Pairing, crystallization and string correlations of mass-imbalanced atomic mixtures in one-dimensional optical lattices

TL;DR

This paper explores the complex phase diagram of mass-imbalanced binary mixtures of hardcore bosons or fermions in one-dimensional optical lattices, revealing asymmetries between attractive and repulsive interactions and identifying various gapped and crystalline phases.

Contribution

It provides the first detailed numerical analysis of the phase diagram for mass-imbalanced mixtures, highlighting asymmetries and novel phases not predicted by earlier theories.

Findings

Attraction always leads to pairing and pair crystallization at high mass imbalance.

Repulsive interactions keep components gapless over a large parameter range.

Strong mass imbalance induces a spin gap and crystalline phases.

Abstract

We numerically determine the very rich phase diagram of mass-imbalanced binary mixtures of hardcore bosons (or equivalently -- fermions, or hardcore-Bose/Fermi mixtures) loaded in one-dimensional optical lattices. Focusing on commensurate fillings away from half filling, we find a strong asymmetry between attractive and repulsive interactions. Attraction is found to always lead to pairing, associated with a spin gap, and to pair crystallization for very strong mass imbalance. In the repulsive case the two atomic components remain instead fully gapless over a large parameter range; only a very strong mass imbalance leads to the opening of a spin gap. The spin-gap phase is the precursor of a crystalline phase occurring for an even stronger mass imbalance. The fundamental asymmetry of the phase diagram is at odds with recent theoretical predictions, and can be tested directly via time-of-flight experiments on trapped cold atoms.