R.W. White

SENSITIVE DETECTION OF SUPERCONDUCTIVITY BY CHANGES IN INDUCTANCE AND LOSSES OF A RESONANT BRIDGE

THE method of Schawlow and Devlin I is at present the most commonly used for the detection of superconductivity. This method employs the change in selfinductance of a solenoid when a sample located inside the coil becomes superconducting. In order to increase the sensitivity, the solenoid is made a part of a parallel resonant circuit that controls the frequency of an oscillator, and changes in frequency corresponding to small changes in inductance are measured on a frequency counter. The method requires no physical connections to the sample, and irregularly shaped samples may be used. Frequency stability of the oscillator is a problem, however, and, when the solenoid is in a cryogenic bath being cooled by pumping, the oscillator frequency changes--probably because bubbling of the liquid affects the dielectric material surrounding the solenoid. In the method to be described, pumping effects are reduced considerably by a balanced-bridge technique. Also, sensitivity is increased by utilizing both the inductance change and the change in coil loss 2 making it possible to detect superconductivity in less than 1 mg quantities of material. While both of these effects have been noted in the literature, 1-4 they have not previously been used simultaneously for the detection of superconductivity.

5f electrons and superconductivity: Dilute alloys formed between lanthanum and the actinide metals☆

Abstract Small amounts of the metals Th, U, Np, PuI, and Am were dissolved individually into fcc La, and the superconducting transition temperatures T c of the alloys were measured and compared with that of La. Th decreased T c of La only very weakly, and U seemed to do likewise. Np and Pu depressed T c quite strongly, as does the anomalous rare-earth metal Ce. Am does not appear to possess a magnetic moment in La. The present data complement previous work on the 4f metals and illustrate some differences between the first few transactinium metals and their counterparts in the rare-earth series.

Prognosis for superconductivity in Np and Pu

Abstract Np and Pu were substituted for U in bcc γ-U alloys and the depression of Tc measured. By extrapolating the Np and Pu impurity concentrations to 100%, estimates were made for the maximum superconducting transition temperatures that may be expected for bcc γ-Np and ∈-Pu alloys.

Effects of stoichiometry on the superconducting transition temperature of U6Fe and the uranium isotope effect

Determination of the uranium isotope effect on the superconducting transition temperature of U 6 Fe led to studies of the effects of stoichiometry on transition temperature. The isotope effect is T c ∝ M −0.37 .