Normal charge densities in quantum critical superfluids

TL;DR

This paper explores how the normal density in quantum critical superfluids can be non-zero at zero temperature, challenging traditional views, using gauge-gravity duality to analyze different models.

Contribution

It demonstrates that breaking isotropy in quantum critical superfluids can lead to a finite normal density at zero temperature, unlike isotropic models with emergent Lorentz symmetry.

Findings

Models with Lorentz symmetry have zero normal density.

Breaking isotropy can produce a non-zero normal density.

Results may explain experimental observations in cuprate superconductors.

Abstract

The normal density of a translation-invariant superfluid often vanishes at zero temperature, as is observed in superfluid Helium and conventional superconductors described by BCS theory. Here we show that this need not be the case. We investigate the normal density in models of quantum critical superfluids using gauge-gravity duality. Models with an emergent infrared Lorentz symmetry lead to a vanishing normal density. On the other hand, models which break the isotropy between time and space may enjoy a non-vanishing normal density, depending on the spectrum of irrelevant deformations around the underlying quantum critical groundstate. Our results may shed light on recent measurements of the superfluid density and low energy spectral weight in superconducting overdoped cuprates.