Selenium isotope effect in the layered bismuth chalcogenide superconductor $\mathrm{La}{\mathrm{O}}_{\mathbf{0}.\mathbf{6}}{\mathrm{F}}_{\mathbf{0}.\mathbf{4}}\mathrm{Bi}{\left(\mathrm{S},\mathrm{Se}\right)}_{\mathbf{2}}$

We have investigated the Se isotope effect in the layered bismuth chalcogenide ($\mathrm{BiC}{\mathrm{h}}_2$-based) superconductor $\mathrm{La}{\mathrm{O}}_{0.6}{\mathrm{F}}_{0.4}\mathrm{Bi}{\left(\mathrm{S},\mathrm{Se}\right)}_2$ with ${}^{76}\mathrm{Se}$ and ${}^{80}\mathrm{Se}$. For all examined samples, the Se concentration, which is linked to the superconducting properties, is successfully controlled within $x=1.09–1.14$ in $\mathrm{La}{\mathrm{O}}_{0.6}{\mathrm{F}}_{0.4}\mathrm{Bi}{\mathrm{S}}_{2-x}\mathrm{S}{\mathrm{e}}_x$. From the magnetization and electrical resistivity measurements, changes in $T_{\mathrm{c}}$ are not observed for the $\mathrm{La}{\mathrm{O}}_{0.6}{\mathrm{F}}_{0.4}\mathrm{Bi}{\left(\mathrm{S},\mathrm{Se}\right)}_2$ samples with ${}^{76}\mathrm{Se}$ and ${}^{80}\mathrm{Se}$ isotopes. Our result may indicate that the pairing in $\mathrm{La}{\mathrm{O}}_{0.6}{\mathrm{F}}_{0.4}\mathrm{Bi}{\left(\mathrm{S},\mathrm{Se}\right)}_2$ is not mediated by phonons, which is consistent with recent theoretical and experimental studies suggesting unconventional superconductivity in the $\mathrm{BiC}{\mathrm{h}}_2$-based superconductors.

Figure 1

(a) and (b) X-ray-diffraction patterns and Rietveld refinement results for the ${}^{76}\mathrm{Se}$-No. 1 and ${}^{80}\mathrm{Se}$-No. 1 samples. The refinements were performed by two-phase analysis with the secondary phase of $\mathrm{Bi}{\mathrm{F}}_3$ (3%). The blue profiles plotted at the bottom are the differences between observed and calculated patterns. (c) Schematic of the refined crystal structure for the ${}^{76}\mathrm{Se}$-No.1 sample. The Ch1 site is the in-plane chalcogen site.

Figure 2

(a) Temperature dependences of magnetization from 2 to 5 K for the ${}^{76}\mathrm{Se}$-No. 1 and ${}^{76}\mathrm{Se}$-No. 2 samples. (b) Temperature dependences of magnetization from 2 to 5 K for the ${}^{80}\mathrm{Se}$-No. 1 and ${}^{80}\mathrm{Se}$-No. 2 samples. ZFC and FC denote data measured after zero-field cooling and field cooling, respectively. (c)–(f) Enlarged temperature dependences of magnetization around the superconducting transition. The onset $T_{\mathrm{c}}$ is estimated as 3.77, 3.76, 3.76, and 3.77 K for ${}^{76}\mathrm{Se}$-No. 1, ${}^{76}\mathrm{Se}$-No. 2, ${}^{80}\mathrm{Se}$-No. 1, and ${}^{80}\mathrm{Se}$-No. 2, respectively. Irreversible temperature $T_{\mathrm{irr}}$, defined as the temperature at which the difference between ZFC and FC curves emerges, is also estimated from these plots and listed in Table 1.

Figure 3

Temperature dependences of normalized electrical resistivity $\left[\rho \right(T)/\rho (4\mathrm{K}\left)\right]$ for the ${}^{76}\mathrm{Se}$-No. 1 and ${}^{80}\mathrm{Se}$-No. 1 samples around the superconducting transition. The onset $T_{\mathrm{c}}$ is estimated as 3.74 and 3.75 K for ${}^{76}\mathrm{Se}$-No. 1 and ${}^{80}\mathrm{Se}$-No. 1, respectively. For both samples, zero resistivity is observed at $T_{\mathrm{c}}^{\mathrm{zero}}=3.73\mathrm{K}$.