Alexej Pashkin

Infrared, Raman and high-frequency dielectric spectroscopy and the phase transitions in Na1/2Bi1/2TiO3

Infrared reflectivity, time-domain terahertz transmission, high-frequency dielectric measurements and Raman spectroscopy of Na0.5Bi0.5TiO3 complex ferroelectric perovskite were performed in a broad temperature range. The results were analysed considering the macroscopic symmetry as well as symmetry resulting from local Na–Bi ordering in all three known phases. An overdamped infrared soft mode was revealed in the THz range which, together with central-mode type dispersion in the GHz range, contribute to the strong and broad dielectric permittivity maximum around 600 K. Anharmonic Bi and/or Na vibrations and local hopping, respectively, are suggested to be the main origins of these excitations. The broad phonon spectra and the frequency independent dielectric losses at low temperatures are compatible with the existence of nanoscopic polar regions and disorder to liquid He temperatures similar to relaxor ferroelectrics.

Ultrafast insulator-metal phase transition in VO 2 studied by multiterahertz spectroscopy

The ultrafast photoinduced insulator-metal transition in VO2 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.

Effect of high pressure on multiferroic BiFeO3

We report experimental evidence for pressure instabilities in the model multiferroic

Highly tunable photonic crystal filter for the terahertz range

{\text{BiFeO}}_{3}

Ultrafast transient generation of spin-density-wave order in the normal state of BaFe2As2 driven by coherent lattice vibrations

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

Phase-sensitive time-domain terahertz reflection spectroscopy

{\text{BiFeO}}_{3}

Carrier dynamics in low-temperature grown GaAs studied by terahertz emission spectroscopy

is suppressed above 10 GPa. The nonpolar orthorhombic

Femtosecond Response of Quasiparticles and Phonons in Superconducting YBa2Cu3O7-δ Studied by Wideband Terahertz Spectroscopy

Pnma

Frequency-independent dielectric losses (1/f noise) in PLZT relaxors at low temperatures

structure observed above 10 GPa is in agreement with recent theoretical ab initio prediction, while the intermediate monoclinic phase has not been predicted theoretically.

Central-Peak Components and Polar Soft Mode in Relaxor PbMg1/3Nb2/3O3 Crystals

By use of an incipient ferroelectric, SrTiO3, as a defect material inserted into a periodic structure of alternating layers of quartz and high-permittivity ceramic, thermal tuning of a single defect mode over the entire lowest forbidden band was obtained. The tunability of this compact structure reached 60%.