Thorough Raman and infrared (IR) reflectivity investigations of nominally pure ${\mathrm{SrTiO}}_3$ ceramics in the 10–300 K range have revealed a clear presence of the polar phase whose manifestation steeply increases on cooling. The Raman strengths of the Raman-forbidden IR modes are proportional to ${\omega }_{\mathrm{TO}1\mathrm{}}^{-\alpha }(\alpha \approx 1.6)$ where ${\omega }_{\mathrm{TO}1\mathrm{}}$ is the polar soft mode frequency. No pronounced permittivity dispersion is observed below the soft mode frequency so that, as in single crystals, the static permittivity is essentially determined by the soft mode contribution. A theory is suggested which assumes a frozen dipole moment connected with the grain boundaries which induces the polar phase in the grain bulk in correlation with the bulk soft-mode frequency. This stiffens slightly the effective soft mode response and reduces the low-temperature permittivity compared to that of single crystals. Moreover, the polar soft mode strongly couples to the $E_g$ component of the structural soft doublet showing that the polar axis is perpendicular to the tetragonal axis below the structural transition which is shifted to 132 K in our ceramics. Whereas the ${\mathrm{TiO}}_6$ octahedra tilt (primary order parameter) below the structural transition corresponds to that in single crystals, much smaller $A_{1g}-E_g$ splitting of the structural soft doublet shows that the tetragonal deformation (secondary order parameter) is nearly 10 times smaller, apparently due to the grain volume clamping in ceramics.

The polarization dependence of the low field microwave photoconductivity and absorption of a two-dimensional electron system has been investigated in a quasioptical setup in which linear and any circular polarization can be produced in situ. The microwave induced resistance oscillations and the zero resistance regions are notably immune to the sense of circular polarization. This observation is discrepant with a number of proposed theories. Deviations between different polarizations occur only near the cyclotron resonance where an unprecedented large resistance response is observed.

We have performed direct measurements of the low-temperature dynamical conductivity and dielectric permittivity of single crystalline ${\mathrm{SmB}}_6$ in the spectral range from 0.6 to 4.5 meV, i.e., below the hybridization gap. The obtained results together with the data of Hall-effect and infrared reflection measurements give evidence for a 19-meV energy gap in the density of states and an additional narrow donor-type band lying only 3 meV below the bottom of the upper conduction band. It is shown that at temperatures $5\mathrm{K}<T<\mathrm{}20\mathrm{}\mathrm{K}$ the electrodynamic response and the dc conductivity of ${\mathrm{SmB}}_6$ are determined by quasifree carriers thermally excited in the conduction band. We evaluate the microscopic parameters of these carriers: the spectral weight, the concentration, the effective mass, the scattering rate, and the mobility. Below 8 K the concentration of carriers in the conduction band freezes out exponentially and finally the electronic properties of ${\mathrm{SmB}}_6$ are determined by the localized carriers in the narrow band with the typical signature of hopping conductivity.

Infrared reflectivity measurements on several 122 iron pnictides reveal the existence of two electronic subsystems. The one gapped due to the spin-density-wave transition in the parent materials, such as ${\text{EuFe}}_2{\text{As}}_2$, is responsible for superconductivity in the doped compounds, such as $\text{Ba}{\left({\text{Fe}}_{0.92}{\text{Co}}_{0.08}\right)}_2{\text{As}}_2$ and $\text{Ba}{\left({\text{Fe}}_{0.95}{\text{Ni}}_{0.05}\right)}_2{\text{As}}_2$. Analyzing the dc resistivity and scattering rate of this contribution, a hidden $T^2$ dependence is found in the normal state. The second subsystem gives rise to incoherent background, present in all 122 compounds, which is basically temperature independent but affected by the superconducting transition.