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We present a strain-dependent time-resolved reflectometry experiment on $\mathrm{Ba}{({\mathrm{Fe}}_{0.955}{\mathrm{Co}}_{0.045})}_{2}{\mathrm{As}}_{2}$ in the nematic state. Uniaxial external pressure has usually been applied to obtain a single nematic domain. However, reflectivity with and without pumping under uniaxial strain along ${[110]}_{\text{tetr}}$ reveals an unexpected nematicity of which ${C}_{4}$ symmetry-breaking orientation is rotated by ${20}^{\ensuremath{\circ}}$. Our observation suggests a microscopic coexistence of ${B}_{2g}$ and ${B}_{1g}$ nematicity under uniaxial strain and possibly of an $XY$-type nematic order in the electron-doped iron pnictide.

We investigate the ultrafast dynamics of ${\mathrm{Cd}}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}$, a prototype material showing a Lifshitz-type transition as a function of temperature. In the paramagnetic metallic state, the photoreflectivity shows a subpicosecond relaxation, followed by a featureless small offset. In the antiferromagnetic state slightly below ${T}_{N}$, however, the photoreflectivity resurges over hundreds of picoseconds, which goes beyond the usual realm of the effective-temperature model. Our observations are consistent with the Lifshitz phase transition of ${\mathrm{Cd}}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}$ driven by the evolution of the local magnetic moment.

The electrodynamics of free carriers in the semimetallic Dirac material $\mathrm{SrMnB}{\mathrm{i}}_{2}$ was investigated using optical spectroscopy and first-principles calculations. Using a two-carrier-model analysis, the total free-carrier response was successfully decomposed into individual contributions from Dirac fermions and non-Dirac free carriers. Possible roles of chiral pseudospin, spin-orbit interaction (SOI), antiferromagnetism, and electron-phonon $(e\text{\ensuremath{-}}ph)$ coupling in the Dirac fermion transport were also addressed. The Dirac fermions possess a low scattering rate of $\ensuremath{\sim}10\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ at low temperature and thereby experience coherent transport. However, at high temperatures, we observed that the Dirac fermion transport becomes significantly incoherent, possibly due to strong $e\text{\ensuremath{-}}ph$ interactions. The SOI-induced gap and antiferromagnetism play minor roles in the electrodynamics of the free carriers in $\mathrm{SrMnB}{\mathrm{i}}_{2}$. We also observed a seemingly optical-gap-like feature near 120 meV, which emerges at low temperatures but becomes filled in with increasing temperature. This gap-filling phenomenon is ascribed to phonon-assisted indirect transitions promoted at high temperatures.

We examined the temperature ($T$) evolution of the optical conductivity spectra of ${Sr}_{3}$${Ir}_{2}$${O}_{7}$ over a wide range of 10--400 K. The system was barely insulating, exhibiting a small indirect bandgap of $\ensuremath{\le}$0.1 eV. The low-energy features of the optical d-d excitation ($\ensuremath{\hbar}\ensuremath{\omega}$ 0.3 eV) evolved drastically, whereas such evolution was not observed for the O $K$-edge x-ray-absorption spectra. This suggests that the $T$ evolution in optical spectra is not caused by a change in the bare (undressed) electronic structure, but instead presumably originates from an abundance of phonon-assisted indirect excitations. Our results showed that the low-energy excitations were dominated by phonon-absorption processes which involved, in particular, the optical phonons. This implies that phonon-assisted processes significantly facilitate the charge dynamics in barely insulating ${Sr}_{3}$${Ir}_{2}$${O}_{7}$.

We have investigated the effects of atomic disorder on the electronic structure and some physical properties of ${\text{Co}}_{2}\text{CrAl}$ Heusler alloy (HA) films. Flash evaporation onto glass substrates at different temperatures (from 150 to 750 K) and postannealing at various temperatures were employed to manipulate the structural order in ${\text{Co}}_{2}\text{CrAl}$ HA films. The $L{2}_{1}$-ordered ${\text{Co}}_{2}\text{CrAl}$ HA films exhibit the magnetic, the transport, and the optical properties close to those of the bulk ordered sample. Increase in the structural disorder ($L{2}_{1}\ensuremath{\rightarrow}B2\ensuremath{\rightarrow}A2\ensuremath{\rightarrow}\text{amorphous}$ state) causes the reduction in the saturation magnetization and the Curie temperature $({T}_{C})$ down to the paramagnetic state for amorphous films. The recrystallization of amorphous ${\text{Co}}_{2}\text{CrAl}$ HA films is accompanied by an increase in resistivity by about 10%, which is interpreted to be related to the energy gap formation at the Fermi level for the minority bands in the $L{2}_{1}/B2$-ordered state of ${\text{Co}}_{2}\text{CrAl}$ HA. The energy-band structures of $L{2}_{1}$-ordered ${\text{Co}}_{2}\text{CrAl}$ HA have been calculated by using an all-electron full-potential linearized-augmented-plane-wave method. The optical and the magneto-optical properties of $L{2}_{1}$-ordered ${\text{Co}}_{2}\text{CrAl}$ HA have been experimentally investigated and explained in terms of the band structures. It was experimentally shown that the optical properties of crystalline $L{2}_{1}$ (or $B2$)-type ordered alloy at a temperature much higher than ${T}_{C}$ do not significantly change. Structural disorder in ${\text{Co}}_{2}\text{CrAl}$ HA from the crystalline to the amorphous state also does not radically alter the states responsible for the interband-absorption-peak formation.

Angle-resolved photoemission and optical spectroscopy are used to probe the electronic structure of the one-dimensional Mott insulator ${\mathrm{Sr}}_{2}\mathrm{Cu}{\mathrm{O}}_{3+\ensuremath{\delta}}$, both at half-filling and with small concentrations of excess oxygen doping. Spin-charge separation can be seen as evidenced by the existence of spinon and holon branches in the photoemission spectra. Optical studies reveal no significant doping dependence, while photoemission studies show a large energy shift in the spinon and holon states with respect to the main valence band states which remain nearly unaffected. The results suggest the excess dopant carriers are incorporated solely within the one-dimensional Hubbard band which is electronically isolated from the rest of the states in the system.