I. Yu. Arapova

Anharmonic processes of scattering and absorption of slow quasi-transverse modes in cubic crystals with positive and negative anisotropies of second-order elastic moduli.

The quasi-transverse ultrasound absorption during anharmonic processes of the scattering in cubic crystals with positive (Ge, Si, diamond and InSb) and negative (KCl and NaCl) anisotropies of the second-order elastic moduli is studied. Mechanisms underlying the relaxation of the slow quasi-transverse mode by two slow (the SSS mechanism) or two fast (the SFF) modes are discussed in the long-wavelength approximation. Angular dependences of the ultrasound absorption for the SSS, SFF and Landau-Rumer relaxation mechanisms are analyzed in terms of the anisotropic continuum model. The full absorption of the slow quasi-transverse mode is determined. It is shown that the SSS and SFF relaxation mechanisms are due to the cubic anisotropy of the crystals, leading to the interaction between noncollinear phonons. Two most important cases-the wavevectors of phonons are in the cube face plane or the diagonal planes-are considered. In crystals with a considerable anisotropy of the elastic energy (Ge, Si, InSb, KCl and NaCl) the total contribution of the SSS and SFF relaxation mechanisms to the full absorption is either several times or one to two orders of magnitude larger than the contribution from the Landau-Rumer mechanism depending on the direction. Much of the dominance of the former relaxation mechanisms over the Landau-Rumer mechanism is explained by second-order elastic moduli.

Magnetic structure of the low-dimensional magnet NaCu2O2: 63,65Cu and 23Na NMR studies

The magnetic structure of a quasi-one-dimensional frustrated NaCu2O2 magnet single crystal is studied by NMR. The spatial orientation of the planar spin spirals in the copper-oxygen Cu2+-O chains is determined, and its evolution as a function of the applied magnetic field direction is analyzed.

Charge distribution and mobility of lithium ions in Li2TiO3 from 6,7Li NMR data

A comparative analysis of 6,7Li NMR spectra is performed for the samples of monoclinic lithium titanate obtained at different synthesis temperatures. In the 7Li NMR spectra three lines are found, which differ in quadrupole splitting frequencies vQ and according to ab initio EFG calculations are assigned to three crystallographic sites of lithium: Li1 (vQ ∼ 27 kHz); Li2 (vQ ∼ 59 kHz); Li3 (vQ ∼ 6 kHz). The dynamics of lithium ions is studied in a wide temperature range from 300 K to 900 K. It is found that the narrowing of 7Li NMR spectra as a result of thermally activated diffusion of lithium ions in the low-temperature Li2TiO3 sample is observed at a higher temperature in comparison with a sample of high-temperature lithium titanate. Based on the analysis of 6Li NMR spectra it is assumed that there is mixed occupancy of lithium and titanium sites in the corresponding layers of the crystal structure of low-temperature lithium titanate, which hinders lithium ion transfer over regular crystallographic sites.

NMR study of the paramagnetic state of low-dimensional magnets LiCu2O2 and NaCu2O2

A comprehensive NMR study of the magnetic properties of single crystal LiCu2O2 (LCO) and NaCu2O2 (NCO) is carried out in the paramagnetic region of the compounds for various orientations of single crystals in an external magnetic field. The values of the electric-field gradient (EFG) tensor, as well as the dipole and transferred hyperfine magnetic fields for 63,65Cu, 7Li, and 23Na nuclei are determined. The results are compared with the data obtained in previous NMR studies of the magnetically ordered state of LCO/NCO cuprates.

Magnetic properties of Li2RuO3 as studied by NMR and LDA+DMFT calculations

We present results of the combined study of the magnetic properties of Li2RuO3 by means of nuclear magnetic resonance (NMR) spectroscopy and theoretical dynamical mean-field theory (LDA + DMFT) calculations. The NMR data clearly show the onset of a thermal activation process in the high temperature region, T > 560K, which is tentatively ascribed to the formation of the valence bond liquid. The LDA + DMFT calculations demonstrate that the magnetic response at these temperatures is mostly due to the xz/yz orbitals, while the xy orbitals of Ru still form molecular orbitals. Thus, Ru ions are in the orbital-selective state in the high temperature phase of Li2RuO3.

Interaction of collinear and noncollinear phonons in anharmonic scattering processes and their role in ultrasound absorption of fast quasi-transverse modes in cubic crystals

The absorption of fast quasi-transverse modes during anharmonic scattering processes in cubic crystals with positive (Ge, Si, diamond and InSb) or negative (KCl and CaF(2)) anisotropies of the second-order elastic moduli is studied. Mechanisms underlying the relaxation of the fast quasi-transverse mode by two fast (the FFF mechanism) or two slow (the FSS) modes are discussed in the long-wavelength approximation. Angular dependences of the ultrasound absorption for the FFF, FSS and Landau-Rumer relaxation mechanisms are analyzed in terms of the anisotropic continuum model. The full absorption of the fast quasi-transverse mode is determined. The problem of the scattering of collinear and noncollinear phonons in cubic crystals and their role in the ultrasound absorption of the fast quasi-transverse modes is considered. It is shown that the FFF and FSS relaxation mechanisms are due to the cubic anisotropy of the crystals, leading to the interaction between noncollinear phonons. In crystals with a considerable anisotropy of the elastic energy (InSb and KCl), the total contribution of the FFF and FSS relaxation mechanisms to the full absorption is one to two orders of magnitude larger than the contribution from the Landau-Rumer mechanism, depending on the direction. Much of the dominance of the former relaxation mechanisms over the Landau-Rumer mechanism is explained by second-order elastic moduli. The role of the Landau-Rumer mechanism in ultrasound absorption may be considerable in cubic crystals with a smaller anisotropy of the elastic energy. It is demonstrated that when anharmonic scattering processes play the dominant role, the inclusion of one of the relaxation mechanisms (the Landau-Rumer mechanism or the FFF or FSS mechanisms of relaxation) is insufficient for the quantitative description of the anisotropy of the full absorption of the fast quasi-transverse modes in cubic crystals.

Inhomogeneous state of the electron system in BaPb1 − xSbxO3 superconducting perovskites: The 207Pb NMR study

Comprehensive measurements of the 207Pb NMR spectra, Knight shifts 207Ks, spin-lattice relaxation rate 207T1−1, and the Pb spin echo decay 207E(2t) in BaPb1 − xSbxO3 superconducting oxides (x ≤ 0.33) have been performed. A considerable increase in the shift distribution width when Sb is substituted for Pb indicates the formation of the inhomogeneous state of the electron system in the oxide conduction band. The shift magnitude proportional to the density of states near the Fermi energy, 207Ks ∝ N(EF), reaches its maximum in the compounds with x = 0.18–0.25 with the maximum temperatures Tc of the superconducting transition. In the BaPb1 − xSbxO3 compounds, the Korringa relation is met, indicating that all of the samples are in the metal phase. The data on the Pb spin echo decay and the relevant estimates of the indirect coupling constants of the nuclei of the nearest neighbor Pb atoms clearly demonstrate the development of the electron system state in the oxide metal phase, which is microscopically inhomogeneous over the crystal.

Reduction in the rate of phonon scattering by a spatially correlated system of iron ions and low-temperature “anomaly” of the thermoelectric phenomena in HgSe:Fe crystals

A new effect of the reduction in the rate of phonon scattering by the spatially correlated system of iron ions in HgSe:Fe crystals is detected experimentally and calculated theoretically. The thermoelectric power is measured using HgSe:Fe samples with different iron content in the temperature range 7.5–60 K. It is found that the dependence of the thermoelectric power on iron content exhibits remarkable features at T<10 K: the quantity |α(NFe)| increases as the iron concentration increases to NFe=5×1018 cm−3, reaches a maximum at NFe≈(1–2)×1019 cm−3, but then monotonically decreases with further increases in NFe. It is shown that the obseved increase in the thermoelectric power is due to a reduction in the rate of phonon scattering by the spatially correlated system of Fe3+ ions. This new effect is analyzed theoretically, and the theoretical results are compared with the experimental data.

Spin dynamics in LiCu2O2 and NaCu2O2 low-dimensional helical magnets

Comprehensive NMR investigation of low-frequency spin dynamics of LiCu2O2 (LCO) and NaCu2O2 (NCO) low-dimensional helical magnets in the paramagnetic state has been carried out for the first time. Temperature dependences of the spin–lattice relaxation rate and anisotropy on various LCO/NCO nuclei have been determined at various orientations of single crystals in an external magnetic field. The spatial asymmetry of spin fluctuations in LCO multiferroic has been discovered. The quantitative analysis of the anisotropy of spin–lattice relaxation in LCO/NCO has allowed estimating the contributions of individual neighboring Cu2+ ions to the transferred hyperfine field on Li+(Na+) ions.

Electron density distribution in BaPb1 − xSbxO3 superconducting oxides studied by double nuclear magnetic resonance methods

The effect of charge disorder on the formation of an inhomogeneous state of the electron system in the conduction band in BaPb1 − xSbxO3 superconducting oxides is investigated experimentally by NMR methods. The NMR spectra of 17O are measured systematically, and the contributions from 17O atoms with different cation nearest surroundings are identified. It is found that microscopic regions with an elevated spin density of charge carriers are formed within two coordination spheres near antimony ions. Nuclei of the superconducting phase of the oxide (regions with an elevated antimony concentration) microscopically distributed over the sample are detected in compounds with x = 0.25 and 0.33. Experiments in which a double resonance signal of the spin echo of 17O-207Pb and 17O-121Sb are measured in the metal phase of BaPb1 − xSbxO3 oxides are carried out for the first time. The constants of indirect heteronuclear spin-spin 17O-207Pb interaction are determined as functions of the local Knight shift 207Ks. The estimates of the constants of the indirect interaction between the nuclei of the nearest neighbors (O-Pb and Pb-Pb atoms) and analysis of evolution of the NMR spectra of 17O upon a change in the antimony concentration are convincing evidence in favor of the development of a microscopically inhomogeneous state of the electron system in the metal phase of BaPb1 − xSbxO3 oxides.