R. V. Yusupov

EPR studies of the mechanochemically Er3+-activated fluorite nanoparticles

In the present work we show with EPR spectroscopy that the simultaneous grinding of the crystalline CaF2 and ErF3 salts leads to the mechanochemical doping of the single Er3+ ions in the CaF2 host. Dependence of the EPR spectra intensity on the grain size indicates clearly that the Er3+ centers are created at the surface of the particles. Dominant part of the observed paramagnetic centers represent the Er3+ ions substituting the regular quasicubic Ca2+ lattice site (g = 6.7) perturbed by the vicinity of the surface. Fine particles of CaF2 reveal also the nearly-isotropic EPR signal at g ~ 1.97 that originates from the surface defects and can be used for characterization of the obtained samples with respect of the average grain size. Grinding of the of the CaF2 and ErF3 mixture in the ratio of 100:1 during 12 hours gives the surface density of the Er3+ ions of ~ 0.1 nm−2, or ~ 104 Er3− ions per the 190-nm size particle.

Selective laser spectroscopy of Mn4+-Mn4+ pair centers in SrTiO3 crystal

Zero-phonon lines of a pair center of Mn4+ ions are observed in the luminescence and luminescence-excitation spectra of SrTiO3:Mn crystal. Based on the experimental data, the energy-level structure of the ground state ∣4A2g,4A2g〉 and excited state ∣4A2g,2Eg〉 of the Mn4+-Mn4+ pair center is constructed. It is shown that the exchange interaction in the ground state of the Mn4+-Mn4+ pair is antiferromagnetic. Energies of the levels are calculated assuming that the pair is formed by Mn4+ ions occupying neighboring octahedral positions of Ti4+ ions along the [110] axis. Experimental values of the exchange integral in the ground state ∣4A2g,4A2g〉 and energies of spin multiplets in the excited state ∣4A2g,2Eg〉 agree well with calculation of the exchange interaction carried out within the framework of the channel model with the parameters Jξη = 32 cm4-1 and Jζζ = −45.5 cm4-1. Experimental data and calculations unambiguously demonstrate that zero-phonon lines in the luminescence and luminescence-excitation spectra have magnetic-dipole nature.

A Study of Mechanochemical Doping of Fluoride Crystals with a Fluorite Structure by Er3+ Ions via Electron Paramagnetic Resonance Spectra

Using electron paramagnetic resonance (EPR) spectroscopy, we have shown that, upon mecha- noactivated doping of powders of compounds CaF2, SrF2, and BaF2 with Er3+ ions, impurity centers of single erbium ions with cubic symmetry are formed. Investigations of dependences of EPR spectra intensities on the particle size show that the process of mechanochemical doping with Er3+ ions proceeds differently for CaF2, SrF2, and BaF2 host matrices. In the case of CaF2, impurity centers are localized in a very thin near-surface layer of CaF2 particles, in SrF2, the impurity is distributed over the volume of particles, while, in BaF2, there is a layer of a finite thickness for which the probability of doping in the course of mechanosynthesis is very small and the impurity of the rare-earth element is localized in the core of large particles. These data can be explained assuming that the result of mechanosynthesis of particles of fluorides with a fluorite structure doped with Er3+ ions at room temperature is governed by two processes—mechanoactivated diffusion of rare-earth ions into particles and segregation of impurity ions at grain boundaries. In this case, the typical scales for compounds CaF2, SrF2, and BaF2 considerably differ from each other.

The transition from dynamics to statics in the electron-spin-resonance spectra of impurity Mn2+ ions in strontium titanate

The electron-spin-resonance (ESR) spectra of SrTiO3:Mn single crystals have been investigated. Results unambiguously indicate that the impurity center formed by an Mn2+ ion has a dynamic nature. In the high temperature range (T > 100 K), ESR spectra of Mn2+ ions reveal cubic symmetry; the spectrum is found to broaden significantly with a decrease in temperature. Upon cooling to T < 10 K, low-symmetry centers of Mn2+ ions with a strong orientational dependence emerge in the spectra. Temperature evolution of the ESR spectrum can be described within the model of a dynamic off-center Mn2+ ion substituting for the Sr2+ ion, with a transition to the static regime at low temperatures with an average localization energy of ∼2.4 ± 0.4 meV for Mn2+ centers due to random deformations.

Photo-EPR Studies of KTN-1.2: Evidences of the Nb4+−O− Polaronic Excitons

Strongly anisotropic photoinduced electron paramagnetic resonance spectrum was found in the KTa1-x Nb x O3 crystals with x = 0.012 at T < 10 K, which is described well within the model of the center with S = 1, g ∥ = 0.82 ± 0.04, g ⊥ = 0.52 ± 0.04, and D = 0.44 ± 0.03 cm−1. Analysis shows that this spectrum originates from the Nb4+−O− polaronic exciton in the triplet state.

Experimental manifestations of the Nb4+-O−polaronic excitons in KTa0.988Nb0.012O3

The formation of the photo-polaronic excitons in ABO_{3} perovskite type oxides has been detected experimentally by means of the photoinduced electron paramagnetic resonance studies of KTa_{0.998}Nb_{0.012}O_{3} crystals. The corresponding microwave X-band spectrum at T < 10 K consists of a narrow, nearly isotropic signal located at g ~ 2 and a strongly anisotropic component. The first signal, which has a rich structure due to hyperfine interactions with the lattice nuclei, is attributed to the single trapped charge carriers: the electrons and/or the holes. The anisotropic spectrum is caused by the axial centers oriented along the C_{4} pseudo-cubic principal crystalline axes. The spectrum angular dependence can be described well by an axial center with S = 1, g_{\parallel) = 0.82, g_{\perp} = 0.52 and D = 0.44 cm^{-1}. The anisotropic spectrum is attributed to the Nb^{4+}-O^{-} polaronic excitons. The temperature dependence of the anisotropic component is characterized by two activation energies: the internal dynamics activation E_{a1} = 3.7\pm0.5 meV, which makes the EPR spectrum unobservable above 10 K, and the destruction energy E_{a2} = 52\pm4 meV. By comparing the anisotropic photo-EPR spectrum and the photoinduced optical absorption temperature dependencies, we found that the Nb^{4+}-O^{-} polaronic excitons also manifested themselves via the ~0.7 eV wide absorption band arising under UV light excitation in the weakly concentrated KTaO_{3}:Nb crystals.

Observation of Persistent Currents in Finely Dispersed Pyrolytic Graphite

The trapped magnetic flux in the finely ground pyrolytic graphite sample annealed at 670 K in air has been observed. Flux trapping occurs on cooling of the sample from room temperature to 10 K in a magnetic field of 1 T. The magnitude and sign of the induced trapped moment remain unchanged when the applied magnetic field is varied within ±1 T at T K. The trapped magnetic flux is manifested in the displacement of the magnetization curve relative to that of the sample cooled in zero field. Displacement magnitude gradually decreases with the temperature increase up to 350 K, not reaching zero. The set of experimental observations probably reflects the presence in the sample of a granular high-temperature superconducting phase.

Photoconductivity and photostimulated currency in PMN-PT

ABSTRACT The photoconductivity and the photostimulated currency were measured in PMN-PT single crystals. Based on the experimental data, the characteristics of the density of states of the defect levels and the possibility of their participation in the formation of a diffuse phase transition are discussed.

Phase separation and locally induced states in manganites

ABSTRACT Local charged states have been induced at the surface of lanthanum strontium manganite single crystals as a result of the local bias application by a conducting tip of atomic force microscope. These results show the dependence of the surface potential of induced area on the writing time similar to that for ferroelectrics. Phenomenon of phase separation with charge segregation is considered for low-doped manganites. It indicates the tendency of manganites toward charge segregation stimulated by the magnetic ordering.

Optical studies of the uniaxial stress-induced orbital alignment of the Cr2+ centers in KZnF3 single crystal

Observation of an intense optical linear dichroism arising in cubic KZnF3:Cr crystal at low temperatures under uniaxial stress applied along the four-fold axis is reported. Dichroism occurs in the range of the wide vibronic absorption band corresponding to (5)Eg → (5)T2g transition of the Cr(2+) ions. Strain dependences of the dichroism value were studied at the temperatures of 2.0 K, 4.2 K, and 77 K. We associate our observations with the Jahn-Teller effect in the (5)Eg ground state of the Cr(2+) ion. The model is proposed based on a redistribution of the centers between the minima of the E⨂e problem adiabatic potential that become inequivalent under uniaxial stress applied along the four-fold axis of the crystal. It is shown that random strains in the sample have to be taken into account to achieve the quantitative agreement of the model predictions with experimental data. It is found that random strains in the studied sample originate predominantly from point defects. Obtained parameter values are inversion splitting δ = 9.2 ± 1.6 cm(-1), electron-strain coupling constant qVES = 16 500 ± 600 cm(-1), width of the random strain distribution w = (6.9 ± 0.5) ⋅ 10(-5). It is shown also that the minima of the [CrF6](4-) cluster adiabatic potential correspond to the elongated along the four-fold axes configurations.