On the Anisotropy of the Superconducting Order Parameter of Cu Oxides Observed by STM/STS

Abstract

The anisotropic nature of differential tunnelingconductance of high-T_c oxides has been studied by ScanningTunneling Microscopy/Spectroscopy (STM/STS) with the tip approachalong the [100] and [110] directions of single crystals ofYBa₂Cu₃O₇ and La_2-xSr_xCuO₄ (x=0.10 and0.15). Although the observed gap is larger in the [110] direction thanin the [100] direction, trial calculations of the k-dependenttunneling current which consider the shape of the electron wavefunctions of surface oxygens suggest that the data can be understoodon the basis of the (x²-y²)-like symmetry rather than thexy-like one. Zero-bias conductance peaks have been observed along the[100] direction.