EVIDENCE OF A NEW MAGNETIC-FLUX PINNING CENTER IN ND1BA2CU3OY SINGLE-CRYSTALS

Highly flat, clean, and stable surfaces of as-prepared Nd1Ba2Cu3Oy (Nd 123) single crystals have been studied using an ultrahigh vacuum scann ing tunnelling microscope/spectroscopy (UHV-STM/ STS) system. The resu lts of large area STM scans together with the results of STS measureme nts, magnetization loops measured at several temperatures below T-c, a nd critical current density measurements reveal the presence of a new magnetic Aux pinning center in Nd123 single crystals prepared in a red uced oxygen partial pressure. This pinning center is more conductive o n the surface than the matrix and has a size ranging from several to s everal lens of nanometers as seen by STM. Similar pinning center may b e introduced to other high temperature superconductors to increase the ir critical current densities in an applied magnetic field. (C) 1997 A merican Institute of Physics.