We fabricated a device that controls the motion of flux quanta in a niobium superconducting film grown on an array of nanoscale triangular pinning potentials. The controllable rectification of the vortex motion is due to the asymmetry of the fabricated magnetic pinning centers. The reversal in the direction of the vortex flow is explained by the interaction between the vortices trapped on the magnetic nanostructures and the interstitial vortices. The applied magnetic field and input current strength can tune both the polarity and magnitude of the rectified vortex flow. Our ratchet system is explained and modeled theoretically, taking the interactions between particles into consideration.

No abstract

No abstract

We study the interplay between magnetism and superconductivity in high-quality YBa 2 Cu 3 O 7 (YBCO)/La 0.7 Ca 0.3 MnO 3 (LCMO) superlattices. We find evidence for the YBCO superconductivity depression in the presence of the LCMO layers. We show that due to its short coherence length, superconductivity survives in the YBCO down to a much smaller thickness in the presence of the magnetic layer than in low T c superconductors. We also find that for a fixed thickness of the superconducting layer, superconductivity is depressed over a thickness interval of the magnetic layer in the 100 nm range. This is a much longer length scale than that predicted by the theory of ferromagnetic/superconducting proximity effect. DOI: 10.1103/PhysRevB.67.214511 PACS number͑s͒: 74.78.Fk, 74.50.ϩr, 75.70.Cn The ferromagnetic (F)/superconducting ͑S͒ proximity effect has been a subject of intense research in recent years due to the rich variety of phenomena resulting from the competition between both long range orderings. In this context F/S superlattices have been extensively used in the past because they offer the possibility of tailoring individual thicknesses or modulation length to match characteristic length scales governing ferromagnetism, superconductivity, or their interaction. Most research in this field has involved single element or alloy-based metallic superlattices.1-9 The extension of concepts of the F/S proximity effect to the high-T c superconductors ͑HTS͒ or colossal magnetoresistance ͑CMR͒ oxides is of primary interest since peculiarities like the short superconducting coherence length and full spin polarization could open the door to interesting new effects. Although there has been a theoretical effort recently to examine the F/S interface in oxides, 10 to the best of our knowledge experimental results on the F/S proximity effect are lacking in the literature. In this paper we examine the interplay between magnetism and superconductivity in YBa 2 Cu 3 O 7 (YBCO)/La 0.7 Ca 0.3 MnO 3 (LCMO) superlattices and provide evidence for superconductivity depression due to the presence of magnetic layers. YBCO and LCMO have oxide perovskite structure with very similar in-plane lattice parameters, which allows the growth of superlattices with sharp interfaces, thus strongly reducing extrinsic ͑structural͒ effects which otherwise could obscure the F/S interplay.At the F/S interface, Cooper pairs entering the ferromagnet from the superconductor experience the exchange interaction, which favors one of the spin orientations. This causes the superconducting order parameter to decay in the F layer faster than in a normal metal, within a length scale F ϭបv F /⌬E ex ͑where v F is the Fermi velocity and ⌬E ex is the exchange splitting͒. In single element or alloy ferromagnets, for typical values of ⌬E ex ϭ1 eV and v F of 10 8 cm/s, F is of the order of 1 nm ͑Ref. 3͒, which is shorter than the superconducting coherence length of the low-temperature superconductors ͑usually larger than 10 nm͒. Superconductivity is also depressed in the...

The recent development in the fabrication of artificial oxide heterostructures opens new avenues in the field of quantum materials by enabling the manipulation of the charge, spin and orbital degrees of freedom. In this context, the discovery of two-dimensional electron gases (2-DEGs) at LaAlO3/SrTiO3 interfaces, which exhibit both superconductivity and strong Rashba spin-orbit coupling (SOC), represents a major breakthrough. Here, we report on the realisation of a field-effect LaAlO3/SrTiO3 device, whose physical properties, including superconductivity and SOC, can be tuned over a wide range by a top-gate voltage. We derive a phase diagram, which emphasises a field-effect-induced superconductor-to-insulator quantum phase transition. Magneto-transport measurements show that the Rashba coupling constant increases linearly with the interfacial electric field. Our results pave the way for the realisation of mesoscopic devices, where these two properties can be manipulated on a local scale by means of top-gates.

Conventional superconductivity is incompatible with ferromagnetism, because the magnetic exchange field tends to spinpolarize electrons and breaks apart the opposite-spin singlet Cooper pairs 1. Yet, the possibility of a long-range penetration of superconducting correlations into strong ferromagnets has been evinced by experiments that found Josephson coupling between superconducting electrodes separated afar by a ferromagnetic spacer 2-7. This is considered a proof of the emergence at the superconductor/ferromagnetic (S/F) interfaces of equalspin triplet pairing, which is immune to the exchange field and can therefore propagate over long distances into the F (ref. 8). This effect bears much fundamental interest and potential for spintronic applications 9. However, a spectroscopic signature of the underlying microscopic mechanisms has remained elusive. Here we do show this type of evidence, notably in a S/F system for which the possible appearance of equal-spin triplet pairing is controversial 10-12 : heterostructures that combine a half-metallic F (La 0.7 Ca 0.3 MnO 3) with a d-wave S (YBa 2 Cu 3 O 7). We found quasiparticle and electron interference effects in the conductance across the S/F interfaces that directly demonstrate the long-range propagation across La 0.7 Ca 0.3 MnO 3 of superconducting correlations, and imply the occurrence of unconventional equal-spin Andreev reflection. This allows for an understanding of the unusual proximity behaviour observed in this type of heterostructures 12,13. The proximity effect, usually described as the penetration or 'leakage' of the superconducting condensate from a S into an overlaying normal metal (N), is on a microscopic level the result of two processes. The first one is the Andreev reflection 14 , through which a normal electron incident into the S/N interface is paired with an electron inside the Fermi sea by the S energy gap, leaving a hole excitation that propagates backwards from the interface. In the conventional picture, the incident electron and the reflected hole must have opposite spins. The second process is the coherent propagation into the N material of the resulting hole/electron phase-conjugated pair 15. The latter carries the superconducting correlations into the N, leading to a finite condensation amplitude over a certain length scale ξ N , as schematically shown in Fig. 1a. In the N, such coherent propagation is limited only by the usual dephasing mechanisms and diverges at zero temperature (T): for diffusive systems ξ N = √h D/2π KT and for ballistic ones ξ N =hv F /2π KT , where D is the electronic diffusion constant, K is the Boltzmann constant and v F is the Fermi velocity 15. In clean metals, at low temperatures, ξ N can be micrometres long. If the LETTERS NATURE PHYSICS

Using heterostructures that combine a large-polarization ferroelectric (BiFeO3) and a high-temperature superconductor (YBa2Cu3O(7-δ)), we demonstrate the modulation of the superconducting condensate at the nanoscale via ferroelectric field effects. Through this mechanism, a nanoscale pattern of normal regions that mimics the ferroelectric domain structure can be created in the superconductor. This yields an energy landscape for magnetic flux quanta and, in turn, couples the local ferroelectric polarization to the local magnetic induction. We show that this form of magnetoelectric coupling, together with the possibility to reversibly design the ferroelectric domain structure, allows the electrostatic manipulation of magnetic flux quanta.

A vortex lattice ratchet effect has been investigated in Nb films grown on arrays of nanometric Ni triangles, which induce periodic asymmetric pinning potentials. The vortex lattice motion yields a net dc voltage when an ac driving current is applied to the sample and the vortex lattice moves through the field of asymmetric potentials. This ratchet effect is studied taking into account the array geometry, the temperature, the number of vortices per unit cell of the array, and the applied ac currents. DOI: 10.1103/PhysRevB.71.024519 PACS number͑s͒: 74.78.Ϫw, 05.60.Ϫk, 74.40.ϩk Feynman used, in his Lectures on Physics, 1 a ratchet to show how anisotropy never could lead to net motion in an equilibrium system. Since then, asymmetric sawtooth potentials are called ratchet potentials and, in general, a device with broken inversion symmetry is called a ratchet device. The ratchet effect occurs when asymmetric potentials induce outward particle flow under external fluctuations in the lack of any driving direct outward forces. The ratchet effect changes an ac source in a dc one. Ratchet effect spans from Nature phenomena to laboratory fabricated devices. In a ratchet, the energy necessary for net motion is provided by raising and lowering the barriers and wells, either via an external time-dependent modulation, for example an ac current injected in a superconducting film with asymmetric pinning centers, 2 or by energy input from a nonequilibrium source, such as a chemical reaction, as for instance in biological motors. 3 During the past years, ratchet effect has called the attention of many researchers. A state of the art on the related topics Brownian motion and ratchet potential could be found in Ref. 4.The use of ratchetlike pinning potentials in superconductors has been the subject of theoretical approaches which deal with very different topics, for instance, to remove flux trapped in superconducting devices, 5 fluxon optic, 6 logic devices, 7 etc. From the experimental point of view, some progress has been reported related to superconducting circuits, [8][9][10] and very recently vortex motion ratchet effect has been reported in superconducting films with artificially fabricated arrays of asymmetric pinning centers. 2 In the present paper, we will address some of the properties of this superconducting ratchet effect. We will explore the dependence of the ratchet with the applied alternating current, the array shape, the temperature, and the number of vortices per array unit cell. We will show that periodic asymmetric potentials are crucial to produce the ratchet behavior, that the effect is enhanced decreasing the temperature, and finally, that the effect decreases when the applied magnetic field ͑number of vortices per unit cell of the array͒ increases. The paper is organized as follows: First, we will summarize some results on the behavior of vortex lattice on artificially induced pinning potentials. After this, we will present the fabrication method and main characteristics of the films. Finally, the experim...