No abstract

Santamaria et al. Reply: Schwarzacher and Huo, in their Comment [1] to our recent Letter, claim that electrodeposited Cu films show anomalous scaling with similar roughness exponents as found by us in sputtered Fe=Cr superlattices [2]. They attribute this to the similar microstructure of Cu films and superlattices, and propose that the disagreement of scaling exponents with theoretical predictions is because grain formation is not taken into account.Surface growth models are simplified descriptions of a complex reality in which, aside from short range (interatomic) interactions, long range nonlocal effects determined by surface morphology (grains) and geometry of the incoming particle front (shadowing) play an important role in kinetic roughening. Most data showing agreement with theoretical models correspond to films grown by molecular beam epitaxy, while discrepancies are frequently encountered in surfaces grown by more complex techniques (e.g., sputtering and electrodeposition) [3].

Rectangular pinning arrays of Ni dots define a potential landscape for vortex motion in Nb films. Magnetotransport experiments in which two in-plane orthogonal electrical currents are injected simultaneously allow one to select the direction and magnitude of the Lorentz force on the vortex lattice, thus providing the angular dependence of the vortex motion. The background dissipation depends on the angle at low magnetic fields, which is progressively smeared out with increasing field. The periodic potential locks in the vortex motion along channeling directions. Because of this, the vortex-lattice direction of motion is up to 85 o away from the applied Lorentz force direction.

Rectangular arrays of Ni dots with small interdot separation have been fabricated in superconducting Nb films. The geometry of these rectangular arrays defines deep pinning potential channels. Two effects have been observed. ͑i͒ When the vortices move perpendicular to the channels ͑i.e., along the long side of the rectangular array͒ an enhancement of the background pinning is observed, as predicted by theoretical models. This enhancement occurs for all magnetic fields. ͑ii͒ A new anisotropic matching effect, which has not been theoretically predicted, has been measured. The position of the matching fields depends on the direction of the vortex flow. When the vortices move parallel to the potential channels ͑i.e., along the short side of the rectangular array͒ the vortex lattice matches the rectangular array, but when they move along the long side of the rectangular array only a triangular lattice is observed.

Competition between the vortex lattice and a lattice of asymmetric artificial defects is shown to play a crucial role in ratchet experiments in superconducting films. We present a novel and collective mechanism for current reversal based on a reconfiguration of the vortex lattice. In contrast to previous models of vortex current reversal, the mechanism is based on the global response of the vortex lattice to external forces.