Superuid density in overdoped cuprates: thin films versus bulk samples

TL;DR

This paper investigates the superfluid density in overdoped cuprates, revealing differences between thin films and bulk samples, and suggests quantum phase fluctuations significantly suppress superfluid density in thin films, challenging existing laws like Homes' law.

Contribution

It demonstrates that superfluid density saturates in bulk cuprates and is more suppressed in thin films, highlighting the role of quantum phase fluctuations in overdoped superconductors.

Findings

Superfluid density in bulk cuprates saturates on the overdoped side.

Thin film superfluid density is more strongly suppressed than in bulk samples.

Evidence suggests similar superfluid density saturation in pnictide superconductors.

Abstract

Recent study of overdoped La$_{2-x}$Sr$_x$CuO$_4$ cuprate superconductor thin films by Bo\v{z}ovi\'{c} {\it et al.} has revealed several unexpected findings, most notably the violation of the BCS description which was believed to adequately describe overdoped cuprates. In particular, it was found that the superfluid density in La$_{2-x}$Sr$_x$CuO$_4$ films decreases on the overdoped side as a linear function of critical temperature T$_c$, which was taken as evidence for the violation of the Homes' law. We show explicitly that the law is indeed violated, and as the main reason for violation we find that the superfluid density in Bo\v{z}ovi\'{c}'s films is suppressed more strongly than in bulk samples. Based on the existing literature data, we show that the superfluid density in bulk cuprate samples does not decrease with doping, but instead tends to saturate on the overdoped side. The result is also supported by our recent measurement of a heavily overdoped bulk La$_{2-x}$Sr$_x$CuO$_4$ sample. Moreover, this saturation of superfluid density might not be limited to cuprates, as we find evidence for similar behavior in two pnictide superconductor families. We argue that quantum phase fluctuations play an important role in suppressing the superfluid density in thin films.