Stable pair liquid phase in fermionic systems

TL;DR

This paper predicts a novel pair-liquid phase in lattice fermion systems with finite-range attraction, distinct from bosonic systems, and explores its stability and potential observability in cold atom experiments and high-temperature superconductivity.

Contribution

It introduces the concept of a stable pair-liquid phase in fermionic systems with finite-range interactions, highlighting the role of quantum statistics in its stability.

Findings

Pair-liquid phase exists in fermionic lattice systems with finite-range attraction.

This phase is absent in bosonic systems due to quantum statistics.

Boundaries of pair stability are mapped through four-body Schroedinger equation solutions.

Abstract

We predict the existence of a pair-liquid phase in lattice fermion systems with finite-range attractive interactions. This exotic state competes on one side with a normal Fermi liquid of unpaired fermions and on the other side with a phase-separated state where all fermions are coupled into macroscopic clusters. We show that such a phase is absent in bosonic systems and therefore is protected by the exclusion principle. In contrast with zero-range attractive systems where clustering of more than two fermions is directly prohibited, here quantum statistics acts dynamically and in a more subtle way. Since a many-fermion wave function must have nodes, the cluster formation threshold is larger than the pair formation threshold. By directly solving a four-body Schroedinger equation on one- and two-dimensional lattices, we map the boundaries of pair stability. The pair liquid phase should be observable in cold atom systems. We also discuss implications for the preformed-pair mechanism of high-temperature superconductivity.