R.P. Huebener

High resolution magneto-optical studies of the intermediate state in thin film superconductors

Abstract The structure of the intermediate state in superconducting lead films has been investigated as a function of magnetic field and film thickness. The detection system utilized the high specific Faraday rotation in thin films of a mixture of EuS and EuF 2 in combination with a polarizing microscope, yielding a resolution of about 1 μm. The thickness of the Pb films ranged between 0.7 and 9 μm, thus including the critical film thickness at which the transition from the intermediate state to the vortex state occurs. At low fields a liquid-like mixed state of multi-quanta flux tubes was observed which appeared to be stable up to increasing magnetic fields with decreasing film thickness. The diameter of these flux tubes varied approximately with the square root of the film thickness. At intermediate fields the intermediate state pattern was found to persist down to a film thickness of 0.7 μm, the smallest thickness investigated. The periodicity length of the intermediate state structure was in reasonable agreement with the non-branching model of Landau. Just below H c , small superconducting domains were observed in increasing field, whereas long threads of superconducting material were formed abruptly in decreasing field. These superconducting threads were absent in the specimentsthinner than 1–2μ, being replaced by a liquid-like mixed state of superconducting tubes. After the passage of a sufficiently high electrical current through the specimen, the flux structure was found to be rearranged into long domains oriented predominantly perpendicular to the current, leading to current hysteresis effects. Finally, some dynamic observations were made during current induced flux flow.

Stroboscopic study of flux dynamics in superconductors during current-induced breakdown

Abstract We have studied current-induced breakdown in type-I superconducting films at time resolutions of 100 nsec by combining a laser stroboscope with magneto-optical flux detection. We find that breakdown occurs through the regular, periodic generation of multiquanta flux tubes whose size is determined by properties intrinsic to the sample. Also, a measurement of flux-tube velocity as a function of penetration distance has been performed and compared with a recent theoretical model.

Current-induced breakdown of superconductivity in constricted type I superconducting films

In simultaneous magneto-optical and electrical measurements on constricted type I superconducting films we have correlated step structure in the currentvoltage characteristic with the nucleation of individual flux-tube trains. A central feature of this experiment is the development of a sample geometry with which we can restrict the breakdown behavior to a single, localized flux-tube train running perpendicular to the transport current. Apparently, the step structure in the I–V characteristic results from a Gibbs free-energy barrier against the nucleation of flux-tube trains. The experiments are performed with films of lead and indium. From the I–V characteristics of the indium constrictions we conclude that the flux-tube nucleation rate can vary substantially with applied current beyond a voltage step at temperatures near Tc.

MAGNETO-OPTICAL OBSERVATION OF THE MAGNETIC FLUX STRUCTURE IN SUPERCONDUCTING NIOBIUM.

The magnetic flux structure in superconducting niobium has been studied using the high specific Faraday rotation in thin films of a mixture of EuS and EuF2 at low temperatures. The magneto‐optical detection system had a resolution of about 20 μm. The specimens were niobium foils 18‐μm‐thick and vacuum‐deposited niobium films. In the foils, after raising the magnetic field and returning it to zero, the trapped flux structure consists of large domains of flux with opposite direction in the specimen. The arrangement of the domains is closely related to the sample shape. As in a ferromagnetic body, the domains are established to reduce the total free energy of the system. The Nb films show a very imhomogeneous magnetic flux distribution consisting of regions with very large local magnetic fields.

Self‐heating effects in thin‐film type‐I superconductors

We have investigated the current‐induced electrical resistance for zero applied magnetic field in long superconducting strips of tin 100–200 μm wide and up to a few μm thick. We observe self‐heating effects similar to those reported recently by Skocpol, Beasley, and Tinkham (SBT) for ’’one‐dimensional’’ tin microbridges with dimensions much smaller than those of our samples. Our results agree well with the one‐dimensional SBT hotspot model for long microbridges. In the presence of an inductive load, relaxation oscillations can be observed resulting from the negative differential resistance region in the V‐I characteristic.

APPARATUS FOR MAGNETO-OPTICAL STUDIES OF MAGNETIC STRUCTURES AT LOW TEMPERATURES.

A magneto‐optical apparatus is described which is used for studies of magnetic structures at liquid helium temperatures. The system employs the high specific Faraday rotation of a mixture of EuS and EuF2. A resolution of about 20 μ has been obtained.

ANALOG OF THE NERNST EFFECT IN THE INTERMEDIATE STATE OF TYPE-I SUPERCONDUCTORS.

Abstract Transverse voltages caused by longitudinal temperature gradients were observed in the intermediate state of a Type I superconductor. The transverse voltages occur only above a “critical temperature gradient”, which decreases with increasing magnetic field.

The intermediate state in superconducting mercury

Using the magneto-optic technique, an extensive study has been made of the intermediate state in single crystals of superconducting mercury. A regular laminar structure, closely approaching that envisaged by Landau, was realized in the thinnest sample examined (47 Μm). The field dependence of the lamina periodicity displays excellent agreement with the Landau model and the deduced surface energy is in good agreement with the Ginzburg-Landau theory. At applied magnetic fields very close to the critical one, the stable magnetic structure consists of isolated superconducting filaments. A number of observations are reported on their growth and stability.

Current induced intermediate state in thin film superconductors

Abstract High-resolution magneto-optical studies of the current-induced intermediate state in superconducting lead films indicate a magnetic structure consisting of chains of flux tubes rapidly moving along channels from both edges to the center of the strip. At increasing levels of the transport current new flux tube channels are created abruptly, resulting in relatively large resistive voltage steps.

Enhancement of the lattice heat capacity due to low-frequency resonance modes in dilute Al-Ag alloy

The lattice properties of a solid are changed appreciably-due to the presence of impurities which differ from the host lattice in mass or in the nearest neighbor force constant.(1) The introduction of isolated heavy impurities into a relatively light host lattice causes a change in the phonon spectrum, which is characterized by the existence of low-frequency resonance modes localized at the impurity site. If we neglect the change in the force constant, the resonance frequency associated with a heavy impurity is given by using