Metal substitution effect on superconductivity in barium intercalated ternary metal nitride chlorides

We systematically studied barium cointercalated ternary metal nitride chloride superconductors of Ba_0.8(NH₃) _yZr_1−xHf _xNCl (x=0,0.25,0.5,0.75,1). X-ray diffraction and Raman spectra measurements show a systematical evolution of lattice constants and lattice dynamics upon Hf replacement for Zr. The superconducting transition temperature (T_c) increases monotonically from 11.7 K to 20.2 K with increasing the hafnium content in metal-nitrogen bilayers from 0 to 1. The upper critical field H_c2(0) for Ba_0.8(NH₃) _yHfNCl is estimated to be about 13.2 T. Accompanied by the remarkably increased T_c, the highest energy phonon mode coupled most strongly with electronic system shows a moderate hardening with Hf substitution for Zr. This result suggests the reinforcement of pairing interaction with Hf substitution arises mainly from the evolution of electronic structure of metal d-band rather than from phonon modes. (paper)