Artificial ordered pinning in superconductors

The study of flux pinning in type II superconductors has shown a rich phenomenology, which is interesting from both the fundamental point of view and for its technological applications. The dynamics of vortex-lattices is useful to study the general problem of interacting “particles” (such a colloids, charge density waves, etc) or elastic media moving on different types of pinning potentials. E-beam lithography techniques allow us to obtain ordered arrays of magnetic nanodots with sizes and distances comparable to the superconducting coherence length x and the magnetic penetration depth l. The geometry of the pinning potential landscapes induced with these arrays can be designed at will, and thus a variety of different problems can be studied: commensurability, lattice correlation lengths, order-disorder transitions, ratchets, etc.

Fig. 1

For several years we have been studying the commensurability effects (observed in the magnetoresistance and in the critical current dependence with the applied field, see figure 1) induced by periodic arrays of magnetic nanodots in Nb thin films [2,3], and the different pinning mechanism involved (magnetic, proximity, structural) [4]. We have also investigated vortex-lattice directional motion guided by periodic potentials [5,6], which induce easy-flow channels that lock vortex motion along directions away from the Lorentz force. Our most recent work is dedicated to novel commensurability effects on quasiperiodic and fractal pinning potentials (Figure 2), which induce quasiperiodic vortex-lattices with unexpectedly long correlation lengths [7]. On going research lines include the study of the effect of the nanodot magnetic state and the magnetic order of the array on the pinning of vortices.

Fig. 2

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