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arXiv:cond-mat/9811121 · Physical review. B, Condensed matter

Bose glass in a large- N commensurate dirty boson model

M. B. Hastings

Published Jun 20, 2001• 7 citations
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Abstract

The large-$N$ commensurate dirty boson model, in both the weakly and strongly commensurate cases, is considered via a perturbative renormalization-group treatment. In the weakly commensurate case, there exists a fixed line under renormalization-group flow, with varying amounts of disorder along the line. Including $1/N$ corrections causes the system to flow to strong disorder, indicating that the model does not have a phase transition perturbatively connected to the Mott insulator-superfluid (MI-SF) transition. I discuss the qualitative effects of instantons on the low-energy density of excitations. In the strongly commensurate case, a fixed point found previously is considered and results are obtained for higher moments of the correlation functions. To lowest order, correlation functions have a log-normal distribution. Finally, I prove two interesting theorems for large-$N$ vector models with disorder, relevant to the problem of replica symmetry breaking and frustration in such systems.

DOI
10.1103/physrevb.64.024517

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The large-NNN commensurate dirty boson model, in both the weakly and strongly commensurate cases, is considered via a perturbative renormalization-group treatment. In the weakly commensurate case, there exists a fixed line under renormalization-group flow, with varying amounts of disorder along the line. Including 1/N1/N1/N corrections causes the system to flow to strong disorder, indicating that the model does not have a phase transition perturbatively connected to the Mott insulator-superfluid (MI-SF) transition. I discuss the qualitative effects of instantons on the low-energy density of excitations. In the strongly commensurate case, a fixed point found previously is considered and results are obtained for higher moments of the correlation functions. To lowest order, correlation functions have a log-normal distribution. Finally, I prove two interesting theorems for large-NNN vector models with disorder, relevant to the problem of replica symmetry breaking and frustration in such systems.
We use a large-N renormalization-group (RG) method to study a model of interacting boson system with a quenched random potential. In the absence of impurities, the pure boson system has a critical point that describes the superfluid--Mott-insulator (SF--MI) transition. The SF--MI transition of d-dimensional bosons belongs to the (d+1)-dimensional XY model universality class. In this paper, we study the dirty-boson critical points in the neighborhood of this pure SF--MI critical point. In general, the on-site random potential in the original lattice model gives two types of randomness in the effective-field-theoretic action. One is the randomness in the effective on-site repulsion w(x) and the other is the randomness of the chemical potential u(x). It turns out that d=2 is the critical dimension for both types of disorder but the roles of these two types of disorder are reversed as d=2 is crossed. Applying \ensuremath{\epsilon}=d-2 expansion, we found coupled RG equations for both kinds of randomness which reveal several nontrivial critical points. All the weak random fixed points we found have three or more relevant directions. We conclude that the direct SF--MI transition is unlikely to occur near two dimensions.
We use renormalization-group theory to examine the quantum phase transitions upon exiting the insulating phase of a disordered, strongly interacting boson system. For weak disorder we find a direct transition from this Mott insulator to the superfluid phase. In d>4d>4d>4 a finite region around the particle-hole symmetric point supports this direct transition, whereas for 2< d<42<~d<42< d<4 perturbative arguments suggest that the direct transition survives only precisely at commensurate filling. For strong disorder the renormalization trajectories pass next to two fixed points, describing a pair of distinct transitions; first from the Mott insulator to the Bose glass, and then from the Bose glass to the superfluid. The latter fixed point possesses statistical particle-hole symmetry and a dynamical exponent zzz, equal to the dimension ddd.
The spherical limit of strongly commensurate dirty bosons is studied perturbatively at weak disorder and numerically at strong disorder in two dimensions (2D). We argue that disorder is not perfectly screened by interactions and consequently that the ground state in the effective Anderson localization problem always remains localized. As a result there is only a gapped Mott insulator phase in the theory. Comparisons with other studies and the parallel with disordered fermions in 2D are discussed. We conjecture that while for the physical cases N = 2 (XY) and N = 1 (Ising) the theory should have the ordered phase, it may not for N = 3 (Heisenberg).
The large-NNN $\ensuremath{\sigma}$ model, in D<4D<4D<4 space-time dimensions, with disorder a function of d space dimensions, is analyzed via a renormalization-group treatment. Critical exponents for average quantities are calculated, first to lowest order and then to all orders, in $\ensuremath{\epsilon}=D\ensuremath{-}2\ensuremath{-}d/2.$ In particular, it is found that $\ensuremath{\nu}d=2.$ When D=d+1,D=d+1,D=d+1, this model is equivalent to a large-NNN limit of the strongly commensurate dirty boson problem.
We study the effect of commensurability (integer filling factor) on the superfluid (SF)--Bose-glass (BG) transition in a one-dimensional disordered system in the limit of weak disorder, when the effect is most pronounced and, on the other hand, may be traced via the renormalization-group analysis. The equation for the SF-BG phase boundary demonstrating the effect of disorder-stimulated superfluidity implies that the strength of disorder sufficient to restore superfluidity from Mott insulator (MI) is much larger than that enough to turn MI into BG. Thus we provide an explicit proof of the fact that at arbitrarily small disorder the SF and MI phases are always separated by BG. \textcopyright{} 1996 The American Physical Society.
We investigate the nature of the Bose glass phase of the disordered Bose-Hubbard model in d>2d>2d>2 and demonstrate the existence of a glass-like replica symmetry breaking (RSB) order parameter in terms of particle number fluctuations. Starting from a strong-coupling expansion around the atomic limit, we study the instability of the Mott insulator towards the formation of a Bose glass. We add some infinitesimal RSB, following the Parisi hierarchical approach in the most general form, and observe its flow under the momentum-shell renormalization group scheme. We find a new fixed point with one-step RSB, corresponding to the transition between the Mott insulator and a Bose glass phase with hitherto unseen RSB. The susceptibility associated to infinitesimal RSB perturbation in the Mott insulator is found to diverge at the transition with an exponent of γ=1/dγ=1/dγ=1/d. Our findings are consistent with the expectation of glassy behavior and the established breakdown of self-averaging. We discuss the possibility of measuring the glass-like order parameter in optical lattice experiments as well as in certain spin systems that are in the same universality class as the Bose-Hubbard model.