M. N. Popova

Raman scattering from phonons and magnons in RFe3(BO3)(4)

Inelastic light scattering spectra of several members of the RFe3(BO3)(4) family reveal a cascade of phase transitions as a function of temperature, starting with a structural, weakly first-order, phase transition followed by two magnetic phase transitions. Those consist of the ordering of the Fe-spin sublattice, revealed by all the compounds, and a subsequent spin-reorientational transition for GdFe3(BO3)(4). The Raman data evidence a strong coupling between the lattice and magnetic degrees of freedom in these borates. The Fe-sublattice ordering leads to a strong suppression of the low-energy magnetic scattering, and a multiple peaked two-magnon scattering continuum is observed. Evidence for short-range correlations is found in the paramagnetic phase by the observation of a broad magnetic continuum in the Raman data, which persists up to surprisingly high temperatures.

Cascade of phase transitions in GdFe3(BO3)4

Cascade of phase transitions in GdFe3(BO3)4 at 156, 37, and 9 K has been detected by specific heat measurements and further studied by Raman scattering and Nd3+ spectroscopic probe method. A weakly first-order structural phase transition at 156 K is followed by a second-order antiferromagnetic ordering phase transition at 37 K and a first-order spin-reorientational phase transition at 9 K.

Spectral studies of magnetic ordering in the cuprates R2BaCuO5 (R=Sm, Eu, Tm, Yb, Lu)

Abstract High resolution optical absorption spectra of an intrinsic R 3+ ion or Er 3+ probe have been measured in the “green phases” R 2 BaCuO 5 related to 123 high- T c superconductors. Evidence is given that the phase transitions observed in these compounds earlier are of magnetic nature. Additional spin-reorientation transitions were found in Yb 2 BaCuO 5 and Lu 2 BaCuO 5 . The suggestion is made that the magnetic structures of the copper subsystem are the same in each of the following two groups: (1) Y 2 BaCuO 5 and Lu 2 BaCuO 5 at T R N , Eu 2 BaCuO 5 ; (2) Yb 2 BaCuO 5 at T R N , Y 2 BaCuO 5 at T R , Tm 2 BaCuO 5 .

Germanium–vacancy color center in isotopically enriched diamonds synthesized at high pressures

We report on the high-pressure synthesis of novel nano- and microcrystalline diamonds with germanium–vacancy (Ge–V) color centers emitting at 602 nm. The synthesis was carried out in non-metallic growth systems C–H–Ge and C–H–O–Ge enriched with germanium and carbon isotopes. We demonstrate germanium and carbon isotope shifts in the fine structure of the luminescence, which allows us to unambiguously associate the center with the germanium impurity entering into the diamond lattice. We show that there are two ground-state energy levels with the separation of 0.7 meV and two excited-state levels separated by 4.6 meV in the electronic structure of the center and suggest a split-vacancy structure of this center. High-intensity and narrow-line emission of high-pressure synthesized small diamonds with Ge–V centers makes them promising candidates for single-photon emitters.

Fine structure of spectral lines in LiYF4:Er3+ due to isotopic disorder in the lattice

Abstract We report the first observation of the resolved structure in the 4 I 15/2 → 4 I 13/2 infrared transition of LiYF 4 :Er caused by lithium isotopes of the lattice. Isotope shifts of 0.019 and 0.016 cm −1 and inhomogeneous linewidths of 0.010 and 0.007 cm −1 were measured for the lines 6534.3 and 6538.3 cm −1 , respectively. Unresolved structure due to different erbium isotopes totals ∼0.010 cm −1 .

Crystal field and spectrum of Pr4+ in BaPrO3

Abstract We have measured the absorption spectrum of BaPrO 3 in the spectral range 1900–11000 cm −1 . All the energy levels of the 4f electronic configuration of the Pr 4+ ion in a crystal have been observed for the first time. We describe the total set of electron-nuclear states and the integral intensities of the magnetic dipole transitions of the Pr 4+ in BaPrO 3 in the framework of the crystal field approximation. The crystal field parameters corresponding to the real orthorhombic symmetry of BaPrO 3 are obtained with the exchange charge model.

Magnetic phase transition and short range order in Nd2BaCuO5

Abstract Optical spectra of the “brown phase” Nd 2 BaCuO 5 accompanying the high- T c superconductor NdBa 2 Cu 3 O 7− x have been measured. A magnetic phase transition at T c =7.5 K has been found from the temperature dependences of the spectral line splittings and halfwidths. A short range order present up to 10 T c is supposed to be connected with the quasi-one-dimensional character of magnetism in Nd 2 BaCuO 5 .

High-resolution spectroscopy of rare earth cuprates and nickelates

Abstract This presentation gives a brief review of the work on the R 2 Cu 2 O 5 (R=Tb-Lu, Y, Sc, In) and R 2 BaMO 5 (R=Nd, Sm-Gd, Dy-Lu and Y, M=Cu, Ni) cuprates and nickelates, performed by the authors group. We measured high-resolution optical absorption spectra of an intrinsic R 3+ ion or Er 3+ probe in these compounds, by means of a Fourier transform spectrometer. On the basis of these spectra, we investigated crystal field splittings of the rare-earth energy levels, magnetic ordering, low-dimensional magnetic correlations, the arrangement of magnetic moments of transition metal ions and peculiarities of spin-reorientation phase transitions. We have registered for the first time, magnetic phase transitions, some of them not detectable by other methods, in R 2 BaCuO 5 with R=Nd, Dy, Yb, Lu, Y and in R 2 BaNiO 5 with R=Nd, Sm, Gd, Eu, Dy, Ho, Tm, Lu.

Magnetic phase transitions in the chain nickelates R2BaNiO5 (R = Sm, Eu, Tm) by optical spectroscopy

Abstract Magnetic ordering of Sm2BaNiO5 at 55 ± 4 K, of Eu2BaNiO5 at 23 ± 2 K and of Tm2BaNiO5 at 13 ±1 K was found by high resolution Fourier transform spectroscopy. The Sm3+ ground Kramers doublet is split by Δ=28 cm-1 in the magnetically ordered state of Sm2BaNiO5 while the first excited crystal field level lies at 25 cm-1. Low-dimensional magnetic correlations are present in all the compounds studied.

High-resolution spectroscopy of YbAl3(BO3)4stoichiometric nonlinear laser crystals

We present temperature-dependent high-resolution polarized transmittance spectroscopy as well as luminescence spectroscopy of YbAl3(BO3)4 single crystals. Positions and symmetries of all the Yb3+ crystal-field electronic levels have been found from the analysis of the spectra. Near the main line of the optical transition we observe a lot of narrow weak lines. These lines point to the presence of several different distorted sites of Yb3+. We analyze the possible origin of the distorted sites. Vibronic structure in transmittance and luminescence spectra was observed and compared with room-temperature Raman spectra.