Collective and plastic vortex motion probed by STM

The motion of flux line lattices (VL) in various single crystals of NbSe2 h as been observed using fast scanning tunneling microscopy. In crystals with intrinsic weak point disorder the VL creeps coherently as a large bundle a long one of the principal axes of the VL. However, we observe striking and for the creep regime unanticipated velocity modulations in the direction of motion, related to the VL periodicity, i.e. with the "washboard" frequency . In the transverse direction the fluctuations of the center-of-mass displa cement are random. In crystals with strong line disorder (correlated pinnin g) caused by columnar defects due to swift ion irradiation collective pinni ng is observed in fields up to almost double the dose-equivalent held B phi while interesting plastic flow patterns are detected at 2B phi. (C) 2000 E lsevier Science B.V. All rights reserved.