Successive superconducting transitions and Anderson localization effect in Ta${}_2$S${}_2$C

A complex carbide Ta${}_2$S${}_2$C consists of van der Waals (vdw)-bonded layers with a stacking sequence $\mathrm{.}\mathrm{.}\mathrm{.}$ C-Ta-S-vdw-S-Ta-C- $\mathrm{.}\mathrm{.}\mathrm{.}$ along the c axis. The magnetic properties of this compound have been studied from DC and AC magnetic susceptibility. Ta${}_2$S${}_2$C undergoes successive superconducting transitions of a hierachical nature at $T_{cl}=3.61\pm 0.01$ K [$H_{c1}^{(l)}(0)=28\pm 2$ Oe and $H_{c2}^{(l)}(0)=7.7\pm 0.2$ kOe] and $T_{cu}=9.0\pm 0.2$ K [$H_{c2}^{(u)}(0)=6.0\pm 0.3$ kOe]. The intermediate phase between $T_{cu}$ and $T_{cl}$, where $\delta \chi (= \chi_{FC} - \chi_{ZFC}) \approx 0$, is an intra-grain superconductive state occurring in the Ta-C layers in Ta${}_2$S${}_2$C. The low temperature phase below $T_{cl}$, where $\delta \chi$ clearly appears, is an inter-grain superconductive state. The magnetic susceptibility at $H$ well above $H_{c2}^{(l)}(0)$ is described by a sum of a diamagnetic susceptibility and a Curie-like behavior. The latter is due to the localized magnetic moments of conduction electrons associated with the Anderson localization effect, occurring in the 1T-TaS${}_2$ type structure in Ta${}_2$S${}_2$C.