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

Layer-structured metal nitride chlorides MNC1 (M = Zr, Hf) were electron-doped by the intercalation of alkali metals (A = Li and Na) to form A(x)MNCl, 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)MNCl being expanded by 5.6-18.5 A and the superconducting transition temperatures (T(C)s) increasing by similar to 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 = 4.8-5.4. This finding implies a significant weakening of vortex pinning in the swelled phase. To investigate the irreversibility microscopically, nuclear magnetic resonance (NMR) measurements were carried out on Li-0.5(THF)(y)HfNCl. The H-1- and Li-7-NMR relaxation and chemical shift suggested the relevance of vortex motions to the depinning at T-irr which is the depinning temperature between the thermally excited pancake vortex liquid state at higher temperatures and the pancake vortex glass state at lower temperatures.