V. P. Gnezdilov

Anomalous optical phonons in FeTe chalcogenides: Spin state, magnetic order, and lattice anharmonicity

Polarized Raman-scattering spectra of non-superconducting, single-crystalline FeTe are investigated as function of temperature. We have found a relation between the magnitude of ordered magnetic moments and the linewidth of A1g phonons at low temperatures. This relation is attributed to the intermediate spin state (S=1) and the orbital degeneracy of the Fe ions. Spin-phonon coupling constants have been estimated based on microscopic modeling using density-functional theory and analysis of the local spin density. Our observations show the importance of orbital degrees of freedom for the Fe-based superconductors with large ordered magnetic moments, while small magnetic moment of Fe ions in some iron pnictides reflects the low spin state of Fe ions in those systems.

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.

Light scattering in LiCoPO4 single crystal: analysis of the vibrational spectrum

In a single crystal of LiCoPO4 Raman scattering has been studied in a wide temperature range. Thirty-two of the 36 Raman-active vibrational modes predicted by group-theory analysis were detected. The experimental lines are identified on the basis of their polarization rules, frequency position, and temperature behavior.

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.

Structural phase transition in two-dimensional tetramer-cuprate Na5RbCu4(AsO4)4Cl2

Raman scattering and optical birefringence are used in to investigate a low-temperature phase transition in a single crystal of the two-dimensional Na5RbCu4(AsO4)4Cl2. Phonon anomalies point to a first-order nature of the transition. The observed transition is most probably related to an order-disorder transition of the Rb ion positions along the z axis within the ionic framework of mixed alkali metal chloride lattices.

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.

Phononic and magnetic excitations in the quasi-one-dimensional Heisenberg antiferromagnet KCuF3

The Raman-active phonons and magnetic excitations in the orbitally ordered, quasi one-dimensional Heisenberg antiferromagnet KCuF3 are studied as a function of temperature in the range between 3 and 290 K in different scattering configurations. The low-energy Eg and B1g phonon modes show an anomalous softening (∼25% and ∼13%, respectively) between room temperature and the characteristic temperature TS = 50 K. In this temperature range, a freezing-in of F− ion dynamic displacements is proposed to occur. In addition, the Eg mode at about 260 cm−1 clearly splits below TS. The width of the phonon lines above TS follows an activated behavior with an activation energy of about 50 K. Our observations clearly evidence reduction of the lattice symmetry below TS and indicate strong coupling of lattice, spin, and orbital fluctuations for T > TS. A strongly polarization dependent quasielastic scattering is observed at temperatures above TN = 39 K due to magnetic-energy fluctuations. For temperatures below TN a rich s...

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.