Comment on `Anomalously Large Gap Anisotropy in the a-b Plane of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$'

Abstract

Comment on `Anomalously Large Gap Anisotropy in the a-b Plane of Bi$_{\bf 2}$Sr$_{\bf 2}$CaCu$_{\bf 2}$O$_{{\bf 8+}δ}$', Kazumasa Miyake and Osamu Narikiyo, In a recent Letter, Shen and collaborators [Phys. Rev. Lett. {\bf 70}, 1555 (1993)] reported that angle-resolved photoemission spectra (ARPES) in Bi-2212 cuprate superconductor far below $T_{\rm c}$ has anomalously large anisotropy and then concluded that the symmetry of Cooper pair is likely to be $d_{x^{2}-y^{2}}$, i.e., $Δ_{\bf k}=Δ(\cos k_{x}a-\cos k_{y}a)$. In this Comment we point out that the interpretation done by Shen {\it et al\/.}, where the shift of edge near Fermi level has been identified with the gap $Δ_{\bf k}$, is not unambiguous, and that a possibility of $d_{xy}$ pairing, i.e., $Δ_{\bf k}=2Δ\sin k_{x}a\sin k_{y}a$, cannot be ruled out.