W.W. Fuller

Resistive and Magnetic Susceptibility Transitions in Superconducting (TMTSF)2C104

Abstract We report measurements of the ac magnetic susceptibility and dc resistive superconducting transitions in the organic superconductor (TMTSF)2C104. Inductive measurements show complete diamagnetic shielding below a broad transition and initial flux penetration at very low fields [Hc1(0) < 1 Oe]. The resistive transition is also broad, but occurs at a significantly higher temperature than the inductive transition, Tc = 1.0 K and 0.65 K respectively. Resistance measurements also show evidence of a phase transition in the vicinity of 24 K. Magnetic field induced transitions, measured both inductively and resistively, show marked anistropy both in magnitude and in breadth of the transition. Results suggest that (TMTSF)2C104 is a quasi ID or 2D superconductor at high temperatures and high magnetic fields and an anisotropic bulk superconductor at low temperatures and fields. Associated thermoelectric power measurements suggest that spin density waves coexist with the superconducting state.

Giant resistivity and X-ray diffraction anomalies in low-dimensional ZrTe5 and HfTe5

Abstract Two giant anomalies have been observed in single crystal samples of metallic ZrTe 5 and HfTe 5 : one is an extremely large maximum in the electrical resistivity at low temperature and the other is a correspondingly large maximum in the intensities of certain “forbidden” x-ray reflections, which correlate in temperature with the electrical anomaly.

Study of the magnetic superconductor, Y9Co7, at high pressure and high magnetic field

Abstract We have measured the electrical resistance and magnetic susceptibility of the magnetic superconductor, Y g Co 7 , at pressures up to 20 kbar and in magnetic fields up to 6T. We have found that pressure suppresses the magnetism resulting in a higher superconducting transition temperature and conclude that Y 9 Co 7 is an itinerant ferromagnet, not a spin-glass. Pressure also sharpens the superconducting transition and increases the critical magnetic field, signifying that the long range ferromagnetic and superconducting order parameters co-exist but vary spatially. For pressures greater than 6 kbar, the magnetoresistance is always positive, further indicating the suppresion of magnetism by high pressure.

Synthesis and Study of Electrochemically Grown Salts of Organic II-Donors

Abstract The effects of purity and crystal perfection on the properties of (TMTSF)2ClO4 are examined via SEMs and transition temperatures. Careful electrochemical techniques using gradient sublimed TMTSF starting material and short term oxidation result in high quality crystals. Preliminary work on HMTSF salts of PF − 6 and AsF− 6 is also reported.

Effects of charging energy on the 2D phase transition in granular NbN cermet films

Abstract We have developed quantitative equations for the resistive transition of a superconducting 2-dimensional granular array including the effects of intergranular charging energy. The equations are applied to thin BN-NbN cermet films. Results indicate that charging energy plays a significant role in the high sheet resistance, low T c samples.

High field specific heat and susceptibility measurements on single crystal CeCu6

Abstract Measurements of the specific heat, C , for 2 K T H ⩽23 T and the ac susceptibility, x , for 0.13 K T H ⩽12 T on single crystals of CeCu 6 are presented. The field was aligned along the c -axis where the crystal field does not dominate the magnetic response. This work was motivated by a model prediction that C/T and x at low temperatures should both be proportional to the density of states at the energy μ H . It was found that the model correctly predicts the magnetic field decrease of C/T at low temperatures and that C/T at T = 3 K , H = 23 T is reduced to 0.254J/(mol K 2 ). The measured magnetic field dependence of C/T and x are found to be approximately the same.

Effect of heat treatment and sputtering conditions on the structure and superconducting transition of BiSrCaCuO films

Abstract The morphology of BiSrCaCuO films sputtered on single crystal MgO substrates, after heat treatment, depends upon the sputtering conditions. Sputtering of films in argon without oxygen present results in the formation of crystallites in the form of terraced islands with a preferred orientation. Sputtering with oxygen present results in more uniform films. In both cases, the films in the as-deposited condition are amorphous. Annealing the films in the temperature range from 750 to 865 °C results in highly textured crystalline films and produces changes in the superconducting properties. It has been found that part of the film goes superconducting at 115 K after a three step annealing process in which the highest temperature was 865 °C.

Preparation, Structure, and Magnetic Field Studies of High Tc Superconductors

We have prepared and characterized high Tc single phase samples of La2−xMxCuO4 (where M=Sr, Ba, and Ca and x=0.1, 0.2 or 0.3) and Ba2YCu3O7. Transmission electron microscopy of the Ba2YCu3O7 samples reveals twinning defects in the [110] direction which disappear when the sample is heated above 400°C. We report on the dc susceptibility, magnetization, and magnetoresistance. We have extracted many of the critical superconducting and normal state parameters from the data.

Resistance determination of critical magnetic fields of superconducting (TMTSF)2Cl04

Abstract We report dc resistance measurements on two (TMTSF)2ClO4 samples, both of which undergo a superconducting transition at Tc ≈ 1.0K. Magnetic field induced transitions from the superconducting to the normal state at T

Current-voltage behavior of 2D granular NbN films

Abstract We have measured the I–V characteristics of 2D granular NbN near the superconducting topological transition temperature, T 2D . Characteristic changes in the I–V curves occur at T 2D and the shape of the I–V curve at T 2D is consistent with theoretical predictions. These results lend additional support to the vortex-antivortex model for the 2D topological transition, but suggest refinements in the theory to incorporate the granular nature of the films will be needed for quantitative agreement.