Frustrated Bose condensates in optical lattices

TL;DR

This paper investigates the ground states of frustrated bosons in a 2D optical lattice, revealing that even in dilute conditions, the system remains condensed with superfluid properties, and identifies potential noncondensed phases near the Mott transition.

Contribution

It introduces a large-S magnet mapping to analyze quantum fluctuations in frustrated Bose condensates, highlighting the persistence of condensation and superfluidity near the Mott transition.

Findings

Condensate remains stable even at low densities.

Superfluid density is comparable to condensate density.

Potential noncondensed phase emerges near the Mott transition for small S.

Abstract

We study the Bose-condensed ground states of bosons in a two-dimensional optical lattice in the presence of frustration due to an effective vector potential, for example, due to lattice rotation. We use a mapping to a large-S frustrated magnet to study quantum fluctuations in the condensed state. Quantum effects are introduced by considering a 1/S expansion around the classical ground state. The large-S regime should be relevant to systems with many particles per site. As the system approaches the Mott insulating state, the hole density becomes small. Our large-S results show that, even when the system is very dilute, the holes remain a (partially) condensed system. Moreover, the superfluid density is comparable to the condensate density. In other words, the large-S regime does not display an instability to noncondensed phases. However, for cases with fewer than 1/3 flux quantum per lattice plaquette, we find that the fractional condensate depletion increases as the system approaches the Mott phase, giving rise to the possibility of a noncondensed state before the Mott phase is reached for systems with smaller S.