Testing models of the symmetry of the superconducting pairing state by low-temperature electron irradiation of an untwinned single crystal of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

We have carried out in situ resistivity measurements in a high-voltage electron microscope to determine how the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ and the resistivity ρ(T) change with low-temperature electron irradiation. We find that point defects introduced by oxygen displacements from the ${\mathrm{CuO}}_2$ planes make the dominant contribution to the irradiation-induced suppression of ${\mathit{T}}_{\mathit{c}}$. We find that ${\mathit{dT}}_{\mathit{c}}$/dρ is ∼-0.30±0.04 K/μΩ cm. Analysis of this result indicates the effect of strongly anisotropic superconductivity, and rules out isotropic s-wave pairing. Even though our data cannot determine whether the pairing symmetry is s or d wave, they provide a test for further theoretical developments.