Enhancement of superconductivity in ${\mathrm{La}}_{1-x}{\mathrm{Sm}}_x{\mathrm{O}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$

The superconducting and normal-state properties of ${\mathrm{La}}_{1-x}{\mathrm{Sm}}_x{\mathrm{O}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2 \left(0.1\le x\le 0.9\right)$ have been studied via electrical resistivity, magnetic susceptibility, and specific heat measurements. By using suitable synthesis conditions, Sm exhibits considerable solubility into the ${\mathrm{CeOBiS}}_2$-type ${\mathrm{LaO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$ lattice. In addition to a considerable enhancement of the superconducting volume fraction, it is found that the superconducting transition temperature $T_c$ is dramatically enhanced with increasing Sm concentration to 5.4 K at $x=0.8$. These results suggest that $T_c$ for ${\mathrm{SmO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$ could be as high as $\sim 6.2$ K, and comparably high $T_c$ values might also be obtained in the compounds $Ln{\mathrm{O}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2 \left(Ln=\text{Eu}-\text{Tm}\right)$ if they can be synthesized.

Figure 1

X-ray diffraction pattern of ${\mathrm{La}}_{0.3}{\mathrm{Sm}}_{0.7}{\mathrm{O}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$ as a representative example. Black crosses denote the experimental data. Red and blue lines are the calculated XRD pattern and the difference between the observed and calculated patterns, respectively. Tick marks represent calculated peak positions of the main phase (purple) and ${\mathrm{LaF}}_3$ (orange). The reliability factors are wRp = 5.27% and Rp = 3.99%. (Inset) XRD profiles of the {102} and {004} peaks of $x=0.1$ to 0.8. The dashed lines are guides to the eye.

Figure 2

Dependence of (a) lattice parameters $a$ (left axis) and $c$ (right axis), and (b) unit-cell volume $V$ on nominal Sm concentration $x$. (c) $V$ for $Ln{\mathrm{O}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$ ($Ln=\mathrm{La}$, Ce, Pr, Nd) from Ref. [7] and estimated $V$ for ${\mathrm{SmO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$. Dashed lines are guides to the eye.

Figure 3

(a) Temperature dependence of the electrical resistivity, $\rho \left(T\right)$, normalized by its value at 290 K, $\rho$(290 K), for ${\mathrm{La}}_{1-x}{\mathrm{Sm}}_x{\mathrm{O}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$. The inset displays the data in panel (a) from 2 to 6 K, emphasizing the superconducting transitions. (b) and (c) Electrical resistivity $\rho \left(T\right)$ for two samples with $x=0.5$ and $x=0.8$, respectively. The annealing temperature used for each sample is denoted.

Figure 4

Superconducting critical temperature $T_c$ vs nominal Sm concentration $x$ of ${\mathrm{La}}_{1-x}{\mathrm{Sm}}_x{\mathrm{O}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$. Red, blue, and purple symbols represent results for samples annealed at $800{}^{\circ }\mathrm{C},750{}^{\circ }\mathrm{C}$, and $710{}^{\circ }\mathrm{C}$, respectively. The dashed line is a linear fit of $T_{c,\rho }$ from $x=0.1$ to $x=0.8$. (Inset) $T_{c,\rho }$ of $Ln{\mathrm{O}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$ compounds reported in Refs. [7] and [8] together with the estimated $T_{c,\rho }=6.2$ K of ${\mathrm{SmO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$.

Figure 5

(a) Zero-field-cooled (ZFC) (filled symbols) and field-cooled (FC) (open symbols) dc magnetic susceptibility data for ${\mathrm{La}}_{1-x}{\mathrm{Sm}}_x{\mathrm{O}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$ in an applied magnetic field of 10 Oe. (b) Paramagneticlike behavior of selected samples with $x=0.1$ and 0.9 in applied magnetic fields of 5 Oe and 10 Oe, respectively. Magnetic susceptibility data for $x=0.1$ in 10 Oe is also plotted for comparison. (c) Real and (d) imaginary part of the ac magnetic susceptibility for selected samples. (e) Evolution of shielding volume fraction with Sm substitution. Open and filled circles correspond to ac and dc susceptibility; red, blue, and purple colored data points represent measurements on samples annealed at $800{}^{\circ }\mathrm{C},750{}^{\circ }\mathrm{C}$, and $710{}^{\circ }\mathrm{C}$, respectively. (f) $M/H$ and $H/M$ vs $T$ data in the normal state for ${\mathrm{La}}_{0.3}{\mathrm{Sm}}_{0.7}{\mathrm{O}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$, measured from 2 to 300 K in an applied magnetic field of 5 kOe. The solid line is a nonlinear fit using Eq. (1).

Figure 6

(a) Specific heat $C$ divided by temperature, $C/T$, vs $T$ for ${\mathrm{La}}_{1-x}{\mathrm{Sm}}_x{\mathrm{O}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$ with $x=0.1$, 0.8 and for ${\mathrm{LaOBiS}}_2$. (Inset) ($C\text{−}{C}_{ph})/T$ vs $T$, where $C_{ph}$ is the lattice contribution, and a fit of the Schottky anomaly contribution for ${\mathrm{La}}_{0.2}{\mathrm{Sm}}_{0.8}{\mathrm{O}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$ (dashed line). (b) and (c) Electronic contribution $C_{el}/T$ of ${\mathrm{La}}_{1-x}{\mathrm{Sm}}_x{\mathrm{O}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$ with $x=0.1$ and $x=0.8$, respectively. An entropy conserving construction is shown, which is used to define $T_c$.