Raman scattering investigation of the electron-phonon coupling in superconducting Nd(O,F)${\mathrm{BiS}}_2$

We have performed polarized Raman scattering measurements on the newly discovered superconductor Nd(O,F)${\mathrm{BiS}}_2$ ($T_c=4$ K). We observe 2 Raman-active modes, with frequencies in accordance with first-principles calculations. One $A_{1g}$ phonon mode at $112.4 \mathrm{cm}{}^{-1}$ exhibits a Fano line shape due to electron-phonon coupling. We find a resonance for this mode at 2.45 eV excitation energy. We estimate a 0.68 contribution of this mode to the electron-phonon coupling constant $\lambda$. Our Raman results suggest that the ${\mathrm{BiS}}_2$-based superconductors are possibly phonon-mediated BCS superconductors.

Figure 1

(a) Polarization dependence of the $ab$ plane Raman spectra of Nd(O,F)${\mathrm{BiS}}_2$ recorded at 294 K with a 514.5 nm laser excitation. (b) Polarization dependence of the $ac$ plane spectra at 294 K. The dashed lines in (a) and (b) are guides to the eye for the phonon peak positions.

Figure 2

(a) Temperature dependence of the Raman spectra for the $A_{1g}$ mode at $112.4 \mathrm{cm}{}^{-1}$. (b)–(d) Peak position, Fano asymmetry factor $q$, and linewidth (full width at half maximum), respectively, of the $A_{1g}$ mode at $112.4 \mathrm{cm}{}^{-1}$. The curves in (b) and (d) are fits to Eqs. (2) and (3). The line in (c) is the averaged Fano asymmetry factor $q$.

Figure 3

(a) Photon energy dependence of the Raman spectra for the $A_{1g}$ mode at $112.4 \mathrm{cm}{}^{-1}$. (b) Intensity of the $A_{1g}$ mode at $112.4 \mathrm{cm}{}^{-1}$ with different incident laser normalized by the intensity at 488.0 nm. The black curve is a Gaussian fit to the data. (c) Fano asymmetry factor $q$ of the $A_{1g}$ mode at $112.4 \mathrm{cm}{}^{-1}$ at different incident laser lines. The curve in black is a linear fit of the data point.