Electronic structure of carbosulfide superconductors

We present electronic band structure calculations and x-ray emission and photoelectron measurements of the recently discovered van der Waals–type layered superconductors ${\mathrm{Nb}}_2{\mathrm{S}}_2\mathrm{C}$ and ${\mathrm{Cu}}_{2∕3}{\mathrm{Nb}}_2{\mathrm{S}}_2\mathrm{C}$. Good agreement between our local partial density-of-states calculations and our experimental x-ray emission measurements of the constituents as well as between calculated and experimental $E\left(\mathbf{k}\right)$ dispersion curves is found for ${\mathrm{Nb}}_2{\mathrm{S}}_2\mathrm{C}$. For ${\mathrm{Cu}}_{2∕3}{\mathrm{Nb}}_2{\mathrm{S}}_2\mathrm{C}$, we observe a strong interaction between Cu $3d$ and S $3s3p$ states that is responsible for shifting the S $3s3p$ states toward to the bottom of the valence band.

Figure 1

Crystal structure of $1T\text{−}{\mathrm{Nb}}_2{\mathrm{S}}_2\mathrm{C}$ (a), $3R\text{−}{\mathrm{Nb}}_2{\mathrm{S}}_2\mathrm{C}$ (b), and $1T\text{−}{\mathrm{Cu}}_{2∕3}{\mathrm{Nb}}_2{\mathrm{S}}_2\mathrm{C}$ (c) phases.

Figure 2

Total densities of states of ${\mathrm{Nb}}_2{\mathrm{S}}_2\mathrm{C}$ calculated for $1T$ and $3R$ phases and as a weighted sum of $1T$ and $3R$ phases taken in the relation $0.76\left(1T\right)+0.24\left(3R\right)$.

Figure 3

Total and partial density of states of ${\mathrm{Nb}}_2{\mathrm{S}}_2\mathrm{C}$ (solid and dashed lines), Nb $L{\beta }_{2,15}$, S $L_{2,3}$, S $K\beta$, and C $K\alpha$ XES.

Figure 4

Total and partial density of states of ${\mathrm{Cu}}_{2∕3}{\mathrm{Nb}}_2{\mathrm{S}}_2\mathrm{C}$ (solid and dotted lines), Cu $L\alpha$, Nb $L{\beta }_{2,15}$, S $L_{2,3}$, S $K\beta$, and C $K\alpha$ XES.

Figure 5

C $K\alpha$ x-ray emission and C $1s$ x-ray absorption spectra and total density of states of ${\mathrm{Nb}}_2{\mathrm{S}}_2\mathrm{C}$.

Figure 6

C $K\alpha$ x-ray emission spectra of ${\mathrm{Nb}}_2{\mathrm{S}}_2\mathrm{C}$ measured at excitation energies of $283.1\mathrm{eV}$ (curve $a$), $285.0\mathrm{eV}$ (curve $b$), $288.5\mathrm{eV}$ (Curve $c$), and $290.5\mathrm{eV}$ (curve $d$).

Figure 7

Comparison of calculated and experimental dispersion curves based on the analysis of the C $K\alpha$ x-ray emission spectra of ${\mathrm{Nb}}_2{\mathrm{S}}_2\mathrm{C}$ measured at excitation energies of 283.1, 285.0, 288.5, and $290.5\mathrm{eV}$.

Figure 8

Cu $L\alpha$, $\beta$ emission spectra of a carbosulfide, ${\mathrm{Cu}}_{2∕3}{\mathrm{Nb}}_2{\mathrm{S}}_2\mathrm{C}$, and reference compounds Cu metal (solid area), $\mathrm{Cu}{\mathrm{Ir}}_2{\mathrm{S}}_4$, and CuO.