Milind N. Kunchur

Unstable flux flow due to heated electrons in superconducting films.

A flux instability occurs in superconductors at low temperatures, where e-e scattering is more rapid than e-ph, whereby the dissipation significantly elevates the electronic temperature while maintaining a thermal-like distribution function. The reduction in condensate and rise in resistivity produce a nonmonotonic current-voltage response. In contrast to the Larkin-Ovchinnikov instability where the vortex shrinks, in this scenario the vortex expands and the quasiparticle population rises. Measurements in Y(1)Ba(2)Cu(3)O(7-delta) agree quantitatively with the distinct predictions of this mechanism.

Hysteretic internal fields and critical currents in polycrystalline superconductors

The transport critical current Jc in a polycrystalline superconductor is a hysteretic function of applied magnetic field H0 due to flux trapping by grains. This effect has been observed by several groups and attempts have been made to calculate the intergranular field Hi as a function of the applied H0 in terms of an effective geometrical demagnetization factor D. In general a first-principles calculation of D is very difficult, and furthermore, D is not constant but is itself a hysteretic function of H0. We develop a self-consistent scheme to extract the D and Hi directly from the Jc(H0) data itself. Our model exploits the fact that there are two field ranges for which the demagnetizing field is a simple function of H0. At low virgin fields, in the Meissner state, the susceptibility χv≈−1/4π is well defined, leading to a multiplicative correction: Hi=H0/(1+D4πχv). For fields that have returned from high values, a complete critical state is established and M is well defined—although χv is unknown. This le...

NOVEL TRANSPORT BEHAVIOR FOUND IN THE DISSIPATIVE REGIME OF SUPERCONDUCTORS

As the transport current density J in a superconductor is increased beyond its critical value Jc, dissipation sets in resulting in a finite resistivity. The superconducting state itself, however, persists up to the pair-breaking value Jd, where the kinetic energy associated with the current overcomes the condensation energy. Within this dissipative regime between Jc and Jd, the transport behavior displays a series of interesting physical phenomena (free flux flow, pair-breaking effect, etc.) as the Lorentz force and kinetic energy associated with the current sequentially overcome various intrinsic (e.g., pair-condensation and interlayer-coupling) and extrinsic (e.g., flux pinning) interactions relevant to the superconducting state. This review discusses these phenomena in the context of recent experiments where the existence of these effects was demonstrated in high-Tc superconductors for the first time. Also described are the general principles of the pulsed-current technique used to extend transport measurements to the required high current densities and power-dissipation levels.

Current-Induced Pair Breaking in Magnesium Diboride

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 demarcated 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 Δ vanishes. The first threshold defines the conventional critical current density j c and the second defines the depairing (or pair-breaking) current j d . 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 j c required to overcome pinning and mobilize the vortices. Typically j d » j c and a direct experimental measurement of j d over the entire temperature range (0 ≤ T ≤ T c ) 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 MgB 2 s combination of low normal-state resistivity (ρ n ) and high transition temperature (T c ) have permitted a direct estimation of j d over the entire temperature range. This review describes our experimental investigation of current-induced depairing in MgB 2 , and provides an introduction to the phenomenological theories of superconductivity and how the observations fit in their context.

Pair-breaking critical current density of magnesium diboride

We report the investigation of the pair-breaking current density j d in magnesium diboride. At low current densities j, the transition temperature Tc shifts in the classic DTc( j)/Tc(0)}2@ j/ j d(0)# 2/3 manner, with a projected j d(0)’2310 7 A/cm 2 . Current-voltage curves at fixed temperatures yield a similar value for j d(0), with an overall j d(T) dependence consistent with Ginzburg-Landau theory. To our knowledge this is the first complete investigation of j d(T) down to T’0 in any superconductor.

Evaluating free flux flow in low-pinning molybdenum-germanium superconducting films.

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.

Pinning mechanism in electron-doped HTS Nd

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.

_{1.85}

We studied the high driving force regime of the current-voltage transport response in the mixed state of amorphous molybdenum-germanium superconducting films to the point where the flux flow becomes unstable. The observed nonlinear response conforms with the classic Larkin-Ovchinikov (LO) picture with a quasiparticle-energy-relaxation rate dominated by the quasiparticle recombination process. The measured energy relaxation rate was found to have a magnitude and temperature dependence in agreement with theory. PACS: 74.40.Gh, 74.25.Uv, 72.15.Lh, 73.50.Gr, 73.50.Fq Keywords: fluxon, vortices, TDGL, FFF, negative differential conductivity, NDC, non-monotonic IV curve

Ce

Flux flow was studied over an entire temperature range down to T approximately 2% of T(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 approximately rho(n)B/H(c2), with a prefactor of order unity. Contrary to some speculation, rho(n) saturates to a finite residual value as T-->0, indicating a metallic (rho-->finite) rather than insulating (rho-->infinity) normal state, and the vortex dissipation continues to be conventional as T-->0.

_{0.15}

A pulsed‐current technique was used to measure the extended I–V characteristics of a wide variety of prototype high‐temperature‐superconductor (HTS) leads. It was found that the average resistivity rises with J(≳Jc) more gradually than in conventional superconductors—often remaining very small compared to silver, for values of J(≫Jc) that are high enough to be practically useful. This observation, combined with the low thermal conductivity (∼50 times smaller than Ag), should extend the utility of HTS leads to the dissipative regime where J/Jc≫1.