H. Uchiyama

Two-phonon contributions to meV-resolved, inelastic x-ray scattering spectra of ${\mathrm{MgB}}_2$ at $300\mathrm{K}$ are identified, in good agreement, in both intensity and energy, with a harmonic calculation using the force constant matrix from ab initio calculations. This contribution impacts the determination of the linewidth of the $E_{2g}$ phonon mode that is so important for the high $T_{\mathrm{c}}$ of this material. To the best of our knowledge, this is the first observation of peaks in measurements of phonon dispersion $(q>0)$ due to two-phonon scattering in a non-rare-gas solid.

The temperature and doping dependences of photoemission spectra of polycrystalline ${\mathrm{HgBa}}_2{\mathrm{CuO}}_{4+\delta }$ samples are investigated for several doping levels near the optimum ${(T}_c=98\mathrm{}\mathrm{K}).\mathrm{}$ As doping increases, we observed a systematic shift of the core levels and an increase in the density of states at the Fermi level ${(E}_F).\mathrm{}$ Nonlinear shifts of the core levels with doping and a remarkable doping-dependent suppression of the spectral intensity over more than 300 meV below $E_F$ are compatible with the reported results for ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4.$ With decreasing temperature, the density of states was gradually suppressed at energies lower than 30 meV, suggesting a gap opening. The estimated leading edge shift is about 10 meV for all samples at both 20 and 105 K.