It is shown that the available experimental data on the intriguing doping dependence of specific heat singularity and of the magnetic penetration depth, as well as 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.
Considerable debate has arisen over the doping dependence of the transition temperature Tc [1, 2], magnetic penetration depth 2 [3-8] and specific heat singularity [9] of cuprate superconductors as well as the resulting correlation between these properties. As thermal fluctuations are essential in these extreme type II materials and because these fluctuations are known to mediate universal behavior [10,11], we sketch in this work the implications of three-dimensional (3D)-xy critical point universality, supplemented by the experimentally established phase transition line, and its effect on the intriguing doping dependence of the magnetic penetration depth 2 and of specific heat singularity, as well as the relationship between these properties.
The paper is organized as follows. First, we outline the experimental evidence showing that cuprate superconductors and uncharged superfluids like 4He share universal 3D-xy properties in the fluctuation dominated regime. Then we explain how the universal relations between critical amplitudes and To, supplemented by the experimental phase diagram (doping dependence of To), imply unconventional behavior of the penetration depth in the overdoped regime in agreement with recent magnetic induction measurements of high-quality La2-xSrxCuO4 films [12] and laSR data [3,7,8]. The remainder of the paper addresses the implications of these findings.