Guided vortex motion in Nb films on facetted substrate surfaces Oleksiy K. Soroka a, * , Michael Huth a , Valerij A. Shklovskij b , Jens Oster a , Hermann Adrian a a Institute of Physics, Johannes Gutenberg University, Staudinger Weg 7, 55099 Mainz, Germany b Physical Department, Kharkov State University, 4 Svobody Sq., 61077 Kharkov, Ukraine Abstract Anisotropy of the pinning force in a superconductor can cause a guiding effect on the vortices, which leads to the appearance of new components in the galvanomagnetic quantities of the sample. In this case one can observe an ad- ditional odd magnetoresistive component with respect to magnetic field reversal. Furthermore, an even contribution to the Hall voltage is observed. Guided motion of vortices in Nb films on facetted a-Al 2 O 3 ð10  10Þ was found by mea- suring the longitudinal and transversal resistivities of three films with transport current directed parallel, perpendicular and at an angle of 45° with respect to the facet ridges. Field inversion was used to separate the even and odd com- ponentsofthemeasuredmagnetoresistivitiesandthustoobtainthecontributionscausedbytheguidedvortexmotion. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Pinning; Vortex dynamics; Guiding The influence of anisotropic pinning defects on the vortex dynamics in superconductors attracts much at- tention from both theoretical [1,2] and experimental [3– 5]viewpoint.Vortexmotioncanbeinfluencedbyplanar pinning structures, a natural example being unidirected twin boundaries in YBa 2 Cu 3 O 7d . In the presence of such structures the pinning force is anisotropic: it is muchstrongerinthetransversedirectionwithrespectto the twin boundaries than in the parallel direction. This can result in so-called guided vortex motion, when vorticestendtomovealongthepinningplaneevenifthe external force acting on them is not aligned parallel to this plane. For this case theory predicts the appearance of two ÔnewÕ components in the galvanomagnetic re- sponse of the superconductor: an odd longitudinal and an even transversal (Hall) voltage with respect to mag- netic field reversal. These two components are absent in the isotropic superconductor. This was observed exper- imentally on twinned YBa 2 Cu 3 O 7d single crystals [4,5]. In this article we report guided motion in Nb epitaxially grown on facetted a-Al 2 O 3 ð10  10Þ. We emphasize the model character of these samples for the study of an- isotropic vortex pinning under well-controlled and turnable conditions. Three Nb-films with thickness of 390  A were grown during one deposition process on the facetted a-Al 2 O 3 substrate using molecular beam epitaxy techniques [6]. The films replicate the facetted substrate surface as shown in the inset of Fig. 1. The facet ridges act as pinning planes [6]. Micro-bridges were patterned by photolithography and ion-beam etching, having differ- ent orientations to the facet ridges. Three samples were prepared allowing measurements with the transport current directed at the angles a ¼ 0°, 45° and 90° with regard to the facet ridges. The dependencies of the lon- gitudinal and transversal voltages on temperature were measured simultaneously on the three samples with an average transport current density of 13 kA/cm 2 in fields of þ0.030 and )0.030 T. The magnetic field H was aligned perpendicular to the film surface, i.e. parallel to * Corresponding author. E-mail address: soroka@mail.uni-mainz.de (O.K. Soroka). 0921-4534/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-4534(02)02646-1 Physica C 388–389 (2003) 773–774 www.elsevier.com/locate/physc