On the inverse isotope effect in superconducting (Zr, Hf)-(H, D) system

Abstract
Assuming the presence of low-lying local excitations of the two-level (TLS) type in superconducting (Zr, Hf)-(H, D) systems, the authors explain qualitatively the inverse isotope effect for the critical temperature Tc and the amount of Tc enhancement with increasing H (D) concentration.