Yu. S. Ponosov

Raman Scattering in Os: Nonadiabatic Renormalization of the Optical Phonon Self-Energies

The wave vector dependence of the E 2g optical phonon frequencies and linewidths in the hexagonal close-packed metal, osmium, was studied at 10 K. A strong anisotropic dispersion, as well as damping thresholds, were observed for two transverse branches in the high-symmetry directions of the Brillouin zone around a phonon wave vector magnitude of 10 6 cm -1 . Simultaneously, a continuum was found in the Raman spectra, presumably of electronic nature, whose q-dependence correlates with that the phonon self-energies. Numerical simulations of the electronic Raman response in terms of an intraband mechanism were performed with a model energy-band spectrum reproducing the main features of the Fermi surface anisotropy of Os. A large discrepancy was found between the calculated and the measured anisotropy of the electronic scattering with respect to momentum direction and polarization configuration. The possible assignment to a contribution of resonant terms to the Raman vertex is discussed.

Crystal structure of the low-temperature form of K3PO4

The crystal structure of the low-temperature form of K3PO4 has been determined for the first time using neutron diffraction (Rietveld method) and Raman spectroscopy: orthorhombic cell (sp. gr. Pnma, Z = 4), lattice parameters a = 1.12377(2) nm, b = 0.81046(1) nm, c = 0.59227(1) nm. The structure is made up of isolated [PO4] tetrahedra, with the potassium ions in between.

Electronic Raman scattering and the electron–phonon interaction in YB6

Electronic Raman scattering in YB6 and in its structural and electronic analog LaB6 has been studied in the temperature range of 10–730 K. The experimental spectra have been compared to those calculated on the basis of ab initio band structures with renormalization owing to the electron–phonon interaction. Good agreement between the calculation and experiment for LaB6 has been obtained throughout the entire temperature range. This allows the determination of the coupling constant λep = 0.25. To satisfactorily describe the spectra of electronic light scattering in YB6, it is necessary to introduce an additional electron relaxation channel. In this case, the estimate of the electron–phonon coupling constant λep is no more than 0.4; for this reason, a high superconducting transition temperature cannot be explained only by the phonon mechanism.

Antiferromagnetic fluctuations in water-intercalated YBa2Cu3O6.8

Raman scattering by phononic and electronic excitations, as well as the magnetic susceptibility of water-intercalated YBa2Cu3O6.8, has been studied. Direct evidence of the incorporation of OH−-groups into the structure of the crystal has been obtained. The observed changes in the phonon spectrum and the appearance of the spectrum of antiferromagnetic fluctuations with an increased exchange coupling constant imply the localization of carriers and suppression of superconductivity. Change in the resonance behavior of the two-magnon scattering of light indicates the transformation of the electronic structure in crystallites absorbing water.

Q-dependent light scattering by electrons in LaB6

The inelastic light scattering by intraband electronic excitations in metallic lanthanum hexaboride has been studied in the temperature range of 10–300 K. General agreement has been obtained between the measured spectra and the spectra calculated within the band theory taking into account the renormalization of electron energies owing to electron-phonon scattering. The electron-phonon coupling constant λ and electron relaxation frequency Γ have been estimated. The dependence of the electron self-energies on the direction and magnitude of the wave vector has been revealed, implying the anisotropic electron-phonon interaction or the contribution from other electron scattering mechanisms.

Dynamics of the BiTeI lattice at high pressures

Raman measurements of the phonon spectrum of BiTeI at pressures of up to 20 GPa have been performed. A decrease in the linewidth of E2 vibration by almost a factor of 2 with an increase in the pressure to 3 GPa has been detected. The frequencies of all four Raman active modes increase monotonically with the pressure. These lines are observed in spectra up to ∼8 GPa. Sharp change in the spectrum occurs at pressures of 8–9 GPa, indicating a transition to the high-pressure phase, which holds up to 20 GPa. This transition is reversible and hardly has any hysteresis. A sample in the high-pressure phase is single crystal.

Analysis of dispersion of long-wavelength optical phonons in zinc with the help of light scattering

The temperature dependences (5–300 K) of the Raman spectra of E2g phonons and optical constants in zinc single crystals are measured in the excitation energy range 1.4–2.54 eV. It is found that phonon damping decreases upon an increase in the wavelength of exciting radiation. The obtained results are compared with the dependence of the phonon width on the excitation energy (the probed wave vector of the excitations under investigation), which are presented for the first time for the transition metal osmium, as well as with the calculated electron-phonon renormalization of damping, taking into account the actual distribution of wave vectors.

Phonons in YB6 and LaB6: Effects of temperature and pressure

Raman scattering by phonons in YB6 and LaB6 has been studied in wide temperature (8–800 K) and pressure (up to 10 GPa) ranges. Acoustic phonons in both hexaborides exhibit anomalous softening with a decrease in the temperature. The positive isothermal Grüneisen coefficient of these modes in YB6 appears to be an order of magnitude lower than the absolute value of the negative isobaric coefficient, which indicates the dominating role of temperature effects. The temperature shifts of the vibration frequencies of the B sublattice in YB6 caused by phonon interactions have the opposite sign and exceed the respective shifts in LaB6.

Electronic Raman scattering and the renormalization of the electron spectrum in LuB12

The electronic Raman scattering in LuB12 single crystals of various isotope compositions is studied in the temperature range 10–650 K. The shape and the energy position of spectral maxima depend on the direction and magnitude of a probe wavevector, the temperature, and the excitation symmetry and remain unchanged when the isotope composition changes. Experimental spectra are compared with the spectra simulated on the basis of a calculated electronic structure. The experimental results are successfully described when the electron spectrum renormalization effects caused by electron–phonon coupling are taken into account. This confirms that the origin of the observed spectra in LuB12 is due to Raman scattering by electrons. A comparison of the calculated and experimental data makes it possible to determine the coupling constant (λep = 0.32) that gives the correct superconducting transition temperature.

Interaction of YBa2Cu3O6.8 with atmospheric moisture during low-temperature annealing

The interaction of YBa2Cu3O6.8 (123) with water vapors at T = 200°C and the water influence on the structure and electrophysical properties of the compound have been studied using Raman spectroscopy, magnetometry, and X-ray diffraction. It has been found that the penetration of water into the 123 structure leads to its transition to a hydride-oxyhydroxide H2x − zYBa2Cu3Oy + x − z(OH)z containing fragments of the 124-type structure and exhibiting the spectrum of two-magnon scattering characteristic of antiferromagnetic 123 á compositions; in this case, the superconducting properties of the material as a whole are conserved. After short-time recovery annealing and subsequent oxidation, the water is removed from the compound structure, which leads to the disappearance of the spectrum of spin fluctuations. A possible mechanism of change in the 123 structure upon hydrogen and water intercalation has been discussed.