FFLO strange metal and quantum criticality in two dimensions: theory and application to organic superconductors

TL;DR

This paper develops a theory for the quantum phase transition between a 2D metal and an FFLO superconductor driven by spin imbalance, predicting non-Fermi liquid behavior and unique critical signatures relevant to organic superconductors.

Contribution

It introduces a novel universality class for the metal-FFLO transition, deriving an effective action and predicting testable non-Fermi liquid phenomena.

Findings

Prediction of non-Fermi liquid behavior at the transition

Identification of hot spots with no quasi-particles

Distinct universality class with pairing-channel critical fluctuations

Abstract

Increasing the spin imbalance in superconductors can spatially modulate the gap by forming Cooper pairs with finite momentum. For large imbalances compared to the Fermi energy, the inhomogeneous FFLO superconductor ultimately becomes a normal metal. There is mounting experimental evidence for this scenario in 2D organic superconductors in large in-plane magnetic fields; this is complemented by ongoing efforts to realize this scenario in coupled tubes of atomic Fermi gases with spin imbalance. Yet, a theory for the phase transition from a metal to an FFLO superconductor has not been developed so far and the universality class has remained unknown. Here we propose and analyze a spin imbalance driven quantum critical point between a 2D metal and an FFLO phase in anisotropic electron systems. We derive the effective action for electrons and bosonic FFLO pairs at this quantum phase transition. Using this action, we predict non-Fermi liquid behavior and the absence of quasi-particles at a discrete set of hot spots on the Fermi surfaces. This results in strange power-laws in thermodynamics and response functions, which are testable with existing experimental set-ups on 2D organic superconductors and may also serve as signatures of the elusive FFLO phase itself. The proposed universality class is distinct from previously known quantum critical metals and, because its critical fluctuations appear already in the pairing channel, a promising candidate for naked metallic quantum criticality over extended temperature ranges.