The ultrafast photoinduced insulator-metal transition in VO${}_2$ is studied at different temperatures and excitation fluences using multi-THz probe pulses. The spectrally resolved midinfrared response allows us to trace separately the dynamics of lattice and electronic degrees of freedom with a time resolution of 40 fs. The critical fluence of the optical pump pulse, which drives the system into a long-lived metallic state, is found to increase with decreasing temperature. Under all measurement conditions, we observe a modulation of the eigenfrequencies of the optical phonon modes induced by their anharmonic coupling to the coherent wave-packet motion of V-V dimers at 6.1 THz. Furthermore, we find a weak quadratic coupling of the electronic response to the coherent dimer oscillation resulting in a modulation of the electronic conductivity at twice the frequency of the wave-packet motion. The findings are discussed in the framework of a qualitative model based on an approximation of local photoexcitation of the vanadium dimers from the insulating state.

The complex dielectric response of ${\mathrm{Bi}}_{1.5}{\mathrm{Zn}}_{1.0}{\mathrm{Nb}}_{1.5}{\mathrm{O}}_7$ cubic pyrochlore ceramics was investigated between 100 Hz and 100 THz by a combination of low-frequency capacitance bridges, a high-frequency coaxial technique, time domain transmission THz spectroscopy, and infrared spectroscopy. The data obtained between 10 K and 400 K revealed glasslike dielectric behavior: dielectric relaxation is observed over a wide frequency and temperature range, and the dielectric permittivity and loss maxima shift to higher temperature values by almost 200 K with increasing measuring frequency. The distribution of relaxation frequencies broadens on cooling and can be described by a uniform distribution. The high-frequency end of the distribution at $\sim {10}^{11}\hspace{0.5em}\mathrm{Hz}$ is almost temperature independent and its low-frequency end obeys the Arrhenius Law with an activation energy of $\sim 0.2\hspace{0.5em}\mathrm{eV}.$ The relaxation is assigned to the local hopping of atoms in the A and O’ positions of the pyrochlore structure among several local potential minima. The barrier height for hopping is distributed between 0 and 0.2 eV. Such an anomalously broad distribution may have its origin in the inhomogeneous distribution of ${\mathrm{Zn}}^{2+}$ atoms and vacancies on ${\mathrm{Bi}}^{3+}$ sites, which gives rise to random fields and nonperiodic interatomic potential. Frequency independent dielectric losses $(1/f$ noise) are observed at low temperatures, which seems to be a universal behavior of disordered systems at low temperatures.

We report experimental evidence for pressure instabilities in the model multiferroic ${\text{BiFeO}}_3$ and, namely, reveal two structural phase transitions around 3.5 and 10 GPa by using diffraction and far-infrared spectroscopy at a synchrotron source. The intermediate phase crystallizes in a monoclinic space group, with octahedra tilts and small cation displacements. When the pressure is increased further the cation displacements (and thus the polar character) of ${\text{BiFeO}}_3$ is suppressed above 10 GPa. The nonpolar orthorhombic $Pnma$ structure observed above 10 GPa is in agreement with recent theoretical ab initio prediction, while the intermediate monoclinic phase has not been predicted theoretically.

We measure the anisotropic midinfrared response of electrons and phonons in bulk YBa2Cu3O7-$\delta${} after femtosecond photoexcitation. A line shape analysis of specific lattice modes reveals their transient occupation and coupling to the superconducting condensate. The apex oxygen vibration is strongly excited within 150~fs, demonstrating that the lattice absorbs a major portion of the pump energy before the quasiparticles are thermalized. Our results attest to substantial electron-phonon scattering and introduce a powerful concept probing electron-lattice interactions in a variety of complex materials.