The Novel Superconducting Clathrates: Ba_6Ge_(25)

摘要

The newly-discovered clathrates Ba_6Ge_(25) and its relatives consist of a rigid Germanium framework, in which Barium or other metallic atoms are trapped. These 'guest' atoms can 'rattle' freely in the oversized cages at high temperature. In Ba_6Ge_(25), some of Ba atoms are frozen into the split positions randomly below 200 K. This structural phase transition results in a two-step resistivity jump from metallic behavior to semiconductor-like behavior characterized by a mean free path of 3A. Surprisingly, a BCS-like superconducting transition occurs at T_c≈0.24 K in such a 'bad metal'. By applying hydrostatic pressure, the structural phase transition is suppressed but T_c increases drastically. T_c reaches a maximum value of 3.85K at a critical pressure p_c≈2.8GPa, where the structural distortion is completely suppressed and the system exhibits metallic behavior over the whole temperature range. Higher pressures lead to a slight decrease of T_c . On replacing 1β of the Ba atoms with Na, no structural phase transition is observed and the compound displays a similar behavior to Ba_6Ge_(25) above p_c, with T_c≈l.05K at ambient pressure. The relationship between the superconductivity and the structural phase transition is briefly discussed.