The total spectral weight textit{S} of the emergent low-energy quasipaticles in high-temperature superconductors is explored by x-ray absorption spectroscopy. In order to examine the applicability of the Hubbard model, regimes that cover from zero do
ping to overdoping are investigated. In contrast to mean field theory, we found that textit{S} deviates from linear dependence on the doping level textit{p}. The slope of textit{S} versus textit{p} changes continuously throughout the whole doping range with no sign of saturation up to textit{p} = 0.23. Therefore, the picture of Zhang-Rice singlet remains intact within the most prominent doping regimes of HTSCs.
This study shows that a terahertz (THz) wave can be generated from the (001) surface of cleaved Bi$_{textrm{2}}$Se$_{textrm{3}}$ and Cu-doped Bi$_{textrm{2}}$Se$_{textrm{3}}$ single crystals using 800 nm femtosecond pulses. The generated THz power is
strongly dependent on the carrier concentration of the crystals. An examination of the dependence reveals the two-channel free carrier absorption to which Dirac fermions are indispensable. Dirac fermions in Bi$_{textrm{2}}$Se$_{textrm{3}}$ are significantly better absorbers of THz radiation than bulk carriers at room temperature. Moreover, the characteristics of THz emission confirm the existence of a recently proposed surface phonon branch that is normalized by Dirac fermions.
Quasiparticle dynamics of FeSe single crystals revealed by dual-color transient reflectivity measurements ({Delta}R/R) provides unprecedented information on Fe-based superconductors. The amplitude of fast component in {Delta}R/R clearly tells a compe
ting scenario between spin fluctuations and superconductivity. Together with the transport measurements, the relaxation time analysis further exhibits anomalous changes at 90 K and 230 K. The former manifests a structure phase transition as well as the associated phonon softening. The latter suggests a previously overlooked phase transition or crossover in FeSe. The electron-phonon coupling constant {lambda} is found to be 0.16, identical to the value of theoretical calculations. Such a small {lambda} demonstrates an unconventional origin of superconductivity in FeSe.
