Electron-phonon coupling behavior in MgB2 films with various thicknesses of ZnO buffer layer on metallic substrates

Abstract

In order to investigate the effect of buffer layer on the superconducting properties of MgB2, behaviors of electron-phonon coupling (EPC) in MgB2 thin films with various thicknesses of ZnO buffer layer on two different substrates Al2O3 and Hastelloy were studied. As the ZnO thickness increases, the values of Tc of MgB2 films on two substrates show opposite behaviors. Sharply decreased and then saturated Tc of ~38K is observed on Al2O3 substrates while an abrupt increase of Tc with no tendency of decrease is obtained on Hastelloy substrates. These Tc behaviors are closely related to the E2g phonon hardening/softening inside MgB2 which may be induced from a lattice distortion caused by an introduction of ZnO buffer layer. The electron-E2g coupling provides a dominant contribution to EPC from all phonon modes, however, it shows a very weak dependence on ZnO thickness to explain the Tc behavior of our samples. Therefore, in order to establish a mechanism of ZnO buffer layer influencing Tc of MgB2 films, contribution from other phonon modes needs to be considered. Understanding the effect of ZnO buffer layer on the EPC and the Tc behavior opens a possibility to obtain an optimal condition for ZnO buffer layer of MgB2 for the field of application.