ABSTRACT
I regard the already mentioned paper [32], being accomplished as far back as 1944 (it was submitted for publication on 21 December, 1944), as initial. From [32], it is quite clear that the Londons theory is invalid for the description of the behavior of superconductors in strong enough fields and, in particular, for the calculation of the critical field in the case of films. The introduction of the surface energies o-n and a-8 was an artificial technique and these quantities were absurdly large new constants whose values were not predicted by the theory. The same applies to the surface energy a-n s on the boundary between the normal and superconducting phases. It was also absolutely unclear how the critical current should be calculated in the case of small-sized superconductors. Therefore, it was necessary somehow to generalize the Londons' theory to overcome its limits. Unfortunately, advancement in this direction was slow. One of the possible explanations is that like many theoretical physicists of my generation and the previous, I was simultaneously engaged in the solution of various problems and did not concentrate on anything definite (it can be seen, for instance, from the bibliographical index [47]). But there was gradual progress. So, on the basis of the conception of the Landau theory [4], I came to the conclusion [48] that electromagnetic processes in superconductors must be nonlinear and, incidentally, suggested a possible experiment for revealing such nonlinearity. The main point is that, in note [48,] I made the following remark: 'The indication of a possible inadequacy of the classical description of superconducting currents consists in the fact that the zero energy of excitation in a superconductor is equal in order of magnitude to li2njm6 "' 1 erg cm-2 (for 6"' w-5 em and n"' 1022 cm-3 ) and is thus higher than the magnetic energy 6H2 j81r "' 0.1 erg cm-2 (for H "' 500 Oe)'. The feeling that the theory of superconductivity should take into account quantum effects was also reflected in note [49] devoted for the most part to critical velocity in helium II. At the same time, in that paper I also tried to apply the theory of second-order phase transitions to the >.-transition in liquid helium.
