Superconductor 3D Nanoarchitectures: Properties due to Complex Geometry and Nontrivial Topology

Extending nanostructures into the third dimension has become a major research avenue in condensed-matter physics, because of geometry- and topology-induced phenomena. Modern advances of high-tech fabrication techniques have allowed for generating geometrically and topologically nontrivial manifolds at the nanoscale, which determine novel, sometimes counterintuitive, electronic, magnetic, optical and transport properties of such objects and unprecedented potentialities for design, functionalization and integration of nanodevices due to their complex geometry and non-trivial topology [1]. In open superconductor Nb nanotubes with a submicron-scale inhomogeneity of the normal-to-the-surface component of the applied magnetic field, a topological transition between the vortex and phase-slip regimes determines the magnetic-field-voltage and current-voltage characteristics, which imply a possibility to efficiently tailor the superconducting properties of nanostructured materials by inducing a nontrivial topology of superconducting screening currents [2]. Most of peaks in the induced-voltage-magnetic-field characteristics appear in the presence of abrupt switch-on of the transport current or the magnetic field. Such an abrupt switch-on triggers the transition from the vortex to phase-slip regime. The dependence of the superconducting regimes on the switch-on speed and the stability of such regimes hints that there is a barrier between them. As the result, a novel hysteresis effect is unveiled in the current-voltage characteristic of open nanotubes [3]. The hysteresis loop is wider in current, but narrower in voltage for a stronger magnetic field. Normal conductivity has a crucial impact on the hysteresis effect in superconductor open nanotubes. Namely, a higher normal conductivity leads to the disappearance of the hysteresis effect. As a rule, the upper and lower branches of the hysteresis loop are provided by the phase-slip and vortex regimes, correspondingly. Just before the transition of the system from the upper branch to the lower one, the way of nucleation and annihilation of vortices in the region with suppressed superconductivity experiences readjustment [4]. Dynamic topological transitions in open superconductor nanotubes occur under a combined dc+ac transport current [5]. The key effect is a transition between two regimes of superconducting dynamics. The first regime is characterized by a pronounced first harmonic in the FFT spectrum of the induced voltage at the frequency of the ac current. It occurs when the dominant area of the open tube is superconducting at relatively low magnetic fields and/or weak dc currents - or normal at relatively high magnetic fields and/or strong dc currents. The second regime manifests a rich FFT spectrum of the induced voltage with pronounced multiple harmonics of the ac frequency because of an interplay between the internal dynamics of superconducting vortices or phase slips and the dynamics driven by the ac.