Anomalous Superfluid Density in Pair-Density-Wave Superconductors

TL;DR

This paper investigates the superfluid density in pair-density-wave superconductors, revealing intrinsic instabilities and proposing observable signatures, which challenge the stability of PDW states in quantum materials.

Contribution

It provides the first calculation of superfluid density in a generic 2D PDW superconductor, identifying regions of instability and predicting experimental signatures.

Findings

Large parameter space with negative superfluid density indicating instability

Prediction of small longitudinal superfluid response in stable regimes

Unusual temperature dependence of superfluid density

Abstract

Pair-density-wave (PDW) states are a long-sought-after phase of quantum materials, with the potential to unravel the mysteries of high-$T_c$ cuprates and other strongly correlated superconductors. Yet, surprisingly, a key signature of stable superconductivity, namely the positivity of the superfluid density, $n_s(T)$, has not yet been demonstrated. Here, we address this central issue by calculating $n_s(T)$ for a generic model two-dimensional PDW superconductor. We uncover a surprisingly large region of intrinsic instability, associated with negative $n_s(T)$, revealing that a significant portion of the parameter space thought to be physical cannot support a pure PDW order. In the remaining stable regime, we predict two striking and observable fingerprints: a small longitudinal superfluid response and an unusual temperature dependence for $n_s(T)$. These generally model-independent, as well as experimentally relevant findings suggest that the fragility of the superfluid density poses a significant problem for the formation of stable, finite temperature PDW superconductivity.