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 ...

We investigate experimentally a Josephson vortex ratchet -a fluxon in an asymmetric periodic potential driven by a deterministic force with zero time average. The highly asymmetric periodic potential is created in an underdamped annular long Josephson junction by means of a current injector providing efficiency of the device up to 91%. We measured the ratchet effect for driving forces with different spectral content. For monochromatic high-frequency drive the rectified voltage becomes quantized. At high driving frequencies we also observe chaos, sub-harmonic dynamics and voltage reversal due to the inertial mass of a fluxon. The ratchet effect, i.e., the net unidirectional motion of a particle in a spatially asymmetric periodic potential in the presence of deterministic or stochastic forces with zero time average, received a lot of attention during the 20-th century. The second law of thermodynamics does not allow to extract useful work out of equilibrium thermal fluctuations, as was didactically demonstrated by Feynman [1]. Thus, the only way to produce useful work is to supply non-native fluctuations (usually colored noise), which is the basic principle of operation for any ratchet.Particularly during the last decade ratchets were receiving a lot of attention [2,3,4]. Several new implementations, in particular based on the motion of the Josephson phase in SQUIDs [5] or vortices in long Josephson junctions (LJJ) [6,7,8,9] or Josephson junction arrays (JJA) [10,11,12], were suggested and tested. The investigation of quantum ratchets [13,14,15], i.e., a quantum particle moving/tunneling quantum mechanically in an asymmetric potential, is a fascinating new field not very well developed up to now especially experimentally. Advantages of Josephson junction based ratchets are: (I) directed motion results in an average dc voltage which is easily detected experimentally; (II) Josephson junctions are very fast devices which can operate (capture and rectify noise) in a broad frequency range from dc to ∼ 100 GHz, thus capturing a lot of spectral energy; (III) by varying junction design and bath temperature both overdamped and underdamped regimes are accessible; and (IV) one can operate Josephson ratchets in the quantum regime [15].In this letter we investigate experimentally the deterministic underdamped Josephson vortex ratchet (JVR), in which a Josephson vortex (fluxon) moves along a LJJ. We implemented a novel, effective way to construct a strongly asymmetric potential by means of a current injector and systematically study a quasi-statically driven ratchet with different spectral content of the driver. For non-adiabatic drive we observe quantized rectification, voltage reversal, sub-harmonic, and chaotic dynamics. Our system can be described by the following perturbed sine-Gordon equation [6] φwhere φ is the Josephson phase, the curvilinear coordinate x along the LJJ and the time t are normalized to the Josephson penetration depth λ J and inverse plasma frequency ω −1 p , accordingly, α is the dimensionless damping pa...

We investigate the creation of an arbitrary κ-discontinuity of the Josephson phase in a long NbAlOx-Nb Josephson junction (LJJ) using a pair of tiny current injectors, and study the formation of fractional vortices formed at this discontinuity. The current Iinj, flowing from one injector to the other, creates a phase discontinuity κ ∝ Iinj. The calibration of injectors is discussed in detail. The small but finite size of injectors leads to some deviations of the properties of such a 0-κ-LJJ from the properties of a LJJ with an ideal κ-discontinuity. These experimentally observed deviations in the dependence of the critical current on Iinj and magnetic field can be well reproduced by numerical simulation assuming a finite injector size. The physical origin of these deviations is discussed.

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