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

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Layer-structured metal nitride chlorides $M$NCl ($M=\text{Zr}$, Hf) were electron-doped by the intercalation of alkali metals ($A=\text{Li}$ 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 Å and the superconducting transition temperatures ($T_{\text{c}}$s) increasing by ${\sim}2$ K. When the magnetization data were plotted as a function of temperature, an irreversibility temperature ($T_{\text{irr}}$) was observed, where the zero-field-cooled (ZFC) and field-cooled (FC) magnetization superimpose. The reversible temperature range ($T_{\text{c}}-T_{\text{irr}}$) was markedly increased upon swelling. The slope of the plot of $\log H$ (applied magnetic field) vs $\log(1-T_{\text{irr}}/T_{\text{c}})$ was increased for the swelled system. The slope $n$ in the power-law relationship for Na0.28(PC)yHfNCl and Li0.5(THF)yHfNCl 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 Li0.5(THF)yHfNCl. The 1H- and 7Li-NMR relaxation and chemical shift suggested the relevance of vortex motions to the depinning at $T_{\text{irr}}$, 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.
2010-01-15

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

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