Superglass Phase of Interacting Bosons

Abstract

We introduce a Bose-Hubbard Hamiltonian with random disordered interactions as a model to study the interplay of superfluidity and glassiness in a system of three-dimensional hard-core bosons at half-filling. Solving the model using large-scale quantum Monte Carlo simulations, we show that these disordered interactions promote a stable superglass phase, where superflow and glassy density localization coexist in equilibrium without exhibiting phase separation. The robustness of the superglass phase is underlined by its existence in a replica mean-field calculation on the infinite-dimensional Hamiltonian.Despite the simplicity of its constituent atoms, the phase diagram of bulk helium has proven to be compelling and controversial -especially in light of recent observations of supersolid behavior in the crystal phase of Helium-4 ( 4 He) [1,2]. Several experiments have shown that superflow is enhanced by imperfections in the crystal lattice, such as roaming defects and interstitial atoms [3]. Most strikingly, new experiments by Davis and collaborators indicate that the onset of superflow is intimately tied to relaxation dynamics characteristic of glassy, or amorphous, solids [4]. The precise relationship between superflow and glassiness, and the possible existence of a novel superglass (SuG) state, is not without controversy in the experimental community [5]. Even outside of the 4 He context, the possible existence of a bosonic superglass phase is of broad theoretical importanceone that can be explored for example in simple models of interacting bosons. Lattice models have been instrumental in explaining some of the basic phenomenology of strongly-coupled quantum systems, including that of 4 He. The prototypical model is the lattice Bose-Hubbard (BH) model [6]. Its extended phase diagram is known to contain superfluids, Mott-insulating "crystals", and supersolid phases, where the latter refers to a state with coexisting superfluidity and broken translational symmetry in the particle density [7].Models of interacting bosons with disorder have been considered for some time [6,8] typically in the local chemical potential, as might be realized in 4 He absorbed in porous media. In these and related models [9] arises either a "Bose-glass" (BG) phase, with localized disorder in the particle densities but no coexisting superflow, or superfluid phases with locally inhomogeneous superflow [10] -neither of which correspond to a SuG state. This can be understood in part by considering the nature of the states in the BG close to the Mott lobes which, through Anderson localization [11], essentially form single-particle localized states. For this reason, the BG cannot support phase coherence; other types of interactions beyond the random local chemical potential are necessary to induce superglassiness. In this paper we show that a thermodynamic SuG state of bosons can be stabilized via random pairwise boson-boson interactions.It is widely believed that disorder (in the form of dislocations, grain boundaries, impurities, etc.) play...

Quantum charge glasses of itinerant fermions with cavity-mediated long-range interactions

“…(1) with bosonic atoms? Numerical simulations [42,43] and replica calculations [18] for similar models agree on the existence of a stable "superglass" phase with superfluid phase coherence in the presence of glassy random density order. It is tempting to identify the superglass as the bosonic pendant to the metallic fermion glass.…”

“…Such a glassy state with metallic conduction was obtained in dynamical mean field theory by Dobrosavljevic and collaborators [33,34]. Moreover, the fact that glassy phases may also exist in phases with good transport properties was recently shown in models of frustrated bosons [18,42,43] where a 'superglass' phase with microscopically coexisting superfluid and glassy density order exists. In the context of Coulomb frustrated systems in condensed matter (without disorder), an intermediate metallic phase with periodic, striped density order ("conducting crystal") was discussed by Spivak and Kivelson [35].…”

“…7, we plot an exemplary Ω = 0 bubble as function of momenta as occurring in the saddle-point equations (38,39,43). As expected, the dominant contributions come from momenta in the vicinity of the nesting condition q ≈ Q nest = (π, π, π).…”

“…Appendix A: Alternative derivation of the glass instability equations (38,43,42) 1. Replica approach…”

NaCaNi2F7: A frustrated high-temperature pyrochlore antiferromagnet withS=1N<

“…11,12 While single crystals are available of Y 2 Mo 2 O 7 , other transition-metal-based traditional oxide pyrochlores, many of which show antiferromagnetic interactions and exhibit glassy behavior, are not available as large single crystals and do not show the same level of frustration as the fluorides. [13][14][15][16] There is theoretical interest in the FeF 3 pyrochlore, but unfortunately, the methods employed for its synthesis likely preclude the growth of this material as large single crystals. 15,17,18 In contrast, the S = 1 NaCaNi 2 F 7 and S = 3/2 NaCaCo 2 F 7 and NaSrCo 2 F 7 fluoride pyrochlores have now been grown as cm-scale single crystals, suggesting that transition metal fluorides have the potential to develop into a significant and readily accessible new family of geometrically frustrated magnets.…”