Nature of the superconducting state in high-T_c cuprates

It is shown that the available experimental data on the doping dependence of the specific heat singularity and penetration depth, and the relation between transition temperature and magnetic penetration depth can be understood in terms of three-dimensional xy critical point universality, supplemented by the experimental phase transition line. Thus, the nature of the superconducting state in the cuprates appears to be essentially that of superfluidity. © 1994.