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

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

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

Reconstructing the quantum critical fan of strongly correlated systems via quantum correlations

Albeit occurring at zero temperature, quantum critical phenomena are known to have a huge impact on the finite-temperature phase diagram of strongly correlated systems -- an aspect which gives experimental access to their observation. In particular the existence of a gapless, zero-temperature quantum critical point is known theoretically to induce the existence of an extended region in parameter space -- the so-called quantum critical fan -- characterized by power-law temperature dependences of all observables, with exponents related to those of the quantum critical point. Identifying experimentally the quantum critical fan and its crossovers to the other regions (renormalized classical, quantum disordered) remains nonetheless a big challenge. Focusing on paradigmatic models of quantum phase transitions, here we show that quantum correlations - captured by the quantum variance of the order parameter (I. Fr\'erot and T. Roscilde, Phys. Rev. B {\bf 94}, 075121 (2016)) - exhibit the temperature scaling associated with the quantum critical regime over an extended parameter region, much broader than that revealed by ordinary correlations, and with well-defined crossovers to the other regimes. The link existing between the quantum variance and the dynamical order-parameter susceptibility paves the way to an experimental reconstruction of the quantum critical fan using \emph{e.g.} spectroscopy on strongly correlated quantum matter.Comment: 5 + 4 pages, 4 + 5 figures. See also the accompanying paper by Gabbrielli, Smerzi and Pezz\`