Two-fluid coexistence and phase separation in a one dimensional model with pair hopping and density interactions

TL;DR

This paper explores the phase diagram of a one-dimensional spinless fermion model with pair-hopping and interactions, revealing coexistence and separation of fermion and pair fluids, supported by numerical and analytical methods.

Contribution

It introduces a detailed phase diagram for a 1D fermion model, highlighting novel two-fluid coexistence and phase separation phenomena with analytical support.

Findings

Identification of phases with coexisting fermions and pairs

Discovery of spatially segregated phase separation

Support for two-fluid models in pairing phenomena

Abstract

We compute the phase diagram of a one-dimensional model of spinless fermions with pair-hopping and nearest-neighbor interaction, first introduced by Ruhman and Altman, using the density-matrix renormalization group combined with various analytical approaches. Although the main phases are a Luttinger liquid of fermions and a Luttinger liquid of pairs, we also find remarkable phases in which only a fraction of the fermions are paired. In such case, two situations arise: either fermions and pairs coexist spatially in a two-fluid mixture, or they are spatially segregated leading to phase separation. These results are supported by several analytical models that describe in an accurate way various relevant cuts of the phase diagram. Last, we identify relevant microscopic observables that capture the presence of these two fluids: while originally introduced in a phenomenological way, they support a wider application of two-fluid models for describing pairing phenomena.