124 research outputs found
Field-induced quantum disordered phases in S=1/2 weakly-coupled dimer systems with site dilution
In the present paper we discuss the rich phase diagram of S=1/2 weakly
coupled dimer systems with site dilution as a function of an applied uniform
magnetic field. Performing quantum Monte Carlo simulations on a site-diluted
bilayer system, we find a sequence of three distinct quantum-disordered phases
induced by the field. Such phases divide a doping-induced order-by-disorder
phase at low fields from a field-induced ordered phase at intermediate fields.
The three quantum disordered phases are: a gapless disordered-free-moment
phase, a gapped plateau phase, and two gapless Bose-glass phases. Each of the
quantum-disordered phases have distinct experimental signatures that make them
observable through magnetometry and neutron scattering measurements. In
particular the Bose-glass phase is characterized by an unconventional
magnetization curve whose field-dependence is exponential. Making use of a
local-gap model, we directly relate this behavior to the statistics of rare
events in the system.Comment: 13 pages, 7 figure
Comment on "Feshbach-Einstein Condensates" by V. G. Rousseau and P. J. H. Denteneer
In a recent paper (Phys. Rev. Lett. 102, 015301 (2009), arXiv:0810.3763)
Rousseau and Denteneer claim that an unconventional "super-Mott" (SM) phase is
realized by bosons trapped in an optical lattice close to a Feshbach resonance
with a molecular state. The supposed SM phase, observed via quantum Monte Carlo
(QMC) simulations of an atom-molecule Bose-Hubbard model, is an incompressible
phase developing spontaneous atomic/molecular supercurrents which are perfectly
anticorrelated. Here we show that the identification of this phase is based on
a misinterpretation of the estimators of superfluidity in QMC, which break down
in the presence of coherent atom/molecule conversion. Our conclusion is that
the supposed SM phase is in fact a fully normal insulator.Comment: 1+epsilon page, 1 figur
Dirty-boson physics with magnetic insulators
We review recent theoretical and experimental efforts aimed at the
investigation of the physics of interacting disordered bosons (so-called dirty
bosons) in the context of quantum magnetism. The physics of dirty bosons is
relevant to a wide variety of condensed matter systems, encompassing Helium in
porous media, granular superconductors and ultracold atoms in disordered
optical potentials, to cite a few. Nevertheless, the understanding of the
transition from a localized, Bose-glass phase to an ordered, superfluid
condensate phase still represents a fundamentally open problem. Still to be
constructed is also a quantitative description of the highly inhomogeneous and
strongly correlated phases connected by the transition. We discuss how
disordered magnetic insulators in a strong magnetic field can provide a well
controlled realization of the above transition. Combining numerical simulations
with experiments on real materials can shed light on some fundamental
properties of the critical behavior, such as the scaling of the critical
temperature to condensation close to the quantum critical point
Probing correlated phases of bosons in optical lattices via trap squeezing
We theoretically analyze the response properties of ultracold bosons in
optical lattices to the static variation of the trapping potential. We show
that, upon an increase of such potential (trap squeezing), the density
variations in a central region, with linear size of >~ 10 wavelengths, reflect
that of the bulk system upon changing the chemical potential: hence measuring
the density variations gives direct access to the bulk compressibility. When
combined with standard time-of-flight measurements, this approach has the
potential of unambiguously detecting the appearence of the most fundamental
phases realized by bosons in optical lattices, with or without further external
potentials: superfluid, Mott insulator, band insulator and Bose glass.Comment: 4 pages, 4 figure
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