Pairing Instabilities of the Yukawa-SYK Models with Controlled Fermion Incoherence

TL;DR

This paper investigates pairing instabilities in the Yukawa-SYK model, demonstrating how tuning the distribution of Yukawa couplings controls fermion coherence and enhances superconducting transition temperatures in non-Fermi liquids.

Contribution

It introduces a generalized Yukawa-SYK model with variable coupling distributions, systematically exploring how fermion incoherence affects pairing and superconductivity.

Findings

Unconventional spin-triplet pairing is favored with spin-dependent interactions.

All pairing channels show instabilities with spin-independent interactions.

The superconducting transition temperature $T_c$ is higher in non-Fermi liquids than in Fermi liquids.

Abstract

The interplay of non-Fermi liquid and superconductivity born out of strong dynamical interactions is at the heart of the physics of unconventional superconductivity. As a solvable platform of the strongly correlated superconductors, we study the pairing instabilities of the Yukawa-Sachdev-Ye-Kitaev (Yukawa-SYK) model, which describes spin-1/2 fermions coupled to bosons by the random, all-to-all, spin-independent and dependent Yukawa interactions. In contrast to the previously studied models, the random Yukawa couplings are sampled from a collection of Gaussian ensembles whose variances follow a continuous distribution rather than being fixed to a constant. By tuning the analytic behaviour of the distribution, we could control the fermion incoherence to systematically examine various normal states ranging from the Fermi liquid to non-Fermi liquids that are different from the conformal solution of the SYK model with a constant variance. Using the linearised Eliashberg theory, we show that the onset of the unconventional spin-triplet pairing is preferred with the spin-dependent interactions while all pairing channels show instabilities with the spin-independent interactions. Although the interactions shorten the lifetime of the fermions in the non-Fermi liquid, the same interactions also dress the bosons to strengthen the tendency to pair the incoherent fermions. As a consequence, the onset temperature $T_c$ of the pairing is enhanced in the non-Fermi liquid compared to the case of the Fermi liquid.