R.T. Kampwirth

Impurity Effect in the Ionization Dilatation of Vitreous Silica

The normal effect of ionizing radiation is to cause a contraction. Vitreous silica which colors in the visible (presumably because of its aluminum content) shows a very rapid radiation‐induced expansion which soon saturates. Thus, depending on the impurity composition, the vitreous silica may show an initial decreased contraction or an initial expansion followed by subsequent contraction. Data are given for the optical absorption and the dilatation, for x‐ray, gamma‐ray, and electron irradiations. The dilatation was measured by interferometric and photoelastic methods, and for the latter, the method was further developed. The expansion was found to have a different distribution along the range of electrons than the contraction. Spontaneous annealing was found for the expansion, but not for the contraction. The thermal annealing of the dilatation and optical absorption were studied. In the coloring vitreous silica, the expansion annealed at a lower temperature than the contraction. The expansion and the co...

Superconductivity in thin films of the Bi-Ca-Sr-Cu-O system

Abstract Films of Bi-Sr-Ca-Cu-O have been made by multi-target magnetron sputtering. They show a superconductivity of ≈90–110 K and a resistive T c 0 of 72 K. Preliminary X-ray diffraction analysis suggests the material to be predominantly tetragonal with a cell size of 5.41 A × 5.41 A × 30.8 A.

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.

Experimental study of the ultimate limit of flux pinning and critical currents in superconductors

Abstract The effectiveness of planar surface and grain boundary pinning structures in films and multilayers has been determined from measurements of the critical temperature, T c , critical magnetic field, B c2 , and critical current density, J c . The effects of these were separated using a Kramer-like analysis near B c2 . The dramatic increases in J c of the multilayers of NbN over single NbN films, measured at 4.2 K and 20 T parallel field, are found to be a result of both increased B c2 and increased pinning due to the alternating barrier layers. Both pinning structures exhibited strong pinning forces, which were significantly reduced for defect spacings smaller than the flux core diameter, and in an optimized multilayer the pinning force reached ∼22% of the theoretical maximum. A large anisotropy, similar to that observed in high-temperature superconductors, is found in the critical properties of the NbN multilayers, and an anisotropic model is developed. This model predicts a maximum potential J c value of at least 2 x 10 8 A/cm 2 in YBa 2 Cu 3 O 7 at low reduced temperature and 20 T field. This would be reduced to ∼4 x 10 7 A/cm 2 if a pinning strength equal to the highest effectiveness found in NbN could be realized in YBa 2 Cu 3 O 7 .

Large anisotropy in the upper critical field of sputtered thin films of superconducting Tl‐Ba‐Ca‐Cu‐O

The upper critical field Bc2 provides the most direct measure of the intrinsic anisotropy of a superconductor. For highly oriented, sputtered thin films of the high‐temperature superconductor Tl2Ba2CaCu2Ox, we find a Bc2 anisotropy of at least 70, which exceeds similar measurements on thin films and single crystals of the high‐temperature superconductors Bi‐Sr‐Ca‐Cu‐O and YBa2Cu3Oy. We discuss why the midpoints of the resistive transitions, used in these measurements, may be appropriate for defining the intrinsic Bc2.

Effect of oxygen partial pressure on the in situ growth of Y‐Ba‐Cu‐O thin films on SrTiO3

The evolution of YBCO film growth with thickness at various oxygen pressures was observed by in situ synchrotron x‐ray diffraction in real time. When the films were deposited at 2 A/s and 730 °C under higher oxygen partial pressures (in an Ar/O2 mixture of 90 mTorr), the nucleation was observed to have c‐axis orientation. After the films reached a critical thickness, the growth of the YBCO film changed from c axis to a axis and then propagated epitaxially. This provides evidence that a‐axis epitaxial growth nucleates on a c‐axis base. The critical thickness reflects the competition between the growth of the c and a axes, which is determined by the oxygen partial pressure in the process of thin‐film formation. The a‐axis oriented films showed a very sharp rocking curve (less than 0.1°) which indicates a very high structural quality. For very low oxygen partial pressures, the in situ growth process was very similar, but the initial nuclei involve a second phase mixed with a small amount of c‐axis ‘‘123’’ ph...

High critical field anisotropy of superconducting Bi‐Sr‐Ca‐Cu oxide from highly oriented thin films

Superconducting films of Bi‐Sr‐Ca‐Cu oxides have been grown on MgO substrates by multitarget magnetron sputtering. After post‐annealing in an oxygen atmosphere, zero resistance was obtained at about 80 K. X‐ray diffraction indicates a high degree of preferential growth of the c axis perpendicular to the substrate. Measurements of the upper critical field Bc2(T) of these films show critical field slopes (B’c2≡−dBc2/dT at Tc) of 8.5 and 0.56 T/K, respectively, for B parallel and perpendicular to the substrate. The anisotropy of ∼15 and the Bc2∥ are the highest values yet reported for high‐temperature superconductors.

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.

Thermal propagation and stability in superconducting films

Thermal propagation and stable hot spots (normal domains) are studied in various high Tc superconducting films (Nb3Sn, Nb, NbN and Nb3Ge). The prediction of the thermal propagation velocity of the long-standing model of Broom and Rhoderick (1960) is verified quantitatively in the regime of its validity. A new energy balance model is shown to give reasonable quantitative agreement of the dependence of the propagation velocity on the length of short normal domains. The steady state (zero velocity) measurements indicate the existence of two distinct situations for films on high thermal conductivity (sapphire) substrates. For low power per unit area the film and substrate have the same temperature, and the thermal properties of the substrate dominate. However, for higher power densities in short hot spots, the coupling is relatively weak and the thermal properties of the film alone are important. Here a connection is made between the critical current stability of superconducting films and a critical hot spot size for thermal propagation. As a result efficient heat removal is shown to dominate the stabilisation of superconducting films. These models are used to explain quantitatively self-healing of hot spots and a curious switching phenomena in a superconducting film in which a hot spot grows and collapses repeatedly over a very narrow range of current. Finally there is a discussion of the potential use of the thermal propagation model in applications of superconductors, especially switches.

Investigation of magnetism at surfaces by polarized neutron reflection (invited)

It is shown that the spin‐dependent reflectivity of a cold neutron beam from magnetically active surfaces, measured as a function of the neutron wavelength, can provide the magnetic depth profile of the material. Following the construction of a prototype instrument at the Intense Pulsed Neutron Source at Argonne, studies were made of the magnetic disturbances at the surface of superconductors. The results obtained, and discussed here, pertain to the penetration depth in a superconductor (niobium and lead) below the critical flux entry field Hc1, and the surface superconductivity of an incipient type II superconductor (lead‐bismuth alloy) below Hc3. The prospects for this technique, with particular regard to the study of the surface critical phenomena of ferromagnets, are also discussed.