Low temperature superfluid response of high-Tc superconductors

We have reviewed our theoretical and experimental results of the low temperature superfluid response function rho(s)(mu) of high temperature superconductors (HTSC). In clean high-T-c materials the in-plane superfluid density rho(s)(ab) varies linearly with temperature. The slope of this linear T term is found to scale approximately with 1/T-c which, according to the weak coupling BCS theory for a d-wave superconductor, implies that the gap amplitude scales approximately with T-c. A T-5 behavior of the out-of-plane superfluid density rho(s)(c) for clean tetragonal HTSC was predicted and observed experimentally in the single layer Hg-compound HgBa2CuO4+delta. In other tetragonal high-T-c compounds with relatively high anisotropy, such as Hg2Ba2Ca2Cu3O8-delta, rho(s)(c) varies as T-2 due to disorder effects. In optimally doped YBa2Cu3O7-delta, rho(s)(c) varies linearly with temperature at low temperatures, but in underdoped YBa2Cu3O7-delta, rho(s)(c) varies as T-2 at low temperatures; these results are consistent with our theoretical calculations.