Eur. Phys. J. B (2018) 91: 2
https://doi.org/10.1140/epjb/e2017-80477-8

Regular Article

Strong temperature effect on the sizes of the Cooper pairs in a two-band superconductor

¹ Institute of Physics, University of Tartu, W. Ostwaldi Str. 1, 50411 Tartu, Estonia
² Department of Theoretical Physics, The Royal Institute of Technology, 10691 Stockholm, Sweden
³ Lublin University of Technology, Faculty of Mechanical Engineering, Nadbystrzycka 36, 20618 Lublin, Poland
⁴ AGH University of Science and Technology, Department of Process Control, Mickiewicza 30, 30-059 Krakow, Poland

^a e-mail: teet.ord@ut.ee

Received: 17 August 2017
Received in final form: 9 November 2017
Published online: 15 January 2018

Abstract

We study the temperature dependencies of the mean sizes of the Cooper pairs in a two-band BCS-type s-wave superconductivity model with coupling cut-off in the momentum space. It is found that, in contrast to single-band systems, the size of Cooper pairs in the weaker superconductivity band can significantly decrease with a temperature increase due to an interband proximity effect. The relevant spatial behaviour of the wave functions of the Cooper pairs is analyzed. The results also indicate a possibility that the size of Cooper pairs in two-band systems may increase with an increase in temperature.