Interpretation of the temperature dependence of the electromagnetic penetration depth inYBa2Cu3O7−δ

Abstract

The low-temperature behavior of the a-b plane penetration depth, ${\ensuremath{\lambda}}_{\mathit{a}\mathit{b}}$, is a probe of the pairing state in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$. A group-theoretic analysis shows that in orthorhombic or tetragonal crystals all singlet pairing states other than ``s wave'' would lead to \ensuremath{\Delta}${\ensuremath{\lambda}}_{\mathit{a}\mathit{b}}$(T)==${\ensuremath{\lambda}}_{\mathit{a}\mathit{b}}$(T)-${\ensuremath{\lambda}}_{\mathit{a}\mathit{b}}$(0)\ensuremath{\sim}T. In contrast, for an isotropic system, there are combinations of singlet pairing states and field directions that would give rise to \ensuremath{\Delta}\ensuremath{\lambda}(T)\ensuremath{\sim}${\mathit{T}}^{3}$. We reanalyze the surface impedance data of Fiory et al. [Phys. Rev. Lett. 61, 1419 (1988)], and show that these data exhibit neither a BCS temperature dependence nor a linear temperature dependence at low temperature, but instead follow \ensuremath{\Delta}${\ensuremath{\lambda}}_{\mathit{a}\mathit{b}}$(T)\ensuremath{\sim}${\mathit{T}}^{2}$. This behavior is probably not intrinsic, and possible explanations are discussed.