Superconductivity in BiS$_2$-based compounds

Polycrystalline samples of $Ln$O$_{0.5}$F$_{0.5}$BiS$_2$ ($Ln$ = La, Ce, Pr, Nd, Yb) were synthesized by solid-state reaction. These compounds form in a tetragonal structure with space group $P4/nmm$ conforming to the CeOBiS$_2$ crystal structure. Electrical resistivity, magnetic susceptibility and specific heat measurements were performed on all of the samples. All of the compounds exhibit superconductivity in the range 1.9 K - 5.4 K, and the YbO$_{0.5}$F$_{0.5}$BiS$_2$ sample was also found to exhibit magnetic order (probably antiferromagnetic order) at $\sim$2.7 K that appears to coexist with superconductivity below 5.4 K [1]. Electron-doping appears to induce superconductivity in the BiS$_2$-based superconductors as partial substitution of F for O is necessary to observe superconductivity. This was further demonstrated in a study where trivalent La$^{+3}$ was partially substituted with tetravalent Th$^{+4}$, Hf$^{+4}$, Zr$^{+4}$, and Ti$^{+4}$, all of which induced superconductivity [2]. We also observed that substitution of divalent Sr$^{+2}$ for La$^{+3}$ (hole doping) does not induce superconductivity [2]. Electrical resistivity measurements were also performed under applied pressure on $Ln$O$_{0.5}$F$_{0.5}$BiS$_2$ ($Ln$ = La, Ce, Pr, Nd) up to $\sim$3 GPa and down to 1 K. These studies revealed a universal behavior where the systems are tuned away from semi-conducting behavior towards metallic behavior. The superconducting states were stabilized by applied pressure, so that $T_c$ ~increased in all of the rare earth members listed. At a critical pressure $P_c$, $T_c$ increases rapidly from a low $T_c$ phase to a distinct high $T_c$ phase, after which additional pressure no longer suppressed the semiconducting behavior in the normal state [3,4]. In addition, the metallization of NdO$_{0.5}$F$_{0.5}$BiS$_2$ also occurs at $P_c$.\\[4pt] This work was performed in collaboration with M. B. Maple, K. Huang, B. D. White and C. T. Wolowiec. \\[4pt] [1] Yazici et al, Philos. Mag. 93, 673, (2012).\\[0pt] [2] Yazici et al, Phys. Rev. \textbf{B} 87, 174512, (2013).\\[0pt] [3] Wolowiec et al, Phys. Rev. \textbf{B} 88, 064503, (2013).\\[0pt] [4] Wolowiec et al, Journal of Physics: Condensed Matter 25, 422201, (2013).