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Superconductivity in tight-binding approximation

Description: An interpretation of Barisic's relation for transition elements between the d-electron contribution to the cohesive energy and the local atomic parameter eta is presented. This relation is extended to a lattice with more than one atom per unit cell in the tight- binding approximation of rigid ions. It is conjectured that Barisic's relation is correct to first order approximation for transition metal alloys, provided the phonon induced d-d coupling is the dominant mechanism for superconductivity. (auth)
Date: October 1, 1975
Creator: Poon, S.J.
Partner: UNT Libraries Government Documents Department

Hard superconducting materials. Final report

Description: During the period covered by this program (1962-1971). extensive studies of the material parameters and mechanisms controlling critical currents, flux creep and inreversiblity in hand superconducting materials were carried out. Quantitative theories of the pinning and creep processes were developed and tested extensively using equipment and methods developed locally. The role of the superconducting surface sheath was explored in detail and somewhat similar phenomena were found to operate there. The superconducting quantum interferrometry developed for the creep studies proved to be an extremely valuable tool in the study of quantum fluctuations in one-dimensional superconductors. Inltial attempts to study the flux structure by low temperature electron microscopy proved non-productive but later studies using the Bitter pattern technique of Trauble and Essmann were of value fn the study of the flux structure transitions in thin films. Development of instrumentation based on superconducting technology such as a femtovoltmeter and practicable point contact flux detectons also arose under the program. A list of publications is appended. ( auth)
Date: January 1, 1973
Creator: Silcox, J. & Webb, W.W.
Partner: UNT Libraries Government Documents Department

Properties of tunnel junctions with fluorocarbon dielectric barriers

Description: Thesis. The electrical characteristics of In/I/In and In/I/Pb superconducting tunnel junctions have been studied in detail. Since In does not readily form pinhole free oxide layers, a thin insulating dielectric was formed on freshly deposited In film by passing an electric discharge through an atmosphere of fluorocarbon gas. Junctions were then completed by depositing a thin counter electrode of In or Pb. The same process was used to prepare high resistance junctions with Au as the base electrode; these were not however, studied in detail. In/I/In and In/I/Pb junctions were produced with resistances in the range 0.01 ohms to 10/sup 10/ ohms at liquid helium temperatures. Low resistance junctions exhibited nonlinear electrical characteristics associated with good quality oxide'' superconducting junctions including (a) the dc Josephson effcct, (b) quasiparticle tunneling characteristics. (c) phonon structure and (d) inelastic tunneling phenomena. The magnitude of the Josephson current for In/I/In junctions agreed to within a few percent of the value predicted by strong coupling theory. Current voltage (I-V) and first and second derivative curves for In/I/In and In/I/Pb were compared with curves for Al/I/In and Pb/I/Pb junctions. Discrepancies between the characteristics can be, for the most part, explained on the basis of existing theories of phonon mediated superconductivity using recent data from inelastic neutron scattering studies of In. Nonlinear structure at voltages below the phonon spectrum was observed and is most likely associated with Kohn singularities. At higher voltages, second derivative curves exhibited resonances characteristic of CH and OH impurities in the barrier as well as a complex spectrum associated with the vibrational spectrum of the fluorocarbon dielectric. To better characterize this dielectric, a variety of surface analytic techniques were used to determine the complex index of refraction, the chemical composition and chemical homogeneity of the barrier. I-V curves for high resistance junctions were ...
Date: November 1, 1973
Creator: Jack, M.D.
Partner: UNT Libraries Government Documents Department

Radiation effects on superconductivity

Description: The effect of radiation on the superconducting transition temperature (T/ sub c/), upper critical field (H/sub c2/), and volume-pinning-force density (F/ sub p/) were discussed for the three kinds of superconducting material (elements, alloys, and compounds). 11 figures, 3 tables, 86 references. (auth)
Date: January 1, 1975
Creator: Brown, B.S.
Partner: UNT Libraries Government Documents Department

Intrinsic fluctuation effects and certain other aspects of superconducting weak links

Description: Thesis. Recent theoretical developments have led to evermore refined theories for the Josephson effects in a variety of superconducting weak link'' structures. One of the major refinements has been the inclusion of the effects of intrinsic thermal fluctuations upon Josephson-like behavior. Some other developments include the investigation of mechanisms for the origin of the Josephson effects in non-tunneling structures, and equivalent circuit representations. Experimental investigations were made which encompass both of these areas; with special emphaais placed upon fluctuation effects; in pariicular, the effect of fluctuations upon the driven dc Josephson effect has been studied in considerable detail. The weak links studied included niobium point contacts, tin whisker'' crystals, and tin thin film bridges, the latter fabricated via an optical photoresist technique. The experiments were conducted in a temperature regime which extended to within a few mK of T/sub c/, the regime in which fluctuation effects are enhanced (and thus tnore experimentally accessible). The data consisted of relatively high resolution (ln V) low noise 1- -V characteristics obtained with an apparatus which incorporated ( plus or minus 10 mu K) temperature control with wide-band rf transmission, in a highly shielded environment. Experiniental rf-induced step pi-ofiles (the driven ac Josephsorin effect) and zero voltage step profiles (the dc Josephson effect) were compared in detail with the relevant intrinsic fluctuation theories (due to Ambegaokar, Halperin, and Stephen) via a one parameter fit, using the respective no-fluctuation step amplitude as she (natural) fitting parameter. We were able to distinguish between external noise effects and intrinsic effects and effectively excluded external noise; thus we maintain a high degree of confidence that the effects reported are indeed intrinsic to the weak links themselves. The agreement between thcory arind experinient was found to be very good in both the driven and the dc effects over a ...
Date: January 1, 1974
Creator: Henkels, W.H.
Partner: UNT Libraries Government Documents Department

Studies of low temperature phase transformations in high field superconductors and the phonon spectrum and mechanical properties of vanadium. Final report

Description: Extensive studies of the structure and phase transformations of high field superconductors of the BETA -tungsten type were made. A phase transformation was discovered at 43 deg K in Nb/sub 3/Sn and the structure of the low temperature phase was measured. The details of the transformation in terms of the formation of the low temperature tetragonal phase from the high temperature cubic phase was investigated both by x-ray observations and optical microscopy below 40 deg K. The complete phonon dispersion of vanadium was measured by x-ray diffuse scattering, thus complementing the neutron measurements on other body-centered cubic elements. Incidental to the work on phonons in vanadium a strong effect was discovered which was related to hydrogen precipitations in vanadium. In separate electron microscopy studies the structure of the vanadium hydride phase was determined directly and the relative positions of the hydrogen atoms were determined. In a neutron diffraction study complementing the phonon work with x-rays the existence of anharmonic vibrations in single crystals of silicon was shown directly by determining, for the first time, a nuclear forbidden reflection. This reflection is due solely to anharmonic vibration of the nucleus of the atom and it was shown that its intensity increases with increasing temperature roughly as the 4th power of temperature. (auth)
Date: January 1, 1972
Creator: Batterman, B.W.
Partner: UNT Libraries Government Documents Department

Some calorimetric investigations of the role of electrons in superconductivity and magnetism

Description: Thesis. The heat capacity of the system (La,Ce)Al//sub 2/ for 0.0, 0.193, 0.64, and 0.906 at.% Ce has been measured between approximately 0.06 and 22 K and in magnetic fields up to 38 kOe. In the normal state, there is R1n2 entropy associated wtih the Ce spin system and this entropy is removed in the formation of the spin-compensated state. The nonmal state Kondo temperature is 0.42 K. The three more-dilute samples exhibit supenconductivity but cannot be described by the Bardeen - Cooper -- Schrieffer on Abrlkosov -Gor'kov theories. Significant low-energy excitations are evident in the superconducting state and they may be associated with quasibound states deep in the energy gap of the pure superconductor. Fon the 0.193 at.% Ce alloy in the superconducting state the characteristic temperature associated with Ce-spin ordering in the nonmal state is reduced by an estimated order of magnitude. For the 0.64 at.% Ce alloy a small, broadened superconducting anomaly appears in the heat capacity above 1 K, however, no anomaly that could be interpreted as a broadened discontinuity was found at the magnetically-detected transition back into the normal state at lower temperatures. The heat capacities of five samples of alpha -U have been measured between approximately 0.1 and 2 K at zero pressure. The four polycrystalline samples exhibited broad, bulk superconducting transitions. The gamma -values and the smearing of the superconducting transitions are sensitive to grain size--the small-grained samples had the highest gamma -values and the broadest superconducting transitions. A single crystal had the lowest gamma value and appeared to be beginning to enter the superconducting state below 0.25 K. The shape of the heat capacity anomaly near Tc for the large-grained polycrystal indicates that alpha -U is a BCS superconductor and, hence, local moments and pair-breaking mechanisms do not play a role in ...
Date: January 1, 1974
Creator: Bader, S.D.
Partner: UNT Libraries Government Documents Department

Trend of superconductivity in amorphous superconducting transition metals and their alloys

Description: A plausible interpretation for the trend of superconducting transition temperature T/sub c/ in amorphous transition metal TM alloys is presented from a chemical point of view. It is shown that the T/sub c/ behavior is not a reflection of the dependence of the atomic parameter eta on the number of electrons per atom, but rather due to two other effects. One is the changes in the density of states due to the mixing of antibonding and bonding states in the bcc amorphous metals. The other is an increase in the number of valence d electrons participating in the phonon-induced d-d coupling towards the mid series. (auth)
Date: December 1, 1975
Creator: Poon, S.J.
Partner: UNT Libraries Government Documents Department