124 research outputs found
Sliding phase in randomly stacked 2D superfluids/superconductors
Using large scale quantum Monte Carlo simulations of lattice bosonic models,
we precisely investigate the effect of weak Josephson tunneling between 2D
superfluid or superconducting layers. In the clean case, the
Kosterlitz-Thouless transition immediately turns into 3DXY, with phase
coherence and superflow in all spatial directions, and a strong enhancement of
the critical temperature. However, when disorder is present, rare regions
fluctuations can lead to an intermediate finite temperature phase --- the so
called sliding regime --- where only 2D superflow occurs within the layers
without any transverse superfluid coherence, while a true 3D Bose-Einstein
condensate exists. Critical properties of such an unconventional regime are
carefully investigated.Comment: 6 pages, 7 figures, final version (EPL
Spin-resolved entanglement spectroscopy of critical spin chains and Luttinger liquids
Quantum critical chains are well described and understood by virtue of
conformal field theory. Still the meaning of the real space entanglement
spectrum -- the eigenvalues of the reduced density matrix -- of such systems
remains in general elusive, even when there is an additional quantum number
available such as spin or particle number. In this paper we explore in details
the properties and the structure of the reduced density matrix of critical XXZ
spin- chains. We investigate the quantum/thermal correspondence
between the reduced density matrix of a pure quantum state and the
thermal density matrix of an effective entanglement Hamiltonian. Using large
scale DMRG and QMC simulations, we investigate the conformal structure of the
spectra, the entanglement Hamiltonian and temperature. We then introduce the
notion of spin-resolved entanglement entropies which display interesting
scaling features.Comment: 19 pages, 11 figure
Theory of the field-induced BEC in the frustrated spin-1/2 dimer compound BaCuSi2O6
Building on recent neutron and NMR experiments, we investigate the
field-induced exotic criticality observed in the frustrated spin-1/2 dimer
compound BaCuSi2O6 using a frustrated model with two types of bilayers. A
semiclassical treatment of the effective hard-core boson model shows that
perfect inter-layer frustration leads to a 2D-like critical exponent phi=1
without logarithmic corrections and to a 3D low temperature phase with
different but non vanishing triplet populations in both types of bilayers.
These results further suggest a simple phenomenology in terms of a
field-dependent transverse coupling in the context of which we reproduce the
entire field-temperature phase diagram with Quantum Monte Carlo simulations
Many-body localization: an introduction and selected topics
What happens in an isolated quantum system when both disorder and
interactions are present? Over the recent years, the picture of a
non-thermalizing phase of matter, the many-localized phase, has emerged as a
stable solution. We present a basic introduction to the topic of many-body
localization, using the simple example of a quantum spin chain which allows us
to illustrate several of the properties of this phase. We then briefly review
the current experimental research efforts probing this physics. The largest
part of this review is a selection of more specialized questions, some of which
are currently under active investigation. We conclude by summarizing the
connections between many-body localization and quantum simulations.Comment: Review article. 28 pages, 8 figures, Comptes Rendus Physique (2018
Many-body localization in a quasiperiodic Fibonacci chain
We study the many-body localization (MBL) properties of a chain of
interacting fermions subject to a quasiperiodic potential such that the
non-interacting chain is always delocalized and displays multifractality.
Contrary to naive expectations, adding interactions in this systems does not
enhance delocalization, and a MBL transition is observed. Due to the local
properties of the quasiperiodic potential, the MBL phase presents specific
features, such as additional peaks in the density distribution. We furthermore
investigate the fate of multifractality in the ergodic phase for low potential
values. Our analysis is based on exact numerical studies of eigenstates and
dynamical properties after a quench
Quantum and thermal transitions out of the supersolid phase of a 2D quantum antiferromagnet
We investigate the thermodynamic properties of a field-induced supersolid
phase in a 2D quantum antiferromagnet model. Using quantum Monte Carlo
simulations, a very rich phase diagram is mapped out in the temperature -
magnetic field plane, with an extended supersolid region where a diagonal
(solid) order coexists with a finite XY spin stiffness (superfluid). The
various quantum and thermal transitions out of the supersolid state are
characterized. Experimental consequences in the context of field-induced
magnetization plateau materials are briefly discussed.Comment: To appear in Phys. Rev. Let
Quantum spin glass and the dipolar interaction
Systems in which the dipolar energy dominates the magnetic interaction, and
the crystal field generates strong anisotropy favoring the longitudinal
interaction terms, are considered. Such systems in external magnetic field are
expected to be a good experimental realization of the transverse field Ising
model. With random interactions this model yields a spin glass to paramagnet
phase transition as function of the transverse field. Here we show that the
off-diagonal dipolar interaction, although effectively reduced, destroys the
spin glass order at any finite transverse field. Moreover, the resulting
correlation length is shown to be small near the crossover to the paramagnetic
phase, in agreement with the behavior of the nonlinear susceptibility in the
experiments on \LHx. Thus, we argue that the in these experiments a
cross-over to the paramagnetic phase, and not quantum criticality, was
observed.Comment: To appear in Phys. Rev. Let
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