The Vanishing Superfluid Density in Cuprates—and Why It Matters

Recently, we have released the results of a comprehensive study of a couple thousand single-crystal La_2−xSr_xCuO₄ films, covering densely the entire overdoped side of the phase diagram (Božović et al. Nature 536, 309–311, 2016; Wu et al. Nature 547, 432–435, 2017). Here, we review the key experimental findings, place them in the context of other important well-established facts and observations, and discuss their implications for our understanding of high-temperature superconductivity in cuprates. We conclude that it involves some new physics that requires going beyond the standard Fermi liquid description of the normal state and Bardeen-Cooper-Schrieffer description of the superconducting state.