Comment on "Feshbach-Einstein Condensates" by V. G. Rousseau and P. J. H. Denteneer

TL;DR

This paper critiques a previous claim of a novel super-Mott phase in an atom-molecule Bose-Hubbard model, demonstrating that the phase was misinterpreted due to flawed estimators, and concludes it is actually a normal insulator.

Contribution

The paper clarifies the limitations of superfluidity estimators in the presence of atom-molecule conversion, correcting the interpretation of the phase diagram in the prior study.

Findings

The super-Mott phase is a misinterpretation due to estimator breakdown.

The phase is actually a normal insulator.

Superfluidity estimators fail with coherent atom-molecule conversion.

Abstract

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.