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-$\frac{1}{2}$ chains. We investigate the quantum/thermal correspondence between the reduced density matrix of a $T=0$ 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