Symmetry of the gap in Bi2212 from photoemission spectroscopy.

In a recent Letter, Shen et al 1 have detected a large anisotropy of the superconducting gap in Bi 2 Sr 2 CaCu 2 O 8 (Bi2212), consistent with d-wave symmetry, from photoemission spectroscopy. Moreover, they claim that the change in their spectra as a function of aging is also consistent with such an intrepretation. In this Comment, I show that the latter statement is not entirely correct, in that the data as a function of aging are inconsistent with a d-wave gap but are consistent with an anisotropic s-wave gap. In Ref. 1, the data show that the gap is close to zero along the (1,1) direction and maximum along the (1,0) direction, just as expected for a gap with d x 2 −y 2 symmetry. As the sample ages, the gap becomes non-zero along (1,1). This is attributed by them to poorer k resolution due to impurity scattering. This brings up the interesting question of what one theoretically expects for the excitation gap as a function of impurities. I do this for two cases where a spherical Fermi cylinder is assumed for simplicity's sake. For the d-wave case, the gap is assumed to be ∆cos(2φ) and for the anisotropic s-wave case, ∆|cos(2φ)|. Thus, the modulus of the gap is the same, the only difference being the symmetry of the gap under rotations.