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We consider the energy saving approximation of a Wiener process under unilateral constraints. We show that, almost surely, on large time intervals the minimal energy necessary for the approximation logarithmically depends on the interval's length. We also construct an adaptive approximation strategy that is optimal in a class of diffusion strategies and also provides the logarithmic order of energy consumption.

We present the study of the decay $J/ψ\rightarrow ρπ$. The results are based on of 5.2~million $J/ψ$ events collected by the KEDR detector at the VEPP-4M collider. The branching fractions are measured to be $\B(J/ψ\rightarrow ρπ) = \big(2.072\pm 0.017 \pm 0.062 \big)\cdot 10^{-2}$ and $\B(J/ψ\rightarrow π^+π^-π^0) = \big(1.878 \pm 0.013 \pm 0.051 \big)\cdot 10^{-2}$, where the first uncertainties are statistical and the second systematic. Our results are more precise than the previous relative measurements.

A layered triangular lattice with spin-1/2 ions is an ideal platform to explore highly entangled exotic states like quantum spin liquid (QSL). Here, we report a systematic in-field neutron scattering study on a perfect two-dimensional triangular-lattice antiferromagnet, CsYbSe$_2$, a member of the large QSL candidate family rare-earth chalcogenides. The elastic neutron scattering measured down to 70 mK shows that there is a short-range 120$^{\circ}$ magnetic order at zero field. In the field-induced ordered states, the spin-spin correlation lengths along the $c$ axis are relatively short, although the heat capacity results indicate long-range magnetic orders at 3 T $-$ 5 T. The inelastic neutron scattering spectra evolve from highly damped continuum-like excitations at zero field to relatively sharp spin wave modes at the plateau phase. Our extensive large-cluster density-matrix renormalization group calculations with a Heisenberg triangular-lattice nearest-neighbor antiferromagnetic model reproduce the essential features of the experimental spectra, including continuum-like excitations at zero field, series of sharp magnons at the plateau phase as well as two-magnon excitations at high energy. This work presents comprehensive experimental and theoretical overview of the unconventional field-induced spin dynamics in triangular-lattice Heisenberg antiferromagnet and thus provides valuable insight into quantum many-body phenomena., Comment: 8 pages and 3 figures in the main text; 10 pages, 14 figures, and 1 table in the supplemental material

The study of the optical response of gold island films revealed an intense line in the light scattering spectrum near 2100 cm-1. Some possible reasons for the appearance of this line are considered. Comparison of the results obtained for films obtained by various technologies, light scattering spectra on other types of samples, as well as comparison with the results of other authors allow us to interpret the line as the result of inelastic light scattering by one-dimensional carbon chains. Keywords: inelastic light scattering, gold films, SERS, one-dimensional carbon chains.

The product of the electronic width of the $J/��$ meson and the branching fractions of its decay to hadrons and electrons has been measured using the KEDR detector at the VEPP-4M $e^+e^-$ collider. The obtained values are: $��_{ee}(J/��) = 5.550 \pm 0.056 \pm 0.089 \, \text{keV}, $ $��_{ee}(J/��) \cdot \mathcal{B}_\text{hadrons}(J/��) = 4.884 \pm 0.048 \pm 0.078 \, \text{keV}, $ $��_{ee}(J/��) \cdot \mathcal{B}_{ee}(J/��) = 0.3331 \pm 0.0066 \pm 0.0040 \, \text{keV}.$ The uncertainties shown are statistical and systematic, respectively. Using the result presented and the world-average value of the electronic branching fraction, one obtains the total width of the $J/��$ meson: $��= 92.94 \pm 1.83 \, \text{keV}.$ These results are consistent with the previous experiments., 19 pages, 13 figures

Hidden-order phases that occur in a number of correlated f-electron systems are among the most elusive states of electronic matter. Their investigations are hindered by the insensitivity of standard physical probes, such as neutron diffraction, to the order parameter that is usually associated with higher-order multipoles of the f-orbitals. The heavy-fermion compound Ce3Pd20Si6 exhibits magnetically hidden order at subkelvin temperatures, known as phase II. Additionally, for magnetic field applied along the [001] cubic axis, another phase II' was detected, but the nature of the transition from phase II to phase II' remained unclear. Here we use inelastic neutron scattering to demonstrate that this transition is associated with a change in the propagation vector of the antiferroquadrupolar order from (111) to (100). Despite the absence of magnetic Bragg scattering in phase II', its ordering vector is revealed by the location of an intense magnetic soft mode at the (100) wave vector, orthogonal to the applied field. At the II-II' transition, this mode softens and transforms into quasielastic and nearly Q-independent incoherent scattering, which is likely related to the non-Fermi-liquid behavior recently observed at this transition. Our experiment also reveals sharp collective excitations in the field-polarized paramagnetic phase, after phase II' is suppressed in fields above 4 T., Comment: V2: Updated and extended version to be published in Phys. Rev. B (9 pages, 8 figures), S.I. merged with the main text

The product of the electronic width of the $��(2S)$ meson and the branching fraction of its decay to the muon pair was measured in the $e^{+}e^{-} \to ��(2S) \to ��^{+}��^{-}$ process using nine data sets corresponding to an integrated luminosity of about 6.5 pb$^{-1}$ collected with the KEDR detector at the VEPP-4M electron-positron collider: \[ ��_{ee}\times\mathcal{B}_{����} = 19.3 \pm 0.3 \pm 0.5 ~\text{eV}. \] Adding the previous KEDR results on hadronic and leptonic channels, the values of the $��(2S)$ electronic width were obtained under two assumptions: either with the assumption of lepton universality \[ ��_{ee} = 2.279 \pm 0.015 \pm 0.042 ~\text{keV} \] or without it, summing up hadronic and three independent leptonic channels: \[ ��_{ee} = 2.282 \pm 0.015 \pm 0.042 ~\text{keV}. \]

© Published under licence by IOP Publishing Ltd.In the paper results of thin film synthesis and primary characterization of molybdenum and tungsten disulfides are presented. The synthesis is performed by means of the high-temperature sulfurization of the metallic Mo and W films in the flow of argon. Chemical and phase compositions and morphology of thin films are reported. Optical transmission and Raman spectra are presented and discussed.

The experiments reveal the strong narrow band in the spectra of the emission from Au-film on the Cu substrate. The emission was excited by red He-Ne laser in standard geometry for Raman back-scattering experiments. Some additional experiments were made. The set of obtained data did not give enough arguments to make the final conclusion about the nature of this emission. The details of experiments, results, and possible explanations are discussed.

© Published under licence by IOP Publishing Ltd. Reactive dc magnetron sputtering was employed to produce thin films of titanium nitride using titanium metallic target, argon as the plasma gas and nitrogen as the reactive gas. A set of the films was studied deposited on the Si, fused silica and crystalline (001) MgO substrates with various deposition conditions. The films deposited on the Si and SiO 2 substrates are polycrystalline while deposited at slow rate to the heated to 600C MgO substrate are highly epitaxial according both to XRD and LEED data. Electrical resistivity of the films was measured by means of the four-probe van der Pauw method.

© 2016 Author(s).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 5Eg → 5T2g transition of the Cr2+ 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 5Eg ground state of the Cr2+ ion. The model is proposed based on a redistribution of the centers between the minima of the EO×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.

© 2015 American Physical Society. SrEr2O4 is a geometrically frustrated magnet which demonstrates rather unusual properties at low temperatures including a coexistence of long- and short-range magnetic order, characterized by two different propagation vectors. In the present work, the effects of crystal fields (CFs) in this compound containing four magnetically inequivalent erbium sublattices are investigated experimentally and theoretically. We combine the measurements of the CF levels of the Er3+ ions made on a powder sample of SrEr2O4 using neutron spectroscopy with site-selective optical and electron paramagnetic resonance measurements performed on single-crystal samples of the lightly Er-doped nonmagnetic analog, SrY2O4. Two sets of CF parameters corresponding to the Er3+ ions at the crystallographically inequivalent lattice sites are derived which fit all the available experimental data well, including the magnetization and dc susceptibility data for both lightly doped and concentrated samples.

© 2014 Advanced Study Center Co. Ltd. A review on chemical and physical preparation of platinum nanoparticles and some applications with these nanoparticles is presented. Application perspectives such fields as nanocatalysts, electrical conductivity, optics and nonlinear optics are demonstrated. Different parameters and condition of various fabrication technologies are considered and realized specific physical properties of platinum nanoparticles are discussed.

Using the inclusive photon spectrum based on a data sample collected at the $J/��$ peak with the KEDR detector at the VEPP-4M $e^+e^-$ collider, we measured the rate of the radiative decay $J/��\to����_{\rm c}$ as well as $��_{\rm c}$ mass and width. Taking into account an asymmetric photon lineshape we obtained $��^0_{����_{\rm c}}=2.98\pm0.18 \phantom{|}^{+0.15}_{-0.33}$ keV, $M_{��_{\rm c}} = 2983.5 \pm 1.4 \phantom{|}^{+1.6}_{-3.6}$ MeV/$c^2$, $��_{��_{\rm c}} = 27.2 \pm 3.1 \phantom{|}^{+5.4}_{-2.6}$ MeV., 6 pages, 3 figures

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 |4 A2g,4 A2g〉 and excited state |4 A2g,2 Eg〉 of the Mn4+-Mn4+ pair center is constructed. It is shown that the exchange interaction in the ground state of the Mn4+-Mn 4+ 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 |4 A 2g,4 A2g〉 and energies of spin multiplets in the excited state |4 A2g,2 E g〉 agree well with calculation of the exchange interaction carried out within the framework of the channel model with the parameters J ξη = 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. © 2014 Pleiades Publishing, Ltd.

The ratio of the electron and muon widths of the $J/��$ meson has been measured using direct $J/��$ decays in the KEDR experiment at the VEPP-4M electron-positron collider. The result $��_{ee}(J/��)/��_{����}(J/��)=1.0022\pm0.0044\pm0.0048\ (0.65\%)$ is in good agreement with the lepton universality. The experience collected during this analysis will be used for a $J/��$ lepton width determination with up to 1% accuracy., 5 pages, 6 figures, 2 tables

Observation of the intense linear dichroism spectrum on the 5E g → 5T 2g transition of Cr 2+ ions in KZnF 3 crystal under uniaxial stress is reported. The model is proposed which assigns the observed spectrum to alignment of the Jahn-Teller Cr 2+ centers, occurring due to redistribution between the minima of the adiabatic potential that become inequivalent under uniaxial stress applied along the C 4 axis of the crystal. Analysis of the observed dependencies of the dichroism on the applied stress has allowed to estimate the values of the electron-strain interaction constant V ES = 32900 ± 1200 cm -1 and the inversion splitting δ = 9.2 ± 1.3 cm -1, and also to characterize the random strain field.

A high-precision determination of the main parameters of the ��(2S) resonance has been performed with the KEDR detector at the VEPP-4M e^{+}e^{-} collider in three scans of the ��(2S) -- ��(3770) energy range. Fitting the energy dependence of the multihadron cross section in the vicinity of the ��(2S) we obtained the mass value M = 3686.114 +- 0.007 +- 0.011 ^{+0.002}_{-0.012} MeV and the product of the electron partial width by the branching fraction into hadrons ��_{ee}*B_{h} = 2.233 +- 0.015 +- 0.037 +- 0.020 keV. The third error quoted is an estimate of the model dependence of the result due to assumptions on the interference effects in the cross section of the single-photon e^{+}e^{-} annihilation to hadrons explicitly considered in this work. Implicitly, the same assumptions were employed to obtain the charmonium leptonic width and the absolute branching fractions in many experiments. Using the result presented and the world average values of the electron and hadron branching fractions, one obtains the electron partial width and the total width of the ��(2S): ��_{ee} =2.282 +- 0.015 +- 0.038 +- 0.021 keV, ��= 296 +- 2 +- 8 +- 3 keV. These results are consistent with and more than two times more precise than any of the previous experiments.

We report results of a search for narrow resonances in e+ e- annihilation at center-of-mass energies between 1.85 and 3.1 GeV performed with the KEDR detector at the VEPP-4M e+ e- collider. The upper limit on the leptonic width of a narrow resonance Gamma(R -> ee) Br(R -> hadr) < 120 eV has been obtained (at 90 % C.L.).

We report the final results of a study of the ��(3770) meson using a data sample collected with the KEDR detector at the VEPP-4M electron-positron collider. The data analysis takes into account the interference between the resonant and nonresonant $D\bar{D}$ production, where the latter is related to the nonresonant part of the energy-dependent form factor $F_D$. The vector dominance approach and several empirical parameterizations have been tried for the nonresonant $F_D^{\NR}(s)$. Our results for the mass and total width of ��(3770) are M = 3779.2 ^{+1.8}_{-1.7} ^{+0.5}_{-0.7} ^{+0.3}_{-0.3} MeV, ��=24.9 ^{+4.6}_{-4.0} ^{+0.5}_{-0.6} ^{+0.2}_{-0.9} MeV, where the first, second and third uncertainties are statistical, systematic and model, respectively. For the electron partial width two possible solutions have been found: (1) ��_{ee} = 154 ^{+79}_{-58} ^{+17}_{-9} ^{+13}_{-25} eV, (2) ��_{ee} = 414 ^{+72}_{-80} ^{+24}_{-26} ^{+90}_{-10} eV. Our statistics are insufficient to prefer one solution to another. The solution (2) mitigates the problem of non-$D\bar{D}$ decays but is disfavored by potential models. It is shown that taking into account the resonance--continuum interference in the near-threshold region affects resonance parameters, thus the results presented can not be directly compared with the corresponding PDG values obtained ignoring this effect.

We report a new precise determination of the leptonic widths of the J/psi meson performed with the KEDR detector at the VEPP-4M $e^{+}e^{-}$ collider. The measured values of the J/psi parameters are: \Gamma_{ee}\times\Gamma_{ee}/\Gamma = 0.3323 \pm 0.0064 (stat.) \pm 0.0048 (syst.) keV \Gamma_{ee}\times\Gamma_{\mu\mu}/\Gamma=0.3318 \pm 0.0052 (stat.) \pm 0.0063 (syst.) keV. Assuming $e\mu$ universality and using the table value of the branching ratios the leptonic \Gamma_{ll}= 5.59 \pm 0.12 keV width and the total \Gamma=94.1 \pm 2.7 keV widths were obtained. We also discuss in detail a method to calculate radiative corrections at a narrow resonance., Comment: Published in proceedings of the PhiPsi 2009 conference

The nonlinear optical properties of silica glass with copper nanoparticles synthesized by ion implantation were investigated by z-scan method in nanosecond time scale. The reverse saturation absorption (RSA) at the wavelength range of 450-540 nm and saturation absorption (SA) at 550-585 nm were observed. It was supposed that the two-photon electron absorption from bound of d-states determined the RSA effect and the SA is due to saturation of plasmon excitation.

Recently the peak luminosity achieved on the DA��NE collider has been improved by almost a factor three by implementing a novel collision scheme based on large Piwinski angle and Crab-Waist. This encouraging result opened new perspectives for physics research and a new run with the KLOE-2 detector has been scheduled to start by spring 2010. The KLOE-2 installation is a complex operation requiring a careful design effort and a several months long shutdown. The high luminosity interaction region has been deeply revised in order to take into account the effect on the beam caused by the solenoidal field of the experimental detector and to ensure background rejection. The shutdown has been also used to implement several other modifications aimed at improving beam dynamics: the wiggler poles have been displaced from the magnet axis in order to cancel high order terms in the field, the feedback systems have been equipped with stronger power supplies and more efficient kickers and electrodes have been inserted inside the wiggler and the dipole vacuum chambers, in the positron ring, to avoid the e-cloud formation. A low level RF feedback has been added to the cavity control in both rings., 3 pages, 3 figures, presented to the IPAC10 conference

We present a study of the inclusive photon spectrum from 6.3 million J/psi decays collected with the KEDR detector at the VEPP-4M e+e- collider. We measure the branching fraction of the radiative decay J/psi -> eta_c gamma, eta_c width and mass. Taking into account an asymmetric photon line shape we obtain: M(eta_c) = (2978.1 +- 1.4 +- 2.0) MeV/c^2, Gamma(eta_c) = (43.5 +- 5.4 +- 15.8) MeV, B(J/psi->eta_c gamma) = (2.59 +- 0.16 +- 0.31)%$., 6 pages, 1 figure. To be published in the proceedings of the 4th International Workshop on Charm Physics (Charm2010), October 21-24, 2010, IHEP, Beijing

Boron difluoride complexes possessing liquid crystalline and luminescent properties were synthesized for the first time on the basis of non-mesogenic β-enaminoketones. © 2010 Pleiades Publishing, Ltd.

The products of the electron width of the J/��meson and the branching fraction of its decays to the lepton pairs were measured using data from the KEDR experiment at the VEPP-4M electron-positron collider. The results are ��_{ee}(J/��)*Br(J/��->e^+e^-)=(0.3323\pm0.0064\pm0.0048) keV, ��_{ee}(J/��)*Br(J/��->��^+��^-)=(0.3318\pm0.0052\pm0.0063) keV. Their combinations ��_{ee}\times(��_{ee}+��_{����})/��=(0.6641\pm0.0082\pm0.0100) keV, ��_{ee}/��_{����}=1.002\pm0.021\pm0.013 can be used to improve theaccuracy of the leptonic and full widths and test leptonic universality. Assuming e��universality and using the world average value of the lepton branching fraction, we also determine the leptonic ��_{ll}=5.59\pm0.12 keV and total ��=94.1\pm2.7 keV widths of the J/��meson., 7 pages, 6 figures

The paper is dealing with semi-classical asymptotics of a characteristic function for a stochastic process. The main technical tool is provided by the stationary phase method. The extremal range for a stochastic process is defined by limit values of the complex logarithm of the characteristic function. The paper also outlines a numerical method for calculating stochastic extrema.

Along with $��^0$ and $��$ mesons, a resonance structure in the invariant mass spectrum of two photons at $M_{����}= 360 \pm 7 \pm 9$ MeV is observed in the reaction $d C\to��+ ��+X$ at momentum 2.75 GeV/c per nucleon. Estimates of its width and production cross section are $��= 63.7 \pm 17.8$ MeV and $��_{����}=98\pm24^{+93}_{-67} {\rm ��b}$, respectively. The collected statistics amount to $2339 \pm 340$ events of $1.5\cdot 10^6$ triggered interactions of a total number $\sim 10^{12}$ of $d$C-interactions. This resonance structure is not observed in $p$C collisions at the beam momentum 5.5 GeV/c. Possible mechanisms of this ABC-like effect are discussed.

The absorption and luminescence spectra of a LiBaF3:Pb 2+ crystal are studied in the temperature range 10-300 K. At 300 K a structureless absorption band (A band) is found in the energy range 1.4-6.6 eV. The luminescence spectra at 300 and 10 K are composed of two A′ and A1 bands with maxima at ∼5.0 and ∼4.0 eV, respectively. The A′ band originates from the 'regular' Pb2+ centres. The A′ band luminescence decay is single-exponential in the whole temperature range 10-300 K. The properties of the observed absorption and A′ luminescence bands are described well within the semiclassical theory based on the Franck-Condon principle and the Jahn-Teller effect in the excited sp configuration. The possible nature of the A1 band luminescence is discussed. © 2006 IOP Publishing Ltd.

Complex EPR spectra of paramagnetic centers Pb3+ formed in LiBaF3:Pb2+ crystals under X-ray irradiation are studied in the temperature range of 10-150 K. It is shown that lead ions substitute Ba2+ ions in the LiBaF3 crystal and are in the cubic-octahedral 12-fold environment of the fluorine ions. The hyperfine structure constants describing the observed spectrum are determined and parameters of superhyperfine interaction with the nearest fluorine ions are estimated. © Springer-Verlag 2006.

Splitting of the first excited Γ341(H63) doublet state of Tm3+ ions in the LiY1-xTmxF4 crystals in external magnetic fields up to 5.5 T has been studied using measurements of luminescence spectra at low temperatures. In the magnetic field perpendicular to the S4-symmetry axis of the crystal lattice, the splitting of the non-Kramers doublet is caused by the joint action of the nonlinear Zeeman effect and the piezospectroscopic effect related to parastriction. Experimental dependences of the doublet splitting on the magnetic field strength and direction are in good correspondence with the theoretical estimations obtained in the framework of the semiphenomenological crystal field model. It is shown that spectroscopic data present direct information on the forced magnetostriction in the basal plane of uniaxial Van Vleck paramagnets that contain non-Kramers rare earth ions. The measured concentration dependence of the doublet splitting can be explained by taking into account variations of the elastic constants with the concentration of Tm3+ ions. © 2005 Elsevier B.V. All rights reserved.

The optical absorption spectrum, luminescence and photoconductivity on single crystals PbMg1/3Nb2/3O3 and on ceramics Pb0.91La0.09(Zr0.65Ti0.35)O 3 are investigated in the wide temperature range. The luminescence spectrum correlates with the photoconductivity spectrum. The position of the maximum of the luminescent emission spectrum indicates the origin of the charge carriers emitted from the defect centers. On the basis of the data the properties of the local centers are determined, and the phenomenological approach to the relaxor theory is discussed.

Electron paramagnetic resonance (EPR) of Ho3+ single ions and Ho3+-Mg2+-vacancy-Ho3+ associates in holmium-doped forsterite single crystals are studied at 9.4, 37.3 and 65-250 GHz. Crystals were grown from melt by the Czochralski technique in slightly oxidizing atmosphere. For both centers, directions of the principal magnetic axes and parameters of the effective spin Hamiltonians describing dependences of electron-nuclear levels on applied magnetic field are obtained. For Ho 3+ substituting Mg2+ in the M2 site as the single ion and for Ho3+ ions in dimer centers, values of crystal field parameters related to a real crystal lattice structure are estimated in the framework of the exchange charge model. The calculated crystal field energies, values of the g-factors of the ground Ho3+ quasi-doublet and the directions of the corresponding magnetic moments agree satisfactorily with the data obtained from measurements of EPR and optical absorption and site-selective luminescence spectra. © Springer-Verlag 2005.

The optical absorption spectrum, luminescent emission and photoconductivity were investigated in single crystal PbMg1/3Nb2/3O3 and in the Pb0.91La0.09(Zr0.65Ti0.35)O3 ceramics in the wide temperature region. The spectrum of the luminescence correlates with spectrum of photoconductivity emission. The position of the maximum of the luminescence emission indicates the origin of the charge carriers for the emissions from the defect centers. On the basis of these data the properties of the local centers were found, and phenomenological approach to a relaxor theory is discussed.

Phenomenological approach to a relaxor theory is developed and some experi-mental results in the support of this theory are received. The optical absorption spectrum, luminescent emission and photoconductivity are investigated in the wide temperature region in PbMg 1/3 Nb 2/3 O 3 . On the basis of this data the density of states on the impurity band and the velocity of charge carrier localization are found. The probability distribution of relaxation time, g (ln( τ))d ln( τ ), is found analytically as the function of the density of states in the impurity polaron band. The temperature dependences of dielectric response of relaxor ferroelectrics are described in the framework of the Landau-Ginzburg-Devonshire theory of phase transitions. The properties of local centers and polaron localization dynamics are studied.

Phenomenological approach to a relaxor theory is developed and some experimental results in the support of this theory are received. The optical absorption spectrum, luminescent emission and photoconductivity are investigated in the wide temperature region in PbMg 1/3 Nb 2/3 O 3 . On the basis of this data the density of states on the impurity band and the velocity of charge carrier localization are found. The probability distribution of relaxation time, g (ln(τ)) d ln(τ), is found analytically as the function of the density of states in the impurity polaron band. The temperature dependences of dielectric response of relaxor ferroelectrics are described in the framework of the Landau-Ginzburg-Devonshire theory of phase transitions. We show that an additional condition for the relaxor behavior to appear is the coincidence of the phase transition temperature with the temperature region of thermo-localization of charges on the defects. The properties of local centers and polaron localization dynamics are studied.

Assignment of absorption lines of mixed valence Cr3+-Cr2+ pair center has been done by the analysis of the probabilities of exchange-induced electric-dipole transitions. Parameters of exchange interaction J1 = 44.5 ± 5 cm-1 and axial crystal field B2 0 = -19.6±2 cm-1 for the excited state |Cr2+3Ea; Cr3+4A2〉 have been estimated from the fine structure of absorption spectra of the pair. Using microscopic theory of superexchange interaction for the ground and excited states of the pair the value of the transfer integral of eg-electron via the intermediate fluorine ion tvv = 3500 cm-1 has been extracted.

The luminescence spectra of KZnF3:Tl+ and KMgF3:Tl+ crystals with a perovskite structure were investigated in the temperature range of 4.2-300 K and at optical excitation in the A absorption band (∼6 eV). The spectrum of KZnF3:Tl+ at 300 K is a wide band with the maximum Emax at 5.48 eV and the width of about 0.47 eV. At 100 K the band splits into two components: An intensive one with Emax = 5.63 eV and a width of about 0.2 eV and a weak one with Emax = 4.66 eV. At 4.2 K an intensive broad band practically disappears and a narrow line accompanied by a vibration structure is observed at E = 5.725 eV. This line is assigned to a zero-phonon transition from the metastable 3Γ1u level to the ground 1Γ1g level, weakly allowed due to the hyperfine interaction and phonon-assisted mechanisms. The spectrum of KMgF3:Tl+ at 300 K is a band with the maximum at 5.78 eV and a width of about 0.3 eV. This band does not disappear at 4.2 K; its maximum shifts to higher frequencies (5.91 eV) and an intensive narrow line at 5.812 eV is observed on its background. The temperature-dependent luminescence decay was also investigated. At T = 10 K the lifetime of the slow component of luminescence is τs = 11.6 ms for KZnF3:Tl+ and τs = 14.9 ms for KMgF3:Tl+. The main features of the observed luminescence spectra are satisfactorily explained within the framework of the conventional theory, as a manifestation of the Jahn-Teller effect for the excited 6sp electron configuration of an admixture Tl+ ion, with a set of model parameters close to that used earlier to describe absorption spectra of the studied crystals.

The luminescence spectra of a KZnF3: Tl+ crystal are investigated in the energy range from 4.75 to 5.9 eV at temperatures of 10-300 K upon excitation into the A absorption band (5.7-6.3 eV). At T = 300 K, the luminescence spectra exhibit an intense band with a maximum at 5.45 eV, which is attributed to single Tl+ ions substituted for K+ ions. The 5.723-eV intense narrow band observed at T < 20 K is assigned to the 3Γ1u-1Γ1g zero-phonon transition, which is weakly allowed by the hyperfine interaction. The luminescence decay is studied as a function of temperature. The main characteristics of the luminescence spectra are adequately described in terms of the semiclassical theory based on the Franck-Condon principle and the Jahn-Teller effect for an excited sp configuration of the Tl+ ion with the use of the parameters obtained earlier from analyzing the absorption spectra of the system under investigation. © 2002 MAIK "Nauka/Interperiodica".

Experimental and theoretical studies of the optical and EPR spectra, and the spin-lattice relaxation in CsCdBr3 crystals containing 0.1, 0.3 and 1.0 mol.% of impurity Yb3+ ions have been fulfilled. In the optical excitation spectrum, a broad charge transfer band has been detected in the wave-number range of 25000-35000 cm-1. The hybridization of the excited electron configuration corresponding to the charge transfer from the ligand (Br-) 4p closed shells on the ground 4f13 configuration of the ytterbium ion is considered to interpret the anomalously large crystal field splitting of the 2F5/2 multiplet. The EPR spectra with comparable intensities of the axial single-ion and symmetric dimer centres in the sample containing 0.3 mol.% of impurity Yb3+ ions were observed. Spin-lattice relaxation rates and linewidths in the EPR spectra of the single-ion and dimer centres were measured in the temperature range of 1.5 - 35 K at the frequency of 9.5 GHz at different directions of the applied magnetic field relative to the crystal symmetry axis. Experimental results are analyzed in the framework of the microscopic theory of the electron-phonon interaction with taking into account peculiarities of the phonon spectrum of the impurity CsCdBr3 lattice. High-frequency EPR spectra of dimer centres in CsCdBr3 crystals containing 0.2 and 0.5 mol.% of impurity Nd3+ ions were taken in the range of 205-250 GHz at 4.2 K in the magnetic fields up to 0.8 T parallel to the crystal symmetry axis. The crystal field splitting between the first excited and the ground Kramers doublets, and magnetic splitting factors of these doublets were determined. An estimate of the isotropic ferromagnetic exchange constant A = (2.3±0.3) 10-3 cm-1 in symmetric dimer centres formed by impurity Nd3+ ions was obtained from the zero-field splitting of the EPR signals.

Splitting of the first excited Γ34 1 (3H6) doublet state of Tm3+ ions in the LiTmF4 crystal in external magnetic fields up to 11 T has been studied using measurements of luminescence and far infrared absorption spectra at low temperatures. In the magnetic field perpendicular to the S4-symmetry axis of the crystal lattice, the splitting of the non-Kramers doublet is caused by the joint action of the nonlinear Zeeman effect and the piezospectroscopic effect related to parastriction. Experimental dependences of the doublet splitting on the magnetic field strength and direction are in good correspondence with the theoretical estimations obtained in the framework of the semiphenomenological crystal field model. It is shown that spectroscopic data present direct information on the forced magnetostriction in the basal plane of uniaxial Van Vleck paramagnets that contain non-Kramers rare earth ions.

Low-lying energy states of the [Fe3S4]0 cluster have been calculated by taking into account the double exchange, superexchange and vibronic interaction. It was found that the adiabatic potential of the excited state with S = 0 corresponds to the full delocalization of the "excess" charge. From the analysis of experimental data of Mössbauer spectroscopy and the temperature dependence of the magnetic susceptibility the double exchange parameter t ≥ 4000 cm-1 and the vibronic interaction parameter λ2/2k ≥ 2800 cm-1 have been estimated.

Two synthetic beryls (Al2Be3Si6O18) of different color (purple and blue-green) were studied with electron paramagnetic resonance (EPR) and optical spectroscopy. In both crystals, the known spectra of Cu2+ and Fe1+ were observed with the same relative intensity. In the purple sample heated at 700°C in hydrogen atmosphere, two different kinds of Mn2+ EPR spectra were observed. The main one is pseudoaxial, it arises from ions substituted for Al3+ at position 4c of the structure. The weaker one is more complex, it has orthorhombic symmetry and is characterized by an unusually large zero-field splitting (B2 0 = 741 · 10-4 cm-1) and an isotropic hyperfine constant A = 70 G. This spectrum arises from Mn2+ at position 6f in the structure, normally occupied by Be. From optics, the blue-green color arises from Cu2+, while the purple one is due to Mn3+.

The absorption spectra of KZnF3:Tl+ and KMgF3:Tl+ crystals have been measured over the energy range 1.5-6.4 eV at temperatures of 10-300 K. A wide absorption band with a clear doublet structure identified as an absorption A band of Tl+ impurity centres is observed for both crystals. The position of the band, its bandwidth and the distribution of absorption intensity between the two components of the band are temperature dependent in the range T > 65 K. All features of the absorption spectra of the crystals studied are explained within the frameworks of the conventional theory on the basis of the Frank-Condon principle and the semiclassical picture of crystal lattice vibrations. The band shapes calculated by the Monte Carlo method are in satisfactory agreement with observed ones.

The optical absorption spectra of Cr3+-Cr2+ pair centers in KZnF3:Cr3+,Cr2+ crystals were investigated in a wide temperature range. A broad band at 30 800 cm-1 is attributed to a cation-cation eg-electron transfer transition. Narrow lines with maxima at 16 720 and 19 880 cm-1 have been assigned to purely electronic exchange-induced electric-dipole transitions from the ground (Cr3+,4A2g;Cr2+,5Eg) state to the excited (Cr3+,4A2g;Cr2+,3Eg a) and (Cr3+,4A2g;Cr2+,3Eg b) states, respectively. Its vibronic satellites corresponding to the a1g local mode of the Cr3+ fluorine octahedron of the pair are also observed. The energy of the local mode for the ground and the mentioned excited states are 580, 540 and 530 cm-1. The exchange integral J = -14.9±0.4 cm-1 and the Jahn-Teller splitting ΔJT = 340±40 cm-1 for the ground state of the pair were obtained by an analysis of the temperature dependence of the absorption lines. The important features of the crossover double exchange-ferromagnetic superexchange are discussed.

The influence of an external axial electric field on the absorption spectrum of a Cr3+-Cr2+ mixed-valence pair center coupled by the double-exchange mechanism in a KZnF3 crystal is investigated. It is shown experimentally that the Cr3+-Cr2+ pair has an electric dipole moment. The migration of an electron is accompanied by local lattice strain. At the minima of the adiabatic potential, the intermediate fluorine atom is displaced from the lattice site. © 1998 American Institute of Physics.

Studies involving the piezospectroscopy method have shown that the symmetry of the pair centers of Cr3+-Cr2+ ions in the KZnF3 crystal is tetragonal. In this paper we develop a microscopic model of a pair center. We use the temperature dependence of the integrated intensity of the absorption line to find the effective hopping integral for an eg electron, tσσ = 205 ± 10 cm-1, and the polaron reduction factor, equal to 0.11. By analyzing the selection rules for exchange-induced electric dipole transitions under double-exchange conditions we identify all the absorption lines of Cr3+-Cr2+ pairs. © 1998 American Institute of Physics.

In the optical spectrum of the KZnF 3 crystal doped with chromium ions, the absorption lines of Cr 2+-Cr 3+ ion pairs with dynamic electron transfer between chromium ions via the intermediate fluorine ion (double 180 degree exchange) are observed. The value of the transfer integral for the e g-electron was directly measured for the first time using the temperature dependence of the integral intensity of the absorption lines.