 pages (12 figures), Latex. (A Latex macro is corrected
Diagrammatic Monte Carlo
Diagrammatic Monte Carlo (DiagMC) is a numeric technique that allows one to
calculate quantities specified in terms of diagrammatic expansions, the latter
being a standard tool of many-body quantum statistics. The sign problem that is
typically fatal to Monte Carlo approaches, appears to be manageable with
DiagMC. Starting with a general introduction to the principles of DiagMC, we
present a detailed description of the DiagMC scheme for interacting fermions
(Hubbard model), as well as the first illustrative results for the equations of
state.Comment: 15 pages, 7 figures. To appear in: Computer Simulation Studies in
Condensed Matter Physics XXI, Eds. D.P. Landau, S.P. Lewis, and H.B.
Schuttler (Springer Verlag, Heidelberg, Berlin 2008
Supersolid phase of hardcore bosons on triangular lattice
We establish the nature of the supersolid phase observed for hardcore bosons
on the triangular lattice near half-integer filling factor, and study the phase
diagram of the system at finite temperature. We find that the solid order is
always of the (2m,-m',-m') with m changing discontinuously from positive to
negative values at half-filling, contrary to predictions of other phases, based
on an analogy with the properties of Ising spins in transverse magnetic field.
At finite temperature we find two intersecting second-order transition lines,
one in the 3-state Potts universality class and the other of the
Kosterlitz-Thouless type
The Stochastic Green Function (SGF) algorithm
We present the Stochastic Green Function (SGF) algorithm designed for bosons
on lattices. This new quantum Monte Carlo algorithm is independent of the
dimension of the system, works in continuous imaginary time, and is exact (no
error beyond statistical errors). Hamiltonians with several species of bosons
(and one-dimensional Bose-Fermi Hamiltonians) can be easily simulated. Some
important features of the algorithm are that it works in the canonical ensemble
and gives access to n-body Green functions.Comment: 12 pages, 5 figure
Superglass Phase of Helium-four
We study different solid phases of Helium-four, by means of Path Integral
Monte Carlo simulations based on a recently developed "worm" algorithm. Our
study includes simulations that start off from a high-T gas phase, which is
then "quenched" down to T=0.2 K. The low-T properties of the system crucially
depend on the initial state. While an ideal hcp crystal is a clear-cut
insulator, the disordered system freezes into a "superglass", i.e., a
metastable amorphous solid featuring off-diagonal long-range order and
superfluidity
Linear and nonlinear susceptibilities of a decoherent two-level system
The linear and nonlinear dynamical susceptibilities of a two level system are
calculated as it undergoes a transition to a decoherent state. Analogously to
the Glover-Tinkham-Ferrell sum rule of superconductivity, spectral weight in
the linear susceptibility is continuously transferred from a finite frequency
resonance to nearly zero frequency, corresponding to a broken symmetry in the
thermodynamic limit. For this reason, the behavior of the present model (the
Mermin model) differs significantly from the spin-boson model. The third order
nonlinear susceptibility, corresponding to two-photon absorption, has an
unexpected non-monotonic behavior as a function of the environmental coupling,
reaching a maximum within the decoherent phase of the model. Both linear and
nonlinear susceptibilities may be expressed in a universal form.Comment: 10 pages, 9 figure
Suppression of Quantum Phase Interference in Molecular Magnets Fe₈ with Dipolar-Dipolar Interaction
Renormalized tunnel splitting with a finite distribution in the biaxial spin
model for molecular magnets is obtained by taking into account the dipolar
interaction of enviromental spins. Oscillation of the resonant tunnel splitting
with a transverse magnetic field along the hard axis is smeared by the finite
distribution which subsequently affects the quantum steps of hysteresis curve
evaluated in terms of the modified Landau-Zener model of spin flipping induced
by the sweeping field. We conclude that the dipolar-dipolar interaction drives
decoherence of quantum tunnelling in molcular magnets Fe₈, which explains
why the quenching points of tunnel spliting between odd and even resonant
tunnelling predcited theoretically were not observed experimentally.Comment: 5 pages including 3 figure and 1 table. To appear in Physical Review
Commensurate Two-Component Bosons in Optical Lattice: Groundstate Phase Diagram
Two sorts of bosons in an optical lattice at commensurate filling factors can
form five stable superfluid and insulating groundstates with rich and
non-trivial phase diagram. The structure of the groundstate diagram is
established by mapping -dimensional quantum system onto a
-dimensional classical loop-current model and Monte Carlo simulations of
the latter. Surprisingly, the quantum phase diagram features, besides
second-order lines, a first-order transition and two multi-critical points. We
explain why first-order transitions are generic for models with paring
interactions using microscopic and mean-field arguments.Comment: 4 RevTex pages, 3 ps-figures; replaced with revised version accepted
by PRL: results of the MC simulations in 4D are briefly discusse
'Hole-digging' in ensembles of tunneling Molecular Magnets
The nuclear spin-mediated quantum relaxation of ensembles of tunneling
magnetic molecules causes a 'hole' to appear in the distribution of internal
fields in the system. The form of this hole, and its time evolution, are
studied using Monte Carlo simulations. It is shown that the line-shape of the
tunneling hole in a weakly polarised sample must have a Lorentzian lineshape-
the short-time half-width in all experiments done so far should be
, the half-width of the nuclear spin multiplet. After a time
, the single molecule tunneling relaxation time, the hole width begins
to increase rapidly. In initially polarised samples the disintegration of
resonant tunneling surfaces is found to be very fast.Comment: 4 pages, 5 figure
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