Ordered Vortex Lattice and Intrinsic Vortex Core States in Bi₂Sr₂CaCu₂O_x Studied by Scanning Tunneling Microscopy and Spectroscopy

Abstract

The ordered vortex lattice in Bi₂Sr₂CaCu₂O_x (overdoped, T_c=83 K) has been observed for the first time at 4.2 K in 8 T by scanning tunneling spectroscopy (STS). The vortex lattice is short-range ordered in the length scale of 100 nm. The vortices form an almost square lattice with the sides parallel to the diagonal direction of the CuO₂ square lattice, that is, the nodal direction of the d_x²−y² superconductor. In all of the vortex cores of the ordered lattice, the localized states are observed at ±9 meV symmetrically in the superconducting gap and are clearly determined to be intrinsic to the vortex in Bi₂Sr₂CaCu₂O_x. The intensity is found to be electron–hole asymmetric.