A memory cell is proposed which uses vortices in type‐II superconductor thin film as information bits. In the memory cell, vortices are generated by coincident current in two superconductor lines and are read out by a Josephson junction. Preliminary experimental results on vortex generation and detection are also reported.

Trapped vortex memory cells

By means of the magnetooptical method the intermediate-state structures of annealed polycrystalline lead foils having thicknesses between 40 and 120 µm have been investigated. The structures in increasing and decreasing magnetic field are qualitatively described. They mainly have laminar character, but above a critical thicknessd

 s
 , between 115 and 169 µm forT=4.2 K, structures are found which can be interpreted by branching of the normal domains. Rapid increasing of the applied magnetic field at the(SI)

 T
 transition favors the formation of macro flux lines. The period of laminar structures shows good agreement with the “nonbranching” model of Landau for values of the reduced applied magnetic fieldh=H/H

 c
 ≤0.6. A value of the surface energy parameter Δ=86±17 nm is calculated from the experimental data. In contrast to the model of Goren and Tinkham, the mean separation of macro flux lines in lattices of flux lines is proportional to
 
$${1 \mathord{\left/ {\vphantom {1 {\sqrt h }}} \right. \kern-\nulldelimiterspace} {\sqrt h }}$$

 for not too large values ofh.

Nonbranching intermediate-state structures in bulk lead

An experimental investigation of the kinetics of the destruction of superconductivity by rectangular current pulses was made on cylindrical Sn and In wires of different purities and diameters. The study was based on the observation of the variation of the voltage along the sample during the transition. As shown by Rothen and Bestgen, only the timet
1 between the beginning of the transition and the onset of the intermediate state can be determined with this method. The results show that if the currentI is sufficiently larger than the critical currentI

 c
 (typicallyI⪞1.15I

 c
 ), then both shape of the voltage pulses and the transition times agree well with the predictions of the electromagnetic theory of these authors. The transition is, in that case, controlled by the electromagnetic damping associated with the penetration into the normal material of the magnetic field due to the current. IfI is close to
 c
 , then the transition is much slower than predicted by the theory and seems to be essentially governed by thermal effects, i.e., absorption of latent heat, generation of Joule heat, and heat exchange with the helium bath; a small influence of superheating is observed in indium samples.

Kinetics of the current-induced phase transition in type-I superconductors

An gleichstromdurchflossenen Bleischichten werden die Verteilung der senkrechten Magnetfeldkomponente, der Oberflachenenergie-Parameter und der Hall-Koeffizient experimentell ermittelt. Strom-Spannungs-Messungen und gleichzeitige Beobachtung der Topologie der Bereichsstrukturen erbringen Aussagen uber den Widerstandsmechanismus im Zwischenzustand. Mit Hilfe von Wechselstromexperimenten wird die statistische Verteilung der Haftkrafte einer Probe bestimmt. Die mittlere Haftkraft fur normalleitende Bereiche (mit durchschnittlich 100 Flusquanten) wird zu fp = 6 × 10− N cm−1 bestimmt und der Viskositats-Koeffizient fur die Bewegung normalleitender Bereiche in der supraleitenden Matrix zu η = 6 × 10−7 N cm−1 s gefunden.



The distribution of the transversal magnetic field, the parameter of the surface energy, and the Hall coefficient are determined on dc carrying lead films. Current-voltage measurements and the simultaneous observation of the topology of the intermediate state structure allow to clarify the origin of the resistance in the intermediate state. Applying ac the statistical distribution of the pinning forces of a sample is determined. The average pinning force for normal flux spots (with about 100 flux quanta) is fp = 6 × 10−6 N cm−1 and the coefficient of viscosity for the motion of flux spots in the superconducting matrix is η = 6 × 10−7 N cm−1 s.

Magnetooptische Beobachtungen und elektrische Messungen an stromdurchflossenen, supraleitenden Bleischichten im Zwischenzustand

In the previous sections we have dealt with the static properties of magnetic flux structures. An important development of recent years has been the investigation of the dynamic behavior of magnetic flux structures and the discovery of the intimate connection between flux motion and the transport properties of superconductors. Usually, the experimental geometry consists of a superconducting film or plate with a magnetic field applied perpendicular to the broad surfaces. Motion of the magnetic flux structure can be induced by the Lorentz force of an electric current or the thermal force of a temperature gradient. The dynamics of magnetic flux structures becomes important in various technological applications of superconductivity.

Lorentz Force and Flux Motion

The speed at which magnetic flux propagates in superconducting tin and indium disks suddenly subjected to an axial magnetic field lower than the critical has been measured by direct observation. The results are discussed in terms of an energybalance argument based on eddy-current damping and assuming isothermal penetration.

Speed of magnetic flux penetration in type-I-superconducting specimens of nonzero demagnetizing coefficient

The influence of a static perturbation of the magnetic field on the current-induced motion of the normal and superconducting domains in a pure, uncompensated, type I superconductor (indium) in the intermediate state has been observed using a high-resolution magnetooptical technique. We report here these observations, which bring the first direct evidence of the nonlinear “hydrodynamic” flow properties of the intermediate state predicted by Andreev and which are closely related to the additional resistance recently observed by Krylov in the intermediate state of pure indium.

Direct observation of the `hydrodynamic' flow of the intermediate-state structure in pure indium

The thermal conductivity of a regular, optically controlled, intermediate-state structure has been measured in high-purity lead with the heat current nearly perpendicular to the (static) interphase boundaries; this situation is obtained in a particular perpendicular magnetic field range by establishing an electrical, subcritical current through the sample of rectangular cross section. The thermal resistance of a single normal/superconductor interphase boundary is calculated from our measurements. The results agree fairly well with a theoretical calculation by Andreev for the case of electronic heat conduction (T ≳ 3 K). At lower temperatures (T≲3 K), the measured values are smaller than predicted by Andreev, and other factors, such as the reduction of the phonon mean free path in the superconducting lamellas due to the interspaced normal lamellas, have to be taken into account.

P. Laeng

Papers

Speed of magnetic flux penetration in type-I-superconducting specimens of nonzero demagnetizing coefficient

Direct observation of the `hydrodynamic' flow of the intermediate-state structure in pure indium

Thermal resistance of a normal/superconductor interphase boundary in high-purity lead