2, Universita degli Studi di Salerno, Fisciano, , Italy
Nowadays 1D semiconducting nanowire (NW) networks attracted significant interest because of their potential applications in the single-photon detectors, quantum-computing circuits and SQUID-like sensors. There are several approaches to fabricate 1D semiconducting NW network exist. In bottom-up approach ultra thin film of superconducting material deposited on top of the porous template with nanometer sized cells – porous alumina or porous silicon (PS), for example – and due to the very small thickness deposited material cover only surface of the template forming ordered network of superconducting NWs. However in another approach arrays of ferromagnetic NWs could be introduced in ordered porous templates providing regular magnetic arrays with characteristic dimensions, i.e. diameter of the NWs and nearest neighbor distance, in the nanometer range. Ordered array of ferromagnetic NWs serves as a source of periodic magnetic pinning centers in the superconducting film and matching effects between the artificial non-superconducting regular structure and the vortex lattice can be observed. The system behaves like a superconducting NW network when the coherence length is of the same order as the width of the channels between the non-superconducting areas. In this work we investigate the superconducting properties of a 30 nm thick Nb film sputtered on composite material consisting of an array of Ni NWs grown into a PS template. Nb film was electrically isolated from the ferromagnetic composite by an 8 nm insulating alumina layer.
Study of Tc(H) phase diagrams constructed by measuring resistive transitions of obtained samples in various perpendicular magnetic fields for several resistive transition criteria of the normal state resistance (10 – 90 % of normal resistance) revealed pronounced anomaly in the magnetic field values range 0.19-0.23 T where the critical temperature apparently increases with respect to what is conventionally expected, reaching a value that is almost equal to zero field critical temperature. Similar anomaly with lower amplitude is also present at higher fields, in the range 0.53-0.64 T. Samples were Ni NWs electrically interconnected with each other do not show such behavior. Measurements of magnetoresistance and voltammetric characteristics revealed that observed matching effects are more likely caused by the Little-Parks effect more than by the vortex pinning. Therefore, the system behaves like a 1D superconducting NW network where the confinement of the Cooper pairs is not caused by geometrical constrictions, but rather by the stray fields of the magnetic dipoles that modify the topology of the plain film creating regions where the superconductivity is strongly depressed.
This work is supported by Belarusian Republican Foundation for Fundamental Research, grant T16M-012.