High Current Superconductivity Research Articles

High current superconductivity in FeSe0.5Te0.5-coated conductors at 30 tesla

Although high-temperature superconductor cuprates have been discovered for more than 25 years, superconductors for high-field application are still based on low-temperature superconductors, such as Nb(3)Sn. The high anisotropies, brittle textures and high manufacturing costs limit the applicability of the cuprates. Here we demonstrate that the iron superconductors, without most of the drawbacks of the cuprates, have a superior high-field performance over low-temperature superconductors at 4.2 K. With a CeO(2) buffer, critical current densities >10(6) A cm(-2) were observed in iron-chalcogenide FeSe(0.5)Te(0.5) films grown on single-crystalline and coated conductor substrates. These films are capable of carrying critical current densities exceeding 10(5) A cm(-2) under 30 tesla magnetic fields, which are much higher than those of low-temperature superconductors. High critical current densities, low magnetic field anisotropies and relatively strong grain coupling make iron-chalcogenide-coated conductors particularly attractive for high-field applications at liquid helium temperatures.

State of the art in applied high-current superconductivity

State of the art in applied high-current superconductivity

Sostoyanie rabot po sil'notochnoi prikladnoi sverkhprovodimosti

Sostoyanie rabot po sil'notochnoi prikladnoi sverkhprovodimosti

Possible high-current superconductivity at room temperature in oxidised polypropylene and other quasi one-dimensional systems

Application is made to quasi one-dimensional systems of the idea of Parmenter, that, in narrow cylindrical regions with thicknes smaller than the magnetic-field penetration depth, pairing may be enhanced at very high drift velocities. Enhancements for drift velocities equal to the velocity of sound or to an appropriate phase velocity of optical phonons, of excitonic excitations or of plasmons may be considered. Such enhancements can be larger in quasi one-dimensional systems. Calculations made here, based on Eliashberg theory, indicate that the enhancement of interactions is not great enough to produce high-current superconductivity which has an energy gap in the laboratory frame which is comparable to or greater than that at zero current if the Fermi energy E F is large compared with the relevant phonon, plasmon or exciton energy, ħω, but simpler calculations indicate that, if E F≪ħω, high-current superconductivity with strong pairing may occur. A theory for E F≈ħω would be useful. The applicability of these ideas to explain reports of room-temperature superconductivity in narrow channels perpendicular to surfaces of films of oxidised polypropylene is discussed.

Possible high-current superconductivity in ionic semiconductors

The possibility of the occurence of superconductivity at high carrier drift velocities in ionic semiconductors is discussed.

Gapless High-Current Superconductivity

The idea of gaplessness, already familiar in several areas of superconductivity, is considered in connection with high-current superconductivity. A calculation is made of the critical temperature ${T}_{c}$ at which a normal metal with zero current becomes thermodynamically unstable against decay into a high-current super-conducting phase. It is pointed out that at ${T}_{c}$ all the various suggested forms of high-current electron-electron interaction, both retarded and nonretarded, are equivalent, so that a calculation of ${T}_{c}$ does not suffer from ambiguity of choice of interaction. It is found that a transition temperature exists when the Bardeen, Cooper, and Schrieffer (BCS) parameter $N(0){V}_{0}>~0.43$ [assuming $N(0){V}_{c0}=0.1$]. In fact, for $0.43<N(0){V}_{0}<0.63$, two values of ${T}_{c}$ exist, since at sufficiently low temperatures, the normal phase is thermodynamically stable against high-current superconductivity. When $N(0){V}_{0}=0.43$, ${T}_{c}=\frac{0.259\ensuremath{\hbar}\ensuremath{\omega}}{{k}_{B}}$. At $T=0$, a separate calculation is made of the $N(0){V}_{0}$ required to obtain high-current superconductivity with a finite thermal energy gap $2({\ensuremath{\epsilon}}_{0}\ensuremath{-}{p}_{F}{v}_{0})$. This calculation, using a nonretarded interaction, includes the correction of an algebraic error in the 1959 paper by Parmenter. The result, $N(0){V}_{0}>~0.70$, is in close agreement with the recent work of Hone, who made use of a retarded interaction in calculating that one must have $N(0){V}_{0}>~0.67$ in order to get finite-thermal-gap high-current superconductivity. This suggests that retardation effects are not crucial in determining the occurrence or non-occurrence of high-current superconductivity. It is pointed out that experimental measurements of Morin and Maita have suggested that some transition-metal superconductors have large enough values of $N(0){V}_{0}$ for both the gapless form and the finite-thermal-gap form of high-current superconductivity to occur. Either form, if it exists, will exhibit the same form of electrical instability that is known to occur under exceptional conditions in conventional low-current superconductivity.

High-Current Superconductivity?

High-Current Superconductivity?

High-Current Superconductivity

The Bardeen, Cooper, and Schrieffer theory of superconductivity is extended to the case of high current densities by explicitly including in the phonon-induced electron-electron attraction the medification of the phonon spectrum in a moving coordinate system. This modification results from the Doppler effect. In materials having cross-sectional dimensions of the order of the penetration depth it is shown that at current densities of the order of 10/ sup 9/ amp/cm/sup 2/ the superconducting state is stable relative to the normal state, this holding both in superconducting metals and in metals ordinarlly not considered superconducting at all. For this high-current state, it appears possible to obtain superconducting transition temperatures as high as room temperature. Means for achieving such an effect experimentally are discussed. (auth)