Interpretation of the temperature dependence of the electromagnetic penetration depth in YBa₂Cu₃O_7-δ

The low-temperature behavior of the a-b plane penetration depth, λ_ab, is a probe of the pairing state in YBa₂Cu₃O_7-δ. A group-theoretic analysis shows that in orthorhombic or tetragonal crystals all singlet pairing states other than ‘‘s wave’’ would lead to Δλ_ab(T)==λ_ab(T)-λ_ab(0)∼T. In contrast, for an isotropic system, there are combinations of singlet pairing states and field directions that would give rise to Δλ(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 Δλ_ab(T)∼T². This behavior is probably not intrinsic, and possible explanations are discussed.