Commensurability Effects in Superconducting Nb Films with Quasiperiodic Pinning Arrays

Abstract

We study experimentally the critical depinning current Ic versus applied magnetic field B in Nb thin films which contain 2D arrays of circular antidots placed on the nodes of quasiperiodic (QP) fivefold Penrose lattices. Close to the transition temperature Tc we observe matching of the vortex lattice with the QP pinning array, confirming essential features in the Ic(B) patterns as predicted by Misko et al. [Phys. Rev. Lett. 95(2005)]. We find a significant enhancement in Ic(B) for QP pinning arrays in comparison to Ic in samples with randomly distributed antidots or no antidots.PACS numbers: 74.25. Qt, 74.25.Sv, 74.70.Ad, 74.78.Na The formation of Abrikosov vortices in the mixed state of type-II superconductors [1] and their arrangement in various types of "vortex-phases", ranging from the ordered, triangular Abrikosov lattice to disordered phases [2,3,4] has a strong impact on the electric properties of superconductors. Both, in terms of device applications and with respect to the fundamental physical properties of so-called "vortex-matter", the interaction of vortices with defects, which act as pinning sites, plays an important role. Recent progress in the fabrication of nanostructures provided the possibility to realize superconducting thin films which contain artificial defects as pinning sites with well-defined size, geometry and spatial arrangement. In particular, artificially produced periodic arrays of submicron holes (antidots) [5,6,7,8] and magnetic dots [9,10,11,12] as pinning sites have been intensively investigated during the last years, to address the fundamental question how vortex pinning -and thus the critical current density j c in superconductors -can be drastically increased.In this context, it has been shown that a very stable vortex configuration, and hence an enhancement of the critical current I c occurs when the vortex lattice is commensurate with the underlying periodic pinning array. This situation occurs in particular at the so-called first matching field B 1 = Φ 0 /A, i.e., when the applied field B corresponds to one flux quantum Φ 0 = h/2e per unit-cell area A of the pinning array. In general, I c (B) may show a strongly non-monotonic behavior, with local maxima at matching fields B m = mB 1 (m: integer or a rational number), which reflects the periodicity of the array of artificial pinning sites.As pointed out by Misko et al. [13], an enhancement of I c occurs only for an applied field close to matching fields, which makes it desirable to use artificial pinning arrays with many built-in periods, in order to provide either very many peaks in I c (B) or an extremely broad peak in * Electronic address: koelle@uni-tuebingen.de I c (B). Accordingly, Misko et al. studied analytically and by numerical simulation vortex pinning by quasiperiodic chains and by 2D pinning arrays, the latter forming a fivefold Penrose lattice [14], and they predicted that a Penrose lattice of pinning sites can provide an enormous enhancement of I c , even compared to triangular and random pinning arrays.We ...

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