A. V. Yeremenko

Raman scattering in a LiNiPO4 single crystal

The complete Raman spectra of a single crystal of LiNiPO4 for a wide temperature range are reported. Of the 36 Raman-active modes predicted by group theory, 33 have been detected. The spectra are successfully analyzed in terms of internal modes of the (PO4)3− group and external modes. Multiphonon Raman scattering is also discussed. Low-frequency lines, observed in the antiferromagnetic phase, are assigned to magnon scattering and are discussed briefly.

Raman scattering under structural and magnetic phase transitions in terbium ferroborate

The Raman scattering spectrum of single crystal TbFe3(BO3)4 was studied in the frequency range 3–500 cm−1 at temperatures from 2 to 300 K. It was found that in high- and low-temperature phases there exist additional phonon lines which were not known before. In the high-temperature phase, these lines originate from LO–TO splitting of polar phonons. Appearance of the additional lines in the low temperature phase is due to both a reduction of the crystal symmetry under the phase transition and an increase of the primitive cell volume. It was established that the frequencies of some phonon lines in the magneto-ordered phase are shifted towards the high-energy region upon applying an external magnetic field along the third-order axis. The spectrum of two-magnon Raman scattering was investigated. It was shown that at low temperatures the two-magnon band has a complex shape that reflects specific features in the density of state of the magnon branches. The magnon energy at the Brillouin zone boundary was determined.

Phonon Raman scattering in LaMn1−xCoxO3 (x=0, 0.2, 0.3, 0.4, and 1.0)

The Raman-active phonons in perovskite-like LaMn1−xCoxO3 (x=0, 0.2, 0.3, 0.4, and 1.0) are studied by measuring the Raman spectra at temperatures of 295 and 5 K. The changes in the spectra with Co doping are correlated with the decrease of the orthorhombic distortions. Surprisingly, more phonon lines are observed than are allowed for the rhombohedral LaCoO3 structure.

One-magnon light scattering in exchange-noncollinear Nd2CuO4

Abstract It is shown that in an exchange-noncollinear four-sublattice antiferromagnet Nd 2 CuO 4 an anomalously high intensity of light scattering from the exchange magnon should be observed. It makes it possible to determine directly the constant of the biquadratic exchange interaction.

Electronic Raman scattering through a stripe ordering transition in La2−xSrxNiO4

We describe the results of electronic Raman scattering experiments in two differently doped single crystals of La2−xSrxNiO4 (x=0.225 and 1/3). In B1g symmetry a crossover from weakly interacting to pseudogap-like behavior is observed at a charge-ordering temperature Tco. In B2g symmetry a redistribution of electronic continua with decreasing temperature is accompanied by a loss of spectral weight below Tco in the low-frequency region due to opening of a pseudogap. The slope of the Raman response at vanishing frequencies is investigated, too. Its temperature behavior in B2g symmetry, which predominantly selects charge carriers with momenta along the diagonals of the NiO2 bonds, provides clear evidence for one-dimensional charge transport in the charge-ordered phase.

The Raman scattering investigation of the features of low-energy electronic excitations of the terbium ion in the KTb(WO4)2 crystal

Raman scattering of light in the KTb(WO4)2 single crystal is investigated in the frequency range of 3–950 cm−1 at 5 K. The ground multiplet 7F6 of Tb3+ ion is split by the crystal field with symmetry C2, and all the multiplet components are detected. It is found that the first excited electronic quasidoublet consists of two singlet levels of different symmetry and is separated from the ground quasidoublet by ∼75 cm−1. Behavior of all the detected levels is investigated in external magnetic fields H ⊥ C2 and H || C2. Spectroscopic splitting factors are determined for the ground and excited levels of the Tb3+ ion in the KTb(WO4)2 crystal. Experimental data support the view that at low temperatures the case of Ising anisotropy is realized, and the crystal under study should be considered as a system of two-level magnetic ions.

Low-temperature mixed spin state of Co3+ in LaCoO3 evidenced from Jahn–Teller lattice distortions

One-phonon and multiphonon excitations of the single-crystalline LaCoO3 were studied using Raman spectroscopy in the temperature region 5–300K. First-order Raman spectra show a larger number of phonon modes than allowed for the rhombohedral structure. Additional phonon modes are interpreted in terms of activated modes due to lattice distortions, arising from the Jahn–Teller (JT) activity of the intermediate-spin (IS) state of the Co3+ ions. In particular, the 608-cm−1 stretching-type mode shows anomalous behavior in peak energy and scattering intensity as a function of temperature. The anomalous temperature dependence of the second-order phonon excitations spectra is in accordance with the Franck–Condon mechanism that is characteristic for a JT orbital order.

Phonons and magnons in stripe-ordered nickelates. A Raman scattering study

Electronic correlation effects in La2−xSrxNiO4 (x=1/3 and 0.225) lead to spontaneous phase separation into microscopic spin/charge stripes with commensurate and incommensurate order, respectively. Raman scattering experiments on such single-crystalline materials show a rich phenomenology of phonon and magnon anomalies due to the new, self-organized periodicities. These effects are observable as function of temperature but can also be induced by cooling in seemingly small magnetic fields leading to a reorganization of stripe structure.

Low-temperature relaxation of magnetization in manganite Pr0.4Bi0.3Ca0.3MnO3

Low-temperature relaxation of magnetization in Pr0.4Bi0.3Ca0.3MnO3 ceramics was measured after cooling in a magnetic field and aging at temperatures from 5 K to 38 K near the transition temperature to the magnetically ordered state. It was found that the relaxation process has a thermoactivation character at the first stage and is described in terms of the Arrhenius equation. Its activation energy Q ≈ 1.1 meV is comparable with the antiferromagnetic interaction energy in this compound. The temperature–time dependence of the relaxation rate at the second stage is described by a power law. The relaxation slows down as the temperature approaches the critical value, which may be associated with the formation of a cluster structure in the studied compound under these conditions.Low-temperature relaxation of magnetization in Pr0.4Bi0.3Ca0.3MnO3 ceramics was measured after cooling in a magnetic field and aging at temperatures from 5 K to 38 K near the transition temperature to the magnetically ordered state. It was found that the relaxation process has a thermoactivation character at the first stage and is described in terms of the Arrhenius equation. Its activation energy Q ≈ 1.1 meV is comparable with the antiferromagnetic interaction energy in this compound. The temperature–time dependence of the relaxation rate at the second stage is described by a power law. The relaxation slows down as the temperature approaches the critical value, which may be associated with the formation of a cluster structure in the studied compound under these conditions.

Features of the intensity behavior of Kramers doublet components in NdFe3(BO3)4 in the transverse Zeeman geometry

Nontrivial magnetic field dependence of the polarization of the Kramers doublet absorption lines (15 971–15 978 cm−1) was observed in the range of the 4I9/2 → 2H11/2 optical transition of Nd3+ ion in an easy-plane antiferromagnet NdFe3(BO3)4 in the transversal Zeeman effect geometry. Values of the transversal g factor and the effective exchange field with Fe3+ magnetic subsystem were determined for the excited state of Nd3+ from the field dependence of the doublet splitting. A semiempirical equation was proposed for the description of the field dependence of the intensities of the polarized components of the absorption lines corresponding to the Kramers doublet 15 971–15 978 cm−1. Besides the scaling factors, the equation contains only the independently determined magnitudes of exchange fields for the ground and excited states as parameters.