We report the temperature- and magnetic-field-dependent optical conductivity spectra of the heavy electron metal YbIr$_2$Si$_2$. Upon cooling below the Kondo temperature ($T_{rm K}$), we observed a typical charge dynamics that is expected for a forma
tion of a coherent heavy quasiparticle state. We obtained a good fitting of the Drude weight of the heavy quasiparticles by applying a modified Drude formula with a photon energy dependence of the quasiparticle scattering rate that shows a similar power-law behavior as the temperature dependence of the electrical resistivity. By applying a magnetic field of 6T below $T_{rm K}$, we found a weakening of the effective dynamical mass enhancement by about 12% in agreement with the expected decrease of the $4f$-conduction electron hybridization on magnetic field.
We report the anisotropic changes in the electronic structure of a Kondo semiconductor CeOs$_2$Al$_{10}$ across an anomalous antiferromagnetic ordering temperature ($T_0$) of 29 K, using optical conductivity spectra. The spectra along the $a$- and $c
$-axes indicate that a $c$-$f$ hybridization gap emerges from a higher temperature continuously across $T_0$. Along the b-axis, on the other hand, a different energy gap with a peak at 20 meV appears below 39 K, which is higher temperature than $T_0$, because of structural distortion. The onset of the energy gap becomes visible below $T_0$. Our observation reveals that the electronic structure as well as the energy gap opening along the b-axis due to the structural distortion induces antiferromagnetic ordering below $T_0$.
We report the pressure-dependent optical reflectivity spectra of a strongly correlated insulator, samarium monosulfide (SmS), in the far- and middle-infrared regions to investigate the origin of the pressure-induced phase transition from the black ph
ase to the golden phase. The energy gap becomes narrow with increasing pressure in the black phase. A valence transition from Sm2+ in the black phase to mainly Sm3+ in the golden phase accompanied by spectral change from insulator to metal were observed at the transition pressure of 0.65 GPa. The black-to-golden phase transition occurs when the energy gap size of black SmS becomes the same as the binding energy of the exciton at the indirect energy gap before the gap closes. This result indicates that the valence transition originates from an excitonic instability.
