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The search for new superconducting materials approaching room temperature benefits from having a variety of testing methodologies to confirm and characterize the presence of superconductivity. Often the first signatures of new superconducting species occur incompletely and in very small volume fractions. These trace amounts may be too weak to produce an observable Meissner effect and the resistance may not go completely to zero if the percolation threshold is not met. Under these conditions, secondary behavior--such as transitions or cross overs in the temperature dependence of magnetoresistance, magnetic irreversibility, or thermopower--are often used as indications for the presence of superconductivity. Our group has developed a rather unique set of fast-timescale and dissipative transport measurements that can provide another tool set for confirming and characterizing suspected superconductivity. Here we provide some background for these methods and elucidate their collaborative value in the search for new superconducting materials. Keywords: pairbreaking, pair-breaking, vortex, vortices, theory, tutorial, RTS, room-temperature superconductivity, superconductor, detection, characterization, 8 pages, 9 figures, presented at the "2nd International Workshop on Towards Room Temperature Superconductivity: Hydrides and More

We investigated the dissipative regime of the Bi2Te3/FeTe topological insulator-chalcogenide interface superconductor at temperatures well below the Berezinski-Kosterlitz-Thouless transition. We observe a transition in the current-resistance and temperature-resistance curves that quantitatively agrees with the Likharev vortex-explosion phenomenon. In the limit of low temperatures and high current densities, we were able to demonstrate the regime of complete vortex-antivortex dissociation arising from current driven vortex-antivortex pair breaking.

Absence of macroscopic resistance is the most essential trait of superconductors for their practical applications. Yet several mechanisms lead to dissipation in the superconducting state such as weak links, fluctuations, and the motion of flux vortices in the mixed state of type II materials. Thus it is of practical as well as fundamental interest to explore these mechanisms to the fullest extent and to learn how to control them. Regarding the control of flux motion, most research efforts aim at preventing flux motion by introducing a variety of pinning strategies. In the present work, we discuss the circumstances under which it is possible to exclude vortices in the first place, or to short their motion through intrinsic multi-band effects. Experimental results on two model systems, molybdenum-germanium and magnesium diboride films, are presented to support these strategies.

The Bi2Te3/FeTe heterostructure intersects several phenomena and key classes of materials in condensed matter physics: topological insulators, superconductivity, magnetism, and the physics of interfaces. While neither the topological insulator (Bi2Te3) nor the iron chalcogenide (FeTe) are themselves superconductors, superconductivity forms in a thin 7nm interfacial layer between the two. The restricted dimensionality and the extraordinarily conductive normal state, possibly sourced by the topologically protected surface states, have led to the observation of novel phenomena such as the Likharev vortex explosion and transitions in behavior resulting from the interplay between current induced depairing and the Berezinski-Kosterlitz-Thouless regime. Bi2Te3/FeTe also displays the anomalous oscillatory magnetoresistance phenomenon, which we had previously observed in cuprates.

We report oscillatory magnetoresistance in various superconducting films, with a magnetic-field period $\mathrm{\ensuremath{\Delta}}B\ensuremath{\sim}0.1$ T that is essentially independent of sample dimensions, temperature, transport current, and the magnitude and orientation of the magnetic field, including magnetic fields oriented parallel to the film plane. The characteristics of these oscillations seem hard to reconcile with previously established mechanisms for oscillations in magnetoresistance, suggesting the possibility of another type of physical origin.

We investigated current induced depairing in the ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}/\mathrm{FeTe}$ topological insulator-chalcogenide interface superconductor. The measured depairing current density provides information on the magnetic penetration depth and superfluid density, which in turn shed light on the nature of the normal state that underlies the interfacial superconductivity.

Many interesting phenomena and regimes occur when the magnetic field B is oriented parallel to a superconducting film of thickness d much less than the penetration depth λ. The negligible demagnetization effects, together with the nearly complete field penetration, lead to a very large threshold field B c1 ∥ for vortex entry, which is much larger than the bulk B c1. Furthermore, orbital pair breaking is suppressed in parallel fields so that overlap of vortex cores is not the determining condition for the parallel upper critical field B c2 ∥ , but Pauli paramagnetic pair breaking is. In this case B c2 becomes very large and you have novel possibilities such as the Fulde–Ferrell–Larkin–Ovchinnikov phase (FFLO). Finally, when the cross section facing the magnetic field is reduced below a threshold value, Abrikosov vortex cores cease to exist. These features make the determination of the coherence length in the direction of the film thickness problematic. This work demonstrates transport techniques that allow a determination of this important fundamental length through experimental confirmation of the condition when Abrikosov vortices cease to exist.

The electrical transport properties of c-axis oriented Nd1.85Ce0.15CuO4 ? ? superconducting films have been investigated to analyze the pinning mechanism in this material. The samples were grown on SrTiO3 substrates using the dc sputtering high-pressure technique, whereas a detailed analysis of the structure and local composition of the films has been achieved using high-resolution electron microscopy and x-ray microanalysis. Magneto-resistance and current-voltage measurements, in the temperature range from 1.6 to 300 K and in magnetic field up to 9 T, have been reported. In particular, the anisotropic coefficient defined as the ratio between the parallel upper critical field, ab, and the perpendicular one, c, has been evaluated, pointing out the high anisotropy of this compound. Furthermore, the vortex activation energy as a function of the applied magnetic field, parallel and perpendicular to the CuO2 planes, has been derived and compared with the flux-pinning forces to enlighten the peculiar nature of pinning centers in this material.

We report a measurement of the depairing critical current density in the electron-doped Nd 2− x Ce x CuO 4− δ cuprate superconductor. Resistance-versus-temperature transition curves measured at high pulsed current densities show the classic proportionality between the transition-temperature shift and the two-thirds power of the applied current. The measurement provides an alternative method for obtaining the penetration depth purely through transport measurements that is not affected by the large paramagnetic background that arises from the Nd 3+ ions, which affects inductive investigations of this quantity.