Effect of Swelling on the Superconducting Characteristics in Electron-Doped β-ZrNCl and HfNCl

Layer-structured metal nitride chlorides M NCl ( M =Zr, Hf) were electron-doped by the intercalation of alkali metals ( A =Li and Na) to form A x M NCl, and then swelled with organic solvent molecules [tetrahydrofuran (THF) and propylene carbonate (PC)]. Upon swelling, the organic molecules were cointercalated with the alkali metals, resulting in the basal spacing of A x M NCl being expanded by 5.6–18.5 A and the superconducting transition temperatures ( T c s) increasing by ∼2 K. When the magnetization data were plotted as a function of temperature, an irreversibility temperature ( T irr ) was observed, where the zero-field-cooled (ZFC) and field-cooled (FC) magnetization superimpose. The reversible temperature range ( T c - T irr ) was markedly increased upon swelling. The slope of the plot of log H (applied magnetic field) vs log (1- T irr / T c ) was increased for the swelled system. The slope n in the power-law relationship for Na 0.28 (PC) y HfNCl and Li 0.5 (THF) y HfNCl is obtained to be n = 4.8–5...