IMPURITY INDUCED INCREASE IN TC OF THE SUPERCONDUCTOR

We consider a superconductor in which coupling between the conduction band electrons and the electrons of an impurity is realized by the tunneling of the Cooper pairs. We investigate the mode in which such tunneling influences the critical temperature Tc of the superconductor. It is found that in some conditions the presence of impurity may lead to the increase of Tc. Influence of coupling between electrons from different energy bands on superconducting properties of materials has first been studied by V.A. Moskalenko [1]. The model of superconductivity in two-band systems proposed by him is presently widely used worldwide to explain the properties of such compounds as MgB2 [2-4], superconducting pnictides [5]. Introduction of an impurity into a material with a single conduction band creates additional energy levels. In the present work, we will treat such systems assuming that the number of energy levels in one of the two bands has decreased to just a few. The issue to be considered in the present work is to clarify the conditions when the single-band system with impurities behaves similar to the one with two energy bands. It is known that under certai