Quasiparticle Spectra, Charge-Density Waves, Superconductivity, and Electron-Phonon Coupling in $2\mathrm{H}\mathrm{\text{−}}{\mathrm{N}\mathrm{b}\mathrm{S}\mathrm{e}}_2$

High-resolution photoemission has been used to study the electronic structure of the charge-density wave (CDW) and superconducting dichalcogenide, $2\mathrm{H}\mathrm{\text{−}}{\mathrm{N}\mathrm{b}\mathrm{S}\mathrm{e}}_2$. From the extracted self-energies, important components of the quasiparticle interactions have been identified. In contrast to previously studied ${\mathrm{T}\mathrm{a}\mathrm{S}\mathrm{e}}_2$, the CDW transition does not affect the electronic properties significantly. The electron-phonon coupling is identified as a dominant contribution to the quasiparticle self-energy and is shown to be very anisotropic ($k$ dependent) and much stronger than in ${\mathrm{T}\mathrm{a}\mathrm{S}\mathrm{e}}_2$.

Figure 1

The photoemission intensities in the CDW state at $T=10\mathrm{K}$ for several momentum lines indicated in the schematic view of the Brillouin zone (lower left panel) by the dark-gray lines. The light-gray lines represent Nb-derived Fermi sheets. The nine Fermi points are numbered. The MDC derived dispersions are represented by full circles. The high-energy part of the dispersions is fitted with a second-order polynomial (dashed lines), and the low energy part is fitted with straight lines.

Figure 2

(a) Real parts of self-energies $\mathrm{R}\mathrm{e}\Sigma$ extracted from measured dispersions from Fig. 1 for several Fermi points. Temperature dependence of $\mathrm{R}\mathrm{e}\Sigma$ for (b) ${\mathrm{N}\mathrm{b}\mathrm{S}\mathrm{e}}_2$ for point 1 from Fig. 1 and for (c)  ${\mathrm{T}\mathrm{a}\mathrm{S}\mathrm{e}}_2$ (taken from Ref. 13) near the same region on the $\Gamma$-centered FS.

Figure 3

Temperature dependence of the EDCs at Fermi points of the inner (a) and the outer (b) Fermi sheets centered at K.