RESEARCH ON SMALL-SIZE SUPERCONDUCTORS AT THE INSTITUTE FOR PHYSICAL PROBLEMS OF THE ACADEMY OF SCIENCES OF THE USSR1

In research on superconductivity in recent years, two directions have emerged. The first direction includes numerous studies of transitions from the superconducting state to the intermediate and normal states in bodies of various shapes—cylinders, spheres, ellipsoids, and hollow superconductors—in a magnetic field. The other direction is characterized by attempts to penetrate into the very essence of the phenomenon of superconductivity. Studies in this direction set as their goal the search for and investigation of various anomalous phenomena in superconductors. It is most natural to expect these anomalies in superconductors of small size—whose dimensions are comparable with the depth of penetration of the magnetic field into the superconductor—such as colloids, threads, and films. The chief difficulty in studying superconductors of this class is the lack of certainty that the observed anomalous properties are in fact determined by the small dimensions of the superconductors, and not by impurities, which can easily be introduced into a volume consisting practically of a single surface layer. From this point of view, colloids and threads are, naturally, the dirtiest and most inconvenient objects both in the method of preparation itself and because it is impossible, for colloids, to apply electrical methods of investigation—resistance measurement.

The authors set themselves the task of investigating superconductivity in thin layers under conditions that would ensure sufficient purity of the objects. Both the electrical and magnetic properties of thin films of lead, tin, and thallium, obtained under vacuum conditions by evaporation and condensation on the surface of glass, were investigated.

In studying films with thicknesses ranging from \(5 \cdot 10^{-7}\) to \(3 \cdot 10^{-5}\) cm, prepared in the form of strips measuring \(0.3 \cdot 10\) mm, the following was found. Even the thinnest films, only \(15\)–\(20\) atomic layers thick, proved to be superconducting. By passing a small current through the film and measuring its resistance with a potentiometric circuit, it was easy to determine the value of the critical temperatures. By increasing the current flowing through the film, one could observe a sharp jump in the film resistance associated with its transition to the normal state. This sharp jump occurred, for a given film, always at a definite current strength and could serve as a criterion of its superconducting

properties. By studying the dependence of the critical current on temperature, it was also possible to determine the value of the critical temperature. When the superconducting properties of the film were destroyed by applying an external magnetic field, it was found that destruction of superconductivity without passing a current through the film was practically impossible. Destruction could be achieved only by simultaneous action on the film of current and an external field. When the dependence of the critical current destroying superconductivity on the external magnetic field was measured, it became clear that, in the absence of current, the magnitude of the critical magnetic field would reach tens of thousands of oersteds, thus exceeding the normal value for metal in bulk in hundreds of times. At the same time, it was found that films obtained by condensation at \(4.2^\circ\ \mathrm{K}\) have a critical temperature considerably exceeding (by approximately \(1^\circ\ \mathrm{K}\)) the normal value of the critical temperature for the massive metal. The same films, however, after being subjected once to heating to room temperature, gave, on re-examination, critical temperatures close to normal. Here it may be assumed that a metal deposited at such low temperatures has a special structure responsible for the observed shift of the critical temperature. Alongside investigations of the electrical properties of the films, their magnetic properties were also studied; in this case the principal task was to determine the critical magnetic fields corresponding to a change in the magnetic moment of the film. For this purpose, a film in the form of a narrow strip was deposited on a glass plate suspended from a thin quartz thread. After the film had passed into the superconducting state, such a plate tended to become parallel to the field. If the quartz thread was sufficiently thin, then, on increasing the magnetic field, the plate, having become practically parallel to it, remained in this position until the field reached a critical value, after which it sharply returned to its equilibrium position. The return of the film to the equilibrium position occurred as a result of the change in its magnetic moment upon transition from the superconducting to the normal state. From experiments of this kind it was determined that for a film, beginning with thicknesses of the order of \(10^{-5}\ \mathrm{cm}\), the value of the critical field increases sharply. Thus, for films with a thickness of the order of \(10^{-5}\ \mathrm{cm}\) it is approximately twice the critical field for the massive metal, whereas for films with a thickness of approximately \(10^{-6}\ \mathrm{cm}\) the critical field is thirty times greater than for the metal in bulk.

The agreement of the critical fields obtained in this work (and of the corresponding changes in the magnetic moment of the film) with the fields corresponding to the appearance of resistance in it may be interpreted as proof that the increase in the critical field of films occurring with decreasing thickness is indeed a consequence of the small thickness of the film and is not caused by any secondary effects. On the basis of the data obtained, one can construct the dependence of the ratio of the critical field of a film to the critical field of the massive metal as a function of its thickness. The dependence obtained agrees quite closely with the theoretical expression given in the work of Eilhardt, London, et al. [1]. Such agreement speaks in favor of London’s phenomenological theory of superconductivity. The value of the penetration depth of the field for tin at \(2^\circ\ \mathrm{K}\), calculated on the basis of the above-mentioned theoretical considerations of these authors, is \(1.64 \cdot 10^{-5}\ \mathrm{cm}\).

N. Alekseevskii and A. Shalnikov, Moscow

LITERATURE

1. Appleyard, Bristow, London and Misener, Proc. Roy. Soc. 5, 172, 540, 1939.

1. Brief account of a report delivered at the session of the Division of Physical and Mathematical Sciences of the Academy of Sciences of the USSR on April 17, 1940. ↩