Coherent superconductivity with large gap ratio from incoherent metals

TL;DR

This paper explores how incoherent metals can transition directly into a superconducting state with coherent transport and an unusually large gap ratio, using models based on SYK quantum dots.

Contribution

It introduces a new model framework showing direct IM to superconducting transitions with unique properties, including large gap ratios and coherent superfluid transport.

Findings

Superconductivity emerges from incoherent metallic states without quasiparticles.

The superconducting state exhibits coherent superfluid transport despite incoherent normal state.

The gap ratio $2\Delta/T_{sc}$ is significantly larger than the BCS value of 3.53.

Abstract

A mysterious incoherent metallic (IM) normal state with $T$-linear resistivity is ubiquitous among strongly correlated superconductors. Recent progress with microscopic models exhibiting IM transport has presented the opportunity for us to study new models that exhibit direct transitions into a superconducting state out of IM states within the framework of connected Sachdev-Ye-Kitaev (SYK) "quantum dots". Here local SYK interactions within a dot produce IM transport in the normal state, while local attractive interactions drive superconductivity. Through explicit calculations, we find two features of superconductivity arising from an IM normal state: First, despite the absence of quasiparticles in the normal state, the superconducting state still exhibits coherent superfluid transport. Second, the non-quasiparticle nature of the IM Green's functions produces a large enhancement in the ratio of the zero-temperature superconducting gap $\Delta$ and transition temperature $T_{sc}$, $2\Delta/T_{sc}$, with respect to its BCS value of $3.53$.