Cell strong coupling perturbative approach to the phase diagram of ultracold bosons in optical superlattices

TL;DR

This paper introduces a second-order cell strong coupling perturbative method to analyze the phase diagram of ultracold bosons in optical superlattices, accurately capturing complex insulator domains with lower computational effort.

Contribution

The paper develops a novel perturbative approach leveraging cell partitioning to describe complex phase diagrams of ultracold bosons in superlattices, including loophole-shaped insulator domains.

Findings

The method accurately predicts reentrant insulator domains.

Quantum Monte Carlo comparisons show good quantitative agreement.

Loophole-shaped insulating domains are experimentally accessible without extreme conditions.

Abstract

The phase diagram of ultracold bosons in realistic optical superlattices is addressed via second-order {\it cell} strong coupling perturbative expansions for the Bose-Hubbard model describing the system. Taking advantage of the cell partition inherent in the complex periodic modulation of a superlattice, this technique allows for the description of the unusual loophole-shaped insulator domains that may occur in the phase diagram of the system, unlike the standard perturbative approach. Furthermore, comparisons with quantum Monte Carlo simulations show that our approach provides quantitatively satisfactory results at a significantly lower computational cost than brute force numerical methods. We explicitly consider the phase diagrams for two realistic 3-periodic optical superlattices. These show that many insulator domains exhibit an unusual reentrant character, which we discuss, and suggest that the quantum phase transition relevant to the loophole-shaped insulating domains does not require extreme experimental condition in order to be observed.