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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.

Flux flow was studied over an entire temperature range down to T{approx}2% of T{sub c} by using intense pulsed current densities to overcome flux-vortex pinning. The resistivity at high vortex velocities is proportional to B and roughly follows {rho}{approx}{rho}{sub n}B/H{sub c2} , with a prefactor of order unity. Contrary to some speculation, {rho}{sub n} saturates to a finite residual value as T{yields}0 , indicating a metallic ({rho}{yields}finite ) rather than insulating ({rho}{yields}{infinity}) normal state, and the vortex dissipation continues to be conventional as T{yields}0 . (c) 2000 The American Physical Society.

Vortex dynamics in molybdenum-germanium superconducting films were found to well approximate the unpinned free limit even at low driving forces. This provided an opportunity to empirically establish the intrinsic character of free flux flow and to test in detail the validity of theories for this regime beyond the Bardeen-Stephen approximation. Our observations are in good agreement with the mean-field result of time dependent Ginzburg-Landau theory. PACS: 74.25.Sv,74.25.Wx,74.25.Uv,74.25.Op,74.25.F- Keywords: vortices, fluxon, Larkin, Ovchinnikov, upper critical magnetic field, Comment: This is the final revised version of a paper that is currently in press. It is expected to appear in Phys. Rev. B in 2010

The mixed state response of disordered magnesium diboride has a con- spicous absence of the usual flux-density (B) dependent broadening of conductance- temperature and voltage-current curves. This B independence implies a vortex vis- cosity that rises linearly with B. We show that this this anomalous viscosity function can arise from the quasiparticle sea of the quenched weaker band and the unusually long electric field penetration depth that delocalizes the vortex electric fields.

The flux dynamics in magnesium diboride films becomes altered by scattering resulting in an unprecedented quenching of flux motion that is beyond conventional vortex pinning. At currents approaching the pair-breaking value and magnetic fields approaching the upper-critical field, the conductance diverges and the current-resistance and temperature–conductance curves become parallel, showing mainly a shift in the transition due to pair-breaking with minimal signs of flux motion.

The mixed-state transport behavior of magnesium diboride films shows a suppression of flux motion that is more severe than is typical for conventional vortex pinning. At currents approaching the pair-breaking value and magnetic fields approaching the upper-critical field, the conductance diverges and the current-resistance and temperature-conductance curves become parallel, showing mainly a shift in the transition due to pairbreaking with minimal signs of flux motion.

The transport of electrical current through a superconductor falls into three broad regimes: non-dissipative, dissipative but superconducting, and normal or non-superconducting. These regimes are demarkated by two definitions of critical current: one is the threshold current above which the superconductor enters a dissipative (resistive) state; the other is the thermodynamic threshold above which the superconductivity itself is destroyed and the superconducting order parameter vanishes. The first threshold defines the conventional critical current density Jc and the second defines the depairing (or pair-breaking) current Jd. Type II superconductors in the mixed state have quantized flux vortices, which tend to move when acted upon by the Lorentz driving force of an applied transport current. In such a mixed state the resistance vanishes only when vortices are pinned in place by defects and the applied current is below the threshold Jc required to overcome pinning and mobilize the vortices. Typically Jd >> Jc and a direct experimental measurement of Jd over the entire temperature range (0 < T < Tc) is prohibited by the enormous power dissipation densities (p ~ 10^10 -- 10^12 W/cm^3) needed to reach the normal state. In this work, intense pulsed signals were used to extend transport measurements to unprecedented power densities (p ~ 10^9 -- 10^10 W/cm^3). This together with MgB2's combination of low normal-state resistivity and high transition temperature have permitted a direct estimation of \jd over the entire temperature range. This review describes our experimental investigation of current-induced depairing in MgB2, and provides an introduction to the phenomenological theories of superconductivity and how the observations fit in their context., Accepted for publication as a Topical Review in the Journal of Physics: Condensed Matter (Fall 2004)

We have investigated the low-temperature (T < Tc=10) mixed-state current-voltage (IV) response of magnesium diboride films beyond the point where the superconductivity is completely destroyed and the system enters the normal state. The resistance-versus-current R(I) curves are extremely steep and featureless, with a critical current density j c, marking the onset of dissipation, that is unusually high (j c>j d/10) with respect to the depairing current density j d. At large flux densities Hc2/10 $\lessim$ B $\lessim$Hc2, the R(I) curve has a functional shape that is largely independent of B, indicating that the rise in resistivity with increasing current occurs mainly due to pair-breaking rather than flux motion. The macroscopic destruction current I*, which drives the system normal, has a B/Hc2: $\sqrt{B/H_{c2}}$ flux-density dependence, suggesting that the vortices mainly reduce the effective cross section over which a current of effective density j ~ j d flows.

We have conducted pulsed transport measurements on c-axis-oriented magnesium diboride films over the entire relevant ranges of magnetic field $0\ensuremath{\lesssim}H\ensuremath{\lesssim}{H}_{c2}$ (where ${H}_{c2}$ is the upper critical field) and current density $0\ensuremath{\lesssim}j\ensuremath{\lesssim}{j}_{d}$ (where ${j}_{d}\ensuremath{\sim}{10}^{7}\mathrm{A}/{\mathrm{cm}}^{2}$ is the depairing current density). The intrinsic disorder of the films combined with the large coherence length and three dimensionality, compared to cuprate superconductors, results in a sixfold enhancement of ${H}_{c2}$ (to $17\ifmmode\pm\else\textpm\fi{}3\mathrm{T})$ and raises the depinning current density ${j}_{c}$ to within an order of magnitude of ${j}_{d}.$ The current-voltage response has an unusually steep nonlinear shape at all fields, resulting from a combination of depinning and pair breaking, and has no trace of an Ohmic free-flux-flow regime.

The pair-breaking critical current density j_{d} of magnesium diboride was determined over its entire temperature range by a pulsed dc transport measurement. At fixed low values of current density j, the resistive transition temperature Tc shifts in the classic \Delta T_{C}(j)/T_{C}(0) \propto -[j/j_{d}(0)]^{2/3} manner, with a projected j_{d}(0) \approx 2 \times 10^{7}$ A/cm^{2}. The directly measured j_{d}(0), from current-voltage ($I$-$V$)curves at different fixed temperatures, has a similar value and the overall temperature dependence j_{d}(T) and magnitude are consistent with Ginzburg-Landau theory. Keywords: pair, breaking, depairing, superconductor, superconductivity, flux, fluxon, vortex, mgb2, Comment: Submitted to Phys. Rev. Lett. Related preprints/reprints by our group can be found on http://www.physics.sc.edu/kunchur

A model is presented that considers the magnetic relaxation from thermally activated flux creep in a superconductor, after the application of two consecutive field steps separated by a waiting time {ital t}{sub {ital w}}. The time evolution of the resulting superposition of flux distributions is expected to show a characteristic inflection point at a time {ital t}{sub {ital i}}{sup {prime}}. The behavior is reminiscent of, but fundamentally different from, the memory effect of the superconducting-glass model. Measurements on Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} single crystals show the predicted temperature and field dependence of {ital t}{sub {ital i}}{sup {prime}}.

A small amount of bismuth was added to granular aluminum in order to enhance spin-orbit scattering. The metal-insulator transition was found to be shifted in accord with the antilocalization effect on an Anderson transition. The threshold for superconductivity is also shifted, to the same extent, remaining, as in the undoped system, separated from the metal-insulator transition by a narrow range of composition in which the samples are metallic but not fully superconducting. The correlation of the shifts demonstrates that superconductivity is suppressed by the vicinity of the metal-insulator transition.

Granular aluminum near the metal-insulator transition has been found to have a range of metal concentrations over which it is metallic (M) but not superconducting (S). For higher concentrations it is superconducting and at lower concentrations insulating (I). There is a striking resemblance of the temperature dependence of resistivity in the S,M, and I ranges of granular aluminum to the temperature dependence of sheet resistance in very thin quench-condensed films. However there is as yet no theory that gives a satisfactory explanation of this behavior in both two- and three-dimensional systems.

Spin-orbit scattering is enhanced in granular aluminum by the addition of small amounts of bismuth. The metal-insulator transition is displaced to larger values of room-temperature resistivity. The superconducting-normal threshold is displaced by an approximately equal amount.

Incomplete superconducting transitions, often called quasireentrant, have been observed in the resistivity of three-dimensional granular aluminum very close to the metal-insulator transition. An even more pronounced signature of these transitions is evident in the magnetic field dependence at temperatures well below the superconducting transition temperature. The effect in the temperature dependences is similar to behavior observed in discontinuous thin films which has been attributed to macroscopic quantum tunneling. No corresponding measurements or theoretical predictions of the magnetic field dependence have yet been reported.

By changing the deposition parameters 3D granular aluminum (Al-Al2O3) can be prepared with different variations of Tc as a function of resistivity. As the metal-insulator transition is approached Tc decreases gradually to a limiting value independent of the deposition, beyond which complete superconducting transitions are no longer observed. The behavior in Al-Al2O3 is quite different from that observed in Al-Ge (with grain sizes 4 or 5 times larger), where Tc drops abruptly and reaches much lower values. The difference is in accord with the considerations of Deutscher, Goldman, and Micklitz of the competition between localization and percolation.