Superfluid density in overdoped cuprates: Thin films versus bulk samples

Recent study of overdoped La2???xSrxCuO4 cuprate superconductor thin films by Bo??ovic?? 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 La2???xSrxCuO4 films decreases on the overdoped side as a linear function of critical temperature Tc, which was taken as evidence for the violation of 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??ovic?????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 La2???xSrxCuO4 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.

Automated summary

This study investigates the superfluid density of overdoped La2???xSrxCuO4 thin films and reveals that it does not conform to previous theories. It is found that the superfluid density in the thin films is significantly suppressed compared to bulk samples. In contrast, the superfluid density in bulk samples tends to saturate rather than decrease with doping. This saturation phenomenon may not be limited to cuprate superconductors but also observed in pnictide superconductors. Quantum phase fluctuations are argued to play a crucial role in suppressing the superfluid density in thin films.