Schottky Specific Heat due to the Tunneling Rotation of Methyl Groups in Deuterated (DMe-DCNQI)₂Cu System

Abstract

In the course of studies on thermal properties of (DMe-DCNQI)₂Cu systems that exhibit a reentrant metal-insulator transition, we discovered a Schottky type of specific heat with a peak around 8 K. The peak value exceeds 25 J/mol·K. This type of anomaly is observed only on samples composed of molecules containing partially deuterated methyl groups, and was found to be originated from the rotation of deuterated methyl groups that occurs by quantum tunneling. From an analysis of the specific heat curve, we obtained information concerning the low lying energy level scheme of deuterated methyl group CH₂D. It has a non-degenerated ground state and doubly degenerated first excited states located about 20 K above the ground state.