Comment on `Anomalously Large Gap Anisotropy in the a-b Plane of   Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$'
  Comment on `Anomalously Large Gap Anisotropy in the a-b Plane of Bi$_{{\bf 2}}$Sr$_{{\bf 2}}$CaCu$_{{\bf 2}}$O$_{{\bf 8+}{\delta}}$', 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., $\Delta_{{\bf k}}=\Delta(\cos k_{x}a-\cos k_{y}a)$. 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 $\Delta_{{\bf k}}$, is not unambiguous, and that a possibility of $d_{xy}$ pairing, i.e., $\Delta_{{\bf k}}=2\Delta\sin k_{x}a\sin k_{y}a$, cannot be ruled out. 
Kazumasa Miyake and Osamu Narikiyo