Topics in electromagnetic fluctuations at low temperatures and in superconductivity
Creators: name: Fink, Hermann Josef
Publisher: name: University of British Columbia
Year: 2012

description: I. CURRENT FLUCTUATIONS IN A SUPERCONDUCTING CIRCUIT CARRYING A CIRCULATING CURRENT - Persistent currents in superconducting lead are free from fluctuations to less than 1.1 x 10⁻⁹ of full shot noise at approximately 2.4 Mc/s. Superconducting currents are also unaffected by the surface condition of the metal to the same limit as stated above. II. A NEW ABSOLUTE NOISE THERMOMETER AT LOW TEMPERATURES - If three resistors, which are kept at different temperatures, are arranged in form of a π network and if two of the thermal noise voltages appearing across the if network are multiplied together and averaged with respect to time, then under certain conditions the correlation between those voltages can be made zero. This condition is used to calculate the temperature of one noise source provided all the resistance Values and the other temperatures are known. A noise thermometer of this kind was constructed which is capable of measuring temperatures below approximately 140°K. The boiling points of liquid oxygen and liquid nitrogen were determined absolutely within 0.2 percent using the ice-point as reference. Between 1.3°K and 4.2°K the thermometer had to be calibrated due to errors arising in the equipment and the measured temperatures were then accurate within ± 1 percent. III. QUASI-PERSISTENT CURRENTS IN RINGS COMPOSED OF SUPERCONDUCTING AND NON-SUPERCONDUCTING REGIONS - A number of rings composed of a superconductor (Pb, In) apart from a small insert of normal metal (Cu) perpendicular to the current flow have been investigated between 1.30°K and 4.33°K for Pb-Cu and between 1.30°K and 3.20°K for In-Cu. It was found that for samples with good electrical contact the decay of the magnetic field due to the current is exponential and that the effective resistance increased compared with the bulk resistance of Cu by approximately 2.1 for the Pb-Cu rings and by 18.5 for the In-Cu rings. Two different thicknesses of the Cu inserts (0.0125 cm and 0.0053 cm) were used and it was found that the resistivity of the thin Cu insert increased with respect to the thick foil by 16% for the Pb-Cu system and by 36% for the In-Cu system. Part of this relative increase can be explained as a size effect due to electron scattering in the Cu insert. The effective resistance of the Pb-Cu rings shows a maximum at approximately 3.4°K. The resistance of the In-Cu samples decreases by about 10% between 3.2°K and 1.3°K. The resistivity of the Cu foil when measured separately was constant for the above temperature range. For samples with "poor" electrical contact (probably due to some copper oxide on the insert) two definite relaxation times were observed. For these samples the effective resistance was current and temperature dependent and it was decreasing for decreasing currents and decreasing temperatures. This can be explained in terms of a rectification effect of the two oxide layers on the insert. The decay of the magnetic field of the ring is consistent with the decay of a current in an L-R circuit. IV. THE DESTRUCTION OF SUPERCONDUCTIVITY IN TANTALUM WIRES BY A CURRENT - The transition from the superconducting to the normal state of various pre-stretched tantalum wires carrying current was investigated. When the resistance of the wire jumps discontinuously from the superconducting to the normal or intermediate state as a current is passed through it, then this current is defined as the critical current I(c). For temperatures T &lt; (T(c)-5 millidegrees K) the resistance of the wire jumps directly from zero resistance to its normal value at the critical current, such that the total cross section of the wire goes effectively into the normal state. Between (T(c)-5 millidegrees K) and T(c) the resistance of the wire jumps at I(c) to any fraction of the normal resistance between approximately zero and one. For constant temperatures the resistance-current plots show a large hysteresis effect. The transition temperature, T(c), of the various samples is strongly dependent upon their normal resistivity at helium temperatures. If the wires with a small constant current (4.2 ma) flowing in them are cooled from above the transition temperature, the resistance decreases above T(c) and approaches zero at approximately T(c) where T(c) is defined by the extrapolation of the I(c)-T curve to I(c) = 0. If the wires are heated from below T(c) the same resistance-temperature curves are reproduced.