L. Rinderer

Electrodynamics of superconductors. Response to a weak, low-frequency magnetic field

The phenomenon of the penetration of a low-frequency weak magnetic field into a superconductor has stimulated extensive theoretical and experimental studies. These have led to important clarifications regarding the nature of the superconducting phase by, for example, making concrete the idea of the long-range order in terms of the coherence length, which in turn has provided an understanding of the details of the penetration phenomenon under various conditions realized for different relative values of the penetration depth and the coherence length. This work is a review of the theoretical and experimental studies of the penetration phenomenon. The theory of the penetration phenomenon is discussed phenomenologically and microscopically. Different experimental techniques employed to study the phenomenon are discussed. Many of the experimental studies are concerned with the temperature dependence of the penetration depth. These results are reviewed for different elemental superconductors. Practical difficulties in a quantitative analysis of such data are pointed out. A new strategy adopted recently by Egloffet al. for analyzing the penetration depth data of pure lead is discussed. Effects of impurities, anisotropy of the Fermi surface, and the magnetic field on the penetration phenomenon are also discussed.

Penetration of a magnetic field into superconducting lead and lead-indium alloys

The temperature dependence of the magnetic field penetration depth of superconducting lead and lead-indium alloys has been studied over the temperature range between about 2 K andTc. Data are analyzed in terms of the microscopic theory. The difficulties of a unique analysis of the penetration data are pointed out and a strategy for the best analysis is discussed. The penetration depth atT=0 K for pure lead is determined as 525 Å. This value, though higher than the previously accepted value for lead, is nevertheless consistent with the strong coupling character of lead.

Temperature and Randomness Dependence of the Pair-Breaking Parameter in Superconducting Aluminum Films

Magnetoconductance and excess conductance due to superconducting fluctuations in aluminum films are measured in order to study the temperature dependence of the pair-breaking parameter δ at temperatures nearTc. The parameter δM is estimated from the relation δ=πℏ/8kBTτin, where τin is the inelastic scattering time deduced from the analysis of the magnetoconductance. The parameter δF is determined by fitting theories to data on the excess conductance at zero magnetic field. It is shown that: (1) For films with a wide range of the sheet resistanceR▭, 12∼R▭∼200 Ω/□, the temperature dependence of δM nearTcagrees well with the theory of Brenig et al. (2) For clean films withR▭∼100 Ω/□, the value of δF analyzed with theories including the correction term to the Maki-Thompson contribution shows almost the same temperature dependence as δM. In a film withR▭≃200 Ω/□, however, a discrepancy between δM and δF remains.

Apparatus for the study of the kinetics of the destruction of superconductivity by a current

Abstract An apparatus suitable for the study of the kinetic aspects of the destruction of superconductivity by a current is described. It includes a fast, high capacity (100 A) current amplifier, low level sampling circuits for the simultaneous measurement of the current and of the voltage across the sample, a simple analogue divider for directly recording the resistance-current curves, and the necessary synchronizing circuits.

Observation of large diamagnetism in La-Sr-Nb-O films up to room temperature

Large diamagnetic transitions along with sharp resistive transitions were observed in the La-Sr-Nb-O system near room temperature (∼290 K). The amplitude of the diamagnetism reaches 35% of that of a pure Nb sheet. In addition, a behavior similar to weak magnetic spin ordering was observed for some samples at a temperature of about 290 K, over a temperature range of 30 K. The diamagnetism reappears above this temperature and continues up toT ∼320 K. It is not clear what composition ratio of La-Sr-Nb-O is responsible for this large diamagnetism and the high critical temperature. The yield probability of these samples is around 50%. The characteristics of the samples having not passed through many thermal cycles remain stable for about 1 month.

Normal phase propagation along a current carrying superconducting wire and its heat transfer to a cooling He II bath

A theoretical and experimental study of the longitudinal propagation velocity of a normal phase along a superconducting cylindrical wire suspended in a superfluid helium bath is presented. The theoretical model supposes a moving nonplanar separating boundary between the normal and superconducting phases and takes into account the latent heat absorbed during the destruction of the superconducting state. The comparison of the measured and calculated normal zone propagating velocities versus the circulating electrical currents shows an excellent agreement for all the bath temperatures considered. A boundary thermal conductance across the wire wall and superfluid helium bath interface is inferred by adjusting the calculated velocities with the experimental data. The deduced boundary thermal conductance is similar to the Kapitza conductance in the sense that both are proportional to the superfluid helium bath temperature raised to a constant power. Furthermore the deduced boundary conductance seems unaffected by the dynamical aspect of the longitudinal destruction process of the superconducting state and by the heat flux range across the solid and He II separating boundary covered by the normal zone propagation velocity versus the circulating electrical current plots. To the authors knowledge this is the first study of heat transfer using the data obtained from longitudinal normal zone propagation experiments.

Statics of the two-dimensional mixed state in hollow, type I superconductors

A theoretical and experimental study of the statics of the two-dimensional mixed state in hollow, type I superconductors of pure tin has been made without considering thermal or other effects. In the experiments, this state could be moved into the interior of the sample by a magnetic field produced by a current flowing in a coaxial wire placed in the hole. This study shows that the current-voltage characteristics can present horizontal segments as well as discontinuities accompanying the appearance or disappearance of the superconducting, normal, or two-dimensional mixed state domains. Within the experimental error, the agreement between the calculated values and the experimental results is quite good.

A high current density 9 kJ superconducting magnet without degradation

Abstract A co-drawn niobium-titanium copper composite conductor has been used to construct a 36 mm clear inner diameter magnet generating 72·5 kOe at 36 A. The overall winding current density at 36 A is 1.85 x 10 4 A/cm 2 . Individual sections of the winding achieve a current density of 2.85 x 10 4 and 2.5 x 10 4 A/cm 2 at 52.5 kOe. Performance, construction, and a design criteria for the proper choice of copper to superconducting ratio are discussed.

Specific heat and magnetization of superconducting niobium in the mixed state

The thermodynamics of the mixed state of niobium is reconsidered. Critical fieldsHc(t),Hc1(t),and Hc2(t), Maki parameters κ1 and κ2, magnetization, and specific heat are measured with a computer-controlled magnetometer and calorimeter. The results show quantitative discrepancies with the isotropic Gorkov-Brandt theory over almost the whole mixed state. Computations based on Fermi surface anisotropy using a scaling ofHc2 provide a much better agreement with the experimental magnetization curves already in the limit of “weak nonlocality,” thus suggesting a numerical evaluation of the anisotropic terms in the Gorkov-Brandt theory.

Creation of the two-dimensional mixed state and its motion in a type I superconductor

An experimental study of the statics of the two-dimensional mixed state in hollow lead wires is presented. The results confirm the theoretical predictions of a model given in a previous paper and are similar to the measurements already reported on tin specimens. By introducing an axial current-carrying wire into the hole of the tin samples, it was possible to also investigate the motion of this state when it undergoes a displacement from the external surface to the interior. A theoretical model is proposed that agrees well with the experimental results and establishes the conditions required to reach the stationary situation.