From Current Literature

Absence of Superconductivity in Uranium¹

The question of superconductivity in uranium has a short history. Shoenberg, as early as 1940,² while investigating a not very pure sample of uranium, discovered the appearance of a slight diamagnetic effect at a temperature of about 1.5° K (as is known, superconductors possess an anomalously large diamagnetism, owing to which the magnetic induction in the bulk of a superconductor is zero—the Meissner effect). Shoenberg at once attributed this effect to the presence of superconducting impurities. In 1942, in a paper devoted to the superconductivity of rhenium, Aschermann and Justi³ mention experiments that led them to the conclusion that uranium is superconducting. Mott⁴ reports additional information on Justi’s work. Measuring the resistance of uranium, Justi found that at 1.25° K uranium becomes superconducting.

In a short note published in Nature, Shoenberg reports a more careful investigation of uranium that he has recently carried out. Using a magnetic method based on the measurement of magnetic moment, Shoenberg recorded a weak diamagnetic effect in a pure sample of uranium at temperatures below 1.45° K. The magnitude of the effect corresponds to a content of superconductor not exceeding 0.5% of the total volume of the metal. The critical field (i.e., the field that destroys superconductivity) for this effect is about ~250 oersteds at the lowest temperature reached by the author, ~1.08° K. Shoenberg believes that these results, together with the results of chemical analysis of the sample, indicate that the effect is caused by a superconducting impurity and that this impurity is uranium carbide. This conclusion has not yet been confirmed by direct experiments on uranium carbide.

Shoenberg points out the unreliability of the method based on measuring resistance (which Justi used). A small quantity of a superconducting impurity can form a superconducting filament, which will cause a sharp drop in the resistance of the whole sample. When the magnetic method is used, however, the observed effect is always proportional to the volume of the superconducting impurity. On the basis of his experiments, Shoenberg considers the non-superconductivity of uranium above 1.08° K to be beyond doubt.

B. Geilikman

Cited Literature

1. D. Shoenberg, Nature, 159, 303 (March 1947).
2. D. Shoenberg, Proc. Cambr. Phil. Soc. 36, 84, 1940.
3. G. Aschermann, E. Justi, Phys. Zschr. 43, 207, 1942.
4. N. F. Mott, Nature 158, 801, 1946.