Using a previously reported low-energy sputtering technique, Nb-Ge films have been prepared with superconducting T c 's up to ~ 23 K. High-field, x-ray, and annealing results are reported for these highest-T c films. A discussion of the critical parameters required to achieve the highest transition temperatures in the Nb-Ge system is given. Also further arguments in support of the theories that these highest T c 's are due to the formation of the stoichiometric NbJGe phase in the sputtered films are presented.

Measurements of the transport critical-current density (Jc), magnetization Jc, and magnetoresistance in a number of bulk sintered samples of Y1Ba2Cu3Ox from several different laboratories indicate that the transport Jc is limited by weak-link regions between high Jc regions. The weak-link Jc has a Josephson character, decreasing by two orders of magnitude as the magnetic field is increased from 0.1 to 10 mT at 77 K. An examination of the grain-boundary region in Y1Ba2Cu3Ox shows no observable impurities or second phases to the scale of the [001] lattice planes (∼12 Å). The effect of intrinsic conduction anisotropy is discussed. A current-transfer model is proposed in which weak conduction along the c axis plays a role in limiting Jc at grain boundaries. Orienting the grains in the powder state during processing may result in enhanced transport Jc in bulk conductors.

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The importance of geometrical parameters in determining the superconducting properties of a material has recently been a topic of considerable theoretical interest. Experimental investigation in this area so far has been restricted principally to thin-film structures of the non-transition elements, compared of materials possessing relatively small superconducting non-parameters (K). In this paper we discuss some recent studies on thin-film structures of transition metal compounds having both high critical temperatures (Tc≳14 °K) and large upper critical fields (Hc2≳100 kOe). The materials in this work have the rocksalt structure (B1) and are based upon the compound niobium nitride, NbN. The films were prepared by reactive sputtering in argon-nitrogen atmospheres in an ultrahigh vacuum system with a background pressure ≲5×10−10 Torr. Variation of the nitrogen partial pressure over the range 10−3 to 10−6 Torr was used to obtain different film compositions.