Interpretation of the temperature dependence of the electromagnetic penetration depth in YBa2Cu3O7-

The low-temperature behavior of the a-b plane penetration depth, ab, is a probe of the pairing state in YBa2Cu3O7-. A group-theoretic analysis shows that in orthorhombic or tetragonal crystals all singlet pairing states other than s wave would lead to "ab(T)==ab(T)-ab(0)T. In contrast, for an isotropic system, there are combinations of singlet pairing states and field directions that would give rise to "(T)T3. We reanalyze the surface impedance data of Fiory et al. [Phys. Rev. Lett. 61, 1419 (1988)], and show that these data exhibit neither a BCS temperature dependence nor a linear temperature dependence at low temperature, but instead follow "ab(T)T2. This behavior is probably not intrinsic, and possible explanations are discussed.