Upper critical field in the electron-doped layered superconductorZrNCl0.7: Magnetoresistance studies

We report magnetoresistance measurements of layered superconductor $\mathrm{Zr}\mathrm{N}{\mathrm{Cl}}_{0.7}$ at high magnetic fields $(H)$ as high as $27\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ and temperatures $(T)$ as low as $0.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The $T$ dependence of the upper critical field ${H}_{c2}(T)$ for $H\ensuremath{\Vert}c$ and $H\ensuremath{\perp}c$ was obtained. ${H}_{c2}(T)$ starts to increase with a gentle slope near ${T}_{c}(0)$ followed by a steep slope with decreasing $T$, implying the dimensional crossover of the interplane coherence length from three dimensional to two dimensional (2D). The ${H}_{c2}(T)$ is found to be anisotropic for the field direction, where the slope of ${H}_{c2}^{\ensuremath{'}}=(d{H}_{c2}∕dT)$ near $H\ensuremath{\approx}1\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ can be estimated as ${H}_{c2}^{\ensuremath{'}\ensuremath{\perp}}\ensuremath{\approx}\ensuremath{-}22.1\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}∕\mathrm{K}$ and ${H}_{c2}^{\ensuremath{'}\ensuremath{\Vert}}\ensuremath{\approx}\ensuremath{-}4.9\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}∕\mathrm{K}$ for $H\ensuremath{\Vert}c$ and $H\ensuremath{\perp}c$, respectively. The obtained anisotropy parameter $\ensuremath{\gamma}\ensuremath{\approx}4.5$ gives evidence for the 2D characteristic of the superconducting state in $\mathrm{Zr}\mathrm{N}{\mathrm{Cl}}_{0.7}$.