Electrostatic Modulation of the Superfluid Density in a Ultrathin La2-xSrxCuO4 Film

By capacitively charging an underdoped ultrathin La2-xSrxCuO4 film with an electric field applied across a gate insulator with a high dielectric constant, relative changes of the areal superfluid density n_{s\Box}$ of unprecedented strength were observed in measurements of the film kinetic inductance. Although $n_{s\mathrm{\square }}$ appears to be substantially reduced by disorder, the data provide, for the first time on the same sample, direct compelling evidence for the Uemura relation $T_c\propto n_{s\mathrm{\square }}(T=0)$ in the underdoped regime of copper-oxide superconductors.