Interpretation of the temperature dependence of the electromagnetic penetration depth in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">YBa</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Cu</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">O</mml:mi></mml:mrow><mml:mrow><mml:mn>7</mml:mn><mml:mi mathvariant="normal">−</mml:mi><mml:mi mathvariant="normal">δ</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>

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.

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