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Квантовый размерный эффект в сверхпроводниках
In the middle of the last century, it was demonstrated that with a decrease in the size of superconducting
structures, for example, the thickness of a thin film, its critical temperature Tc shifts by a certain amount.
It increases in aluminum, tin, and indium, and decreases in mercury, niobium and lead. However, there is
still no generally accepted theory explaining this effect. In the 70s, during the largest volume of research
on this topic, V.L. Ginzburg assumed that the transition temperature of a sufficiently pure, monoatomic
superconductor film will be exactly the same as in a bulk body. However, this assumption has not been
verified, and the question of the nature of this effect is still open. For the study, we chose aluminum, due
to the fact that the dependence of Tc of the film on its thickness is very predictable and increases with
decreasing size. Despite a number of works on the study of this dependence in aluminum, it is not always
possible to accurately establish a correspondence with the theory. This is because characteristics vary
from sample to sample, even made in the same batch. In our case, polycrystalline films were prepared, the
crystallite sizes in which are comparable to the film thickness, and epitaxial samples with an atomically
smooth surface. The films were fabricated by electron-beam deposition and molecular-beam epitaxy on
various substrates. Within the BCS model, the critical temperature of the superconducting transition
exponentially depends on the density of electronic states at the Fermi level N (EF) and the electron-
phonon interaction constant V: TC ~ exp (-1 / N (EF)) * V. It is shown in this work that, due to QSE in
thin superconducting films, both parameters N (EF) and V change nonmonotonically with the sample
thickness. This behavior is a consequence of form resonance theory. Presumably, the effect caused by the
disordering of crystallites, as well as by the surface or substrate, does not have a dominant role
specifically in our case, since the aluminum films are of high quality, and their thicknesses go far beyond
the limits of ultrathin objects, in which surface phenomena begin to play a decisive role. As a result of
this study, the experimental and theoretical dependences of TC on the thickness of films prepared by
different methods on different substrates were obtained.