A spin-wave (SW) approach for hard-core bosons is presented to treat the problem of two dimensional boson localization in a random potential.After a short review of the method to compute 1/S-corrected observables, the case of random on-site energy is discussed.Whereas the mean-field solution does not display a Bose glass (BG) phase, 1/S corrections do capture BG physics.In particular, the localization of SW excitations is discussed through the inverse participation ratio.
A spin-wave (SW) approach for hard-core bosons is presented to treat the problem of two dimensional boson localization in a random potential.After a short review of the method to compute 1/S-corrected observables, the case of random on-site energy is discussed.Whereas the mean-field solution does not display a Bose glass (BG) phase, 1/S corrections do capture BG physics.In particular, the localization of SW excitations is discussed through the inverse participation ratio.
A spin-wave approach of the zero temperature superfluid-insulator transition for two-dimensional hard-core bosons in a random potential μ=±W is developed. While at the classical level there is no intervening phase between the Bose-condensed superfluid (SF) and the gapped disordered insulator, the introduction of quantum fluctuations leads to a much richer physics. Upon increasing the disorder strength W, the Bose-condensed fraction disappears first, before the SF. Then a gapless Bose-glass phase emerges over a finite region until the insulator appears. Furthermore, in the strongly disordered SF regime, a mobility edge in the spin-wave excitation spectrum is found at a finite frequency Ω(c) decreasing with W, and presumably vanishing in the Bose-glass phase.
A mean-field theory of the localization transition for strongly interacting bosonic systems is developed. Localization is shown to be sensitive to the distribution of the random site energies. It occurs in the presence of a triangular distribution, but not a uniform one. The inverse participation ratio, the single site Green's function, the superfluid order parameter, and the corresponding susceptibility are calculated, and the appropriate exponents determined. All of these quantities indicate the presence of a new phase, which can be identified as the Bose glass.
Recent studies of delocalization-localization transitions in disordered quantum chains have highlighted the role of rare, chain-breaking events that favor localization, in particular for high-energy eigenstates related to many-body localization. In this context, we revisit the random-field XXZ spin-1/2 chain at zero temperature with ferromagnetic interactions, equivalent to interacting fermions or hard-core bosons in a random potential with attractive interactions. We argue that localization in this model can be characterized by chain-breaking events, which are probed by the extreme values of simple local observables, such as the on-site density or the local magnetization, that are readily accessible in both experiments and numerical simulations. Adopting a bosonic language, we study the disorder-induced Berezinskii-Kosterlitz-Thouless (BKT) quantum phase transition from superfluid (SF) to Bose glass (BG), and focus on the strong disorder regime where localization is driven by weak links. Based on high-precision density matrix renormalization group simulations, we numerically show that extreme local densities accurately capture the BKT transition, even for relatively short chains ranging from a few dozen to a hundred sites. We also discuss the SF-BG transition in the weak disorder regime, where finite-size effects pose greater challenges. Overall, our work seeks to establish a solid foundation for using extreme statistics of local observables, such as density, to probe delocalization-localization transitions in disordered quantum chains, both in the ground state and at high energy.
We consider a generalization of the hard-core disordered boson system with the inclusion of random boson hopping. The phase diagrams are obtained, and the effects of the random boson hopping on the zero-temperature quantum phase transitions are studied. The existence of Bose-glass and superfluid glass phases is also investigated. The behavior of both continuous and first-order transitions as well as the tricritical point is discussed as a function of the strength of random on-site energies. The possible Bose glass displayed by the system due to the presence of the disorder is examined. \textcopyright{} 1996 The American Physical Society.
We use quantum Monte Carlo simulations to study the effect of disorder, in the form of a disordered chemical potential, on the phase diagram of the hard-core Bosonic Hubbard model in two dimensions. We find numerical evidence that in two dimensions, no matter how weak the disorder, it will always destroy the long-range density wave order (checkerboard solid) present at half filling and strong nearest-neighbor repulsion and replace it with a bose glass phase. We study the properties of this glassy phase including the superfluid density, energy gaps, and the full Green's function. We also study the possibility of other localized phases at weak nearest-neighbor repulsion, i.e., Anderson localization. We find that such a phase does not truly exist: The disorder must exceed a threshold before the bosons (at weak nearest-neighbor repulsion) are localized. The phase diagram for hard-core bosons with disorder cannot be obtained easily from the soft-core phase diagram discussed in the literature.
Abstract In this work, we theoretically study hard-core bosons on a two-dimensional square optical superlattice at T = 0. First of all, we present the mean field phase diagram of this model in terms of the chemical potential μ and the alternating potential strength Δ. Besides a superfluid (SF) phase at Δ = 0 and a charge density wave (CDW) phase in the large Δ at half filling, we demonstrate that a supersolid (SS) phase emerges in the moderate Δ. Then, we focus on the μ = 0, e.g., half filling case, using large-S semiclassical spin-wave approximation to study the SS to CDW quantum phase transition. In particular, we calculate the ground-state energy and the superfluid density at the level of 1/S correction. We then compare the spin-wave results with the large scale quantum Monte Carlo (QMC) simulations using the cluster stochastic series expansion (CSSE) algorithm, and find that while the spin wave method is intuitive with clear physical pictures, the quantum critical point is quite different from that of numerical results which is believed to be accurate. We suggest that as simple as it is, this model still exhibits strong quantum fluctuations near the quantum critical point beyond the power of semiclassical spin-wave approach.