Optical spectroscopy study of ${\mathrm{Nd}(O,F)\mathrm{BiS}}_2$ single crystals

We present an optical spectroscopy study on F-substituted ${\mathrm{NdOBiS}}_2$ superconducting single crystals grown using the KCl/LiCl flux method. The measurement reveals a simple metallic response with a relatively low screened plasma edge near 5000 ${\mathrm{cm}}^{-1}$ . The plasma frequency is estimated to be 2.1 eV, which is much smaller than the value expected from the first-principles calculations for an electron doping level of $x=0.5$, but very close to the value based on a doping level of $7\%$ of itinerant electrons per Bi site as determined by the angle-resolved photoemission spectroscopy (ARPES) experiment. The energy scales of the interband transitions are also well reproduced by the first-principles calculations. The results suggest an absence of correlation effect in the compound, which essentially rules out the exotic pairing mechanism for superconductivity or scenario based on the strong electronic correlation effect. The study also reveals that the system is far from a charge-density wave (CDW) instability as being widely discussed for a doping level of $x=0.5$.

Figure 1

X-ray diffraction pattern of the ${\mathrm{Nd}(O,F)\mathrm{BiS}}_2$ single crystal. Inset shows the temperature dependence of magnetic susceptibility for the annealed ${\mathrm{Nd}(O,F)\mathrm{BiS}}_2$ single crystal.

Figure 2

The normalized electrical resistivity vs temperature of both as-grown and annealed ${\mathrm{Nd}(O,F)\mathrm{BiS}}_2$ crystals. The inset displays an enlarged region near the superconducting transition.

Figure 3

(a) Temperature dependence of $R\left(\omega \right)$ from 100 to 20 000 ${\mathrm{cm}}^{-1}$ . (b) Temperature dependence of the real part of the optical conductivity ${\sigma }_1\left(\omega \right)$.

Figure 4

The real part of the optical conductivity ${\sigma }_1\left(\omega \right)$ at 300 K, together with the decomposed Drude and Lorentz components. The calculated conductivity based on the first-principle band structure within GGA and an assumed value of scattering rate is also presented.

Figure 5

(a) The band dispersions of ${\mathrm{NdOBiS}}_2$ by first-principle calculations within GGA with a shift of $E_F$ by 0.85 eV. (b) The Fermi surfaces of ${\mathrm{NdOBiS}}_2$ obtained by shifting up $E_F\sim 0.85$ eV, which roughly corresponds to a doping level of $7\%$ per Bi site. (c) The real part and imaginary part of the dielectric function. The interband transition peaks and corresponding transitions in the band dispersions are indicated by the arrows.