Vortex-slip transitions in superconducting a-NbGe mesoscopic channels N. Kokubo,* T. G. Sorop, † R. Besseling, ‡ and P. H. Kes Kamerlingh Onnes Laboratory, Leiden University, P.O. Box 9504, 2300 RA, Leiden, The Netherlands Received 27 February 2006; revised manuscript received 3 May 2006; published 16 June 2006 Intriguing and novel physical aspects related to the vortex flow dynamics have been recently observed in mesoscopic channel devices of a-NbGe with NbN channel edges. In this work we have systematically studied the flow properties of vortices confined in such mesoscopic channels as a function of the magnetic field history, using dc-transport and mode-locking ML measurements. As opposed to the field-down situation, in the field-up case a kink anomaly in the dc I-V curves is detected. The mode-locking measurements reveal that this anomaly is, in fact, a flow induced vortex slip transition: by increasing the external drive either dc or ac a sudden change occurs from n to n + 2 moving vortex rows in the channel. The observed features can be explained in terms of an interplay between field focusing due to screening currents and a change in the predominant pinning mechanism. DOI: 10.1103/PhysRevB.73.224514 PACS numbers: 74.25.Qt, 74.78.Na, 74.25.Fy, 83.50.Ha I. INTRODUCTION Vortex matter in type II superconductors is an ideal sys- tem for studying the motion of periodic media in pinning environments. In recent years, we concentrated our investi- gations on the properties of confined vortex matter in weak pinning mesoscopic channels, which were nanofabricated in double layers of amorphous a-NbGe weakly pinned and NbN strongly pinned thin films. 1,2 Other studies of vortex flow through channels bounded by pinned vortices have been performed in several experimental configurations, e.g., Jo- sephson vortices at low-angle grain boundaries in high-T c superconductors moving through pinned Abrikosov vor- tices. 3,4 Moreover, easy flow channels for Abrikosov vortices were artificially fabricated by using antidot arrays 5,6 or by laser processing. 7 Our system offers the possibility to study the flow properties systematically for both plastic and defec- tive vortex configurations, while continuously tuning the vor- tex lattice structure inside the channels by changing the ap- plied magnetic field. Novel physical aspects related to the commensurability have been recently observed in such chan- nel devices. 8,9 These include unusual oscillations of critical current and flow resistance with magnetic field. 2 The rela- tionship between those oscillations and the commensurabil- ity has been elucidated by mode-locking ML experi- ments. 2,10,11 Furthermore, using the ML technique important information about vortex flow has been obtained and it was possible to detect the dynamic ordering/melting transition predicted by Koshelev and Vinokur. 12 So far our studies were carried out in field-down FD or field-cooled FC conditions. Under these conditions the screening currents along the channel walls are weak and do not play a significant role. However, in the field-up FU case these screening currents cannot be ignored giving rise to in- teresting magnetic field history effects. In this work we ex- plore the consequences of such effects on the dynamics of the confined vortices. It turns out that the screening currents generate a particular vortex configuration and field distribu- tion around the channel which can be characterized by field focusing. This effect is especially strong at the lowest fields. A consequence of this distinct configuration is an entirely different behavior of the flow dynamics compared to that in the field-down case. Using dc-transport, mode-locking, and flux flow resistivity measurements we performed a system- atic study of the vortex dynamics for increasing fields. We observe a change from weak intrinsic pinning to strong shear interaction determining the pinning mechanism and the dy- namic properties. We find compelling evidence for a flow induced vortex slip transition between the vortex rows inside the same channel and also a crossover from one dimensional 1D to two dimensional 2D in the vortex flow behavior. The 2D vortex flow is the Bardeen-Stephen type of flow, in which the flux-flow resistance has a linear dependence in H, whereas the 1D is characterized by a  H dependence. This issue is quite relevant in the view of recent works on con- fined low dimensional flow, for instance, the depinning of a classic quasi-one-dimensional Wigner crystal. 13 The paper is structured as follows. In Sec. II a few details are given about the samples fabrication and geometry, as well as the experimental techniques used. In Sec. III the re- sults of the measurements are presented followed by an ex- tensive discussion. The critical current data are used to pro- pose a simple physical model based on the field focusing effect. Then the model is substantiated by mode-locking and flux flow resistivity experiments. Finally, in Sec. IV the con- clusions are presented. II. EXPERIMENTAL DETAILS The samples used in this study consist of easy flow vortex channels see the inset of Fig. 1. They are fabricated using a similar recipe to the one used in Ref. 1. First double layers consisting of weak pinning a-Nb 1-x Ge x film thickness d =550 nm and x  0.3 and strong pinning NbN film d =50 nm are rf-sputtered successively without breaking the vacuum. Then reactive ion etching, with proper masking, was used to remove the top layer and to make deep trenches down to the middle of the bottom layer. 1 As a result, the remaining NbGe layer in the channels is d ch  300 nm thick. Identical straight channels 300 in parallel, with lengths of 300 m, and at 10 m spacing were fabricated. Applying magnetic field perpendicular to the films induces PHYSICAL REVIEW B 73, 224514 2006 1098-0121/2006/7322/2245148 ©2006 The American Physical Society 224514-1