Vol 61, No 5 (2019)

The optical properties of low-dimensional dielectric cuprates, including parent systems for high-temperature superconductors, such as La₂CuO₄, Sr₂CuO₂Cl₂, and YBa₂Cu₃O₆ are briefly reviewed. The main focus is on the d–d and p–d charge transfer transitions, which determine the fundamental absorption band. The analysis of optical properties shows the instability of parent cuprates against the d–d charge transfer with the formation of metastable electron–hole (EH) dimers of the Cu¹⁺–Cu³⁺-type pairs coupled by the two-particle transfer and characterized by the giant electric polarizability. The formation of a system of stable EH dimers upon nonisovalent substitution determines the unconventional properties of the pseudogap and superconducting phases of doped cuprates.

The results of the study of optical absorption and EPR signals of single crystals of zinc oxide doped with manganese are presented. A broad impurity absorption band with the threshold energy about 2.1 eV, which was treated as a result of charge transfer transitions, has been observed for a long time in ZnO : Mn absorption spectra. In absorption spectra of a polarized light at 4.2 and 77.3 K, we first detected several lines of different intensity in a 1.877–1.936 eV range of energies of the light quanta. The observed lines are attributed to a donor exciton [(d⁵ + h)e] that emerges as a result of the Coulomb binding a free s electron and a hole, which is localized on p–d hybridized states. The EPR spectra of Mn²⁺ ion signals, when corresponding to the impurity absorption band exposed to light, are found to be not photosensitive. The obtained results indicate that the ZnO : Mn impurity absorption is due to transitions from antibonding p–d hybridized DBH states to the conduction band.

The competition between the magnetic and charge orderings in a model cuprate is considered in terms of a simplified static 2D spin–pseudospin model. This model is equivalent to the 2D dilute antiferromagnetic (AFM) Ising model with charged impurities. The mean-field approximation results are presented for the system under study and briefly compared to the classical Monte Carlo (MC) calculations. The numerical simulation shows that the cases of the strong exchange and the strong charge correlation differ qualitatively. In the case of a strong exchange, the AMF phase is instable with respect to the phase separation (PS) into the pseudospin (charge) and magnetic (spin) subsystems that behave as immiscible quantum liquids. The analytical expression has been obtained for the PS temperature.

An NMR analysis of ³¹P nuclei in metallic iron phosphide FeP has been performed both in a zero field and by sweeping the external magnetic field at several fixed frequencies with the aim of revealing and investigating the transformation of the magnetic helical structure of FeP. The NMR lineshape gradually changes with an increase in the field strength from trapezoidal (in weak fields) to pronounced asymmetric with two peaks (in strong fields). The former shape is typical of NMR spectra of powder samples under an applied magnetic field, while the latter is characteristic of NMR spectra of nonmagnetic atoms in single-crystalline helimagnetics. The observed transformation of the FeP NMR spectrum evidences in favor of the spin-reorientation transition of a spin-flop type in FeP at strong external fields.

The spectra of luminescence and magnetic circular polarization of luminescence of praseodymium–yttrium aluminum garnet Pr³⁺ : Y₃Al₅O₁₂ (PrYAG) are studied in the visible spectral region at temperature T = 300 K. An analysis of spectral dependences of magnetooptical and optical spectra makes it possible to identify optical 4 f–4 f-transitions between Stark sublevels of multiplets ³P₀, ³P₁, ³Н₅, and ³Н₆ in PrYAG. It was shown that an important role in the spectrum of the degree of magnetic circular polarization of luminescence of this paramagnetic garnet is played by the effect of quantum-mechanical J–J mixing of states of Stark singlets ³Н₅ and ³Н₆ of non-Kramer rare-earth ion Pr³⁺ in the “green” luminescence band related to forbidden 4 f → 4 f transition ³P₀ → ³Н₅ in the visible spectral region. To interpret the spectra of magnetic circular polarization of luminescence, the energy of experimentally determined Stark sublevels of multiplets under study, their irreducible representations and wave functions determined by numerical simulation of the energy spectrum of the rare-earth ion Pr³⁺ in the garnet structure are used.

Fluoride crystals with the colquiriite structure LiCaAlF₆ doped with Ce³⁺ ions are used as active media for lasers of ultraviolet spectral range with advantages of laser wavelength tuning range and no photoinduced degradation of laser properties. In this work we show that the multicenter character of Ce³⁺ segregation in this matrix agrees with the previously known peculiarities. On the basis of spectral-kinetic properties investigation, we show the increase of segregation coefficient for Ce³⁺ in LiSr_0.8Ca_0.2AlF₆ mixed crystal. In laser experiments with Ce:LiCaAlF₆ crystal, the differential efficiency of 47% and the wavelength tuning from 281 to 312 nm have been reached.

The luminescent characteristics of K₃LuSi₂O₇:Pr³⁺ (1%), which is a promising optical material for the use as a scintillator, have been studied using a set of techniques. The luminescence spectra of K₃LuSi₂O₇:Pr³⁺ (1%) contain two bands in the UV-range with peaks at 284 and 330 nm, which correspond intraconfigurational 5d → 4f transitions in the Pr³⁺ ions. The radiation in the visible and near IR range (480–850 nm) has been represented by the intraconfigurational 4f → 4f transitions. The kinetics of 5d → 4f-luminescence contains a build-up stage (τ_rise ∼ 7–12 ns), nonexponential decay stage (τ_1/2 ∼ 60 ns), and a slow component of the μs-range when excited by high-frequency (∼8 MHz) synchrotron radiation of the X-ray range. The fast component of the decay (τ = 54 ns) dominates in the decay kinetics of luminescence along with the build-up stage while the contribution of the μs decay component is less than 0.5% at the excitation by an pulse electron beam (5Hz). The excitation spectra of d–f- and f–f-photoluminescence in the ultraviolet and vacuum ultraviolet range that are measured using synchrotron radiation reveal features that are caused by both intracenter transitions and processes related to the energy transfer from the intrinsic electronic excitations to the impurity center.

The conversion of the energy absorbed by cubic Gd₂O₃ nanocrystals doped with Er³⁺ and codoped with Yb³⁺ and Zn²⁺ ions in a temperature range of 95–425 K is studied at various concentrations of dopants. The photoluminescence spectra confirm the ability of excitation energy transfer from the Gd³⁺ ions or the matrix towards the Er³⁺ ions. The population of upper excited states in the Er³⁺ ions depends on the way of excitation, which cause the alterations in the probability of multiphonon relaxation from the excited states to the radiative levels. The kinetics of photoluminescence decay exhibit the possible energy transfer from anionic defects created by extra doping with Zn²⁺ ions. The temperature dependences of the upconversion luminescence yield for emission lines of 560 and 660 nm are found to be different.

The relaxation kinetics of electron excitations in single-crystal and crystal-fiber Li₆GdB₃O₉ samples was studied by the time-resolved methods of pulse absorption spectroscopy and pulse cathodoluminescence under excitation with a nanosecond electron beam at 293 K. Based on the results of the numerical modeling of recombination processes, a pulse cathodoluminescence excitation mechanism was proposed.

Processes of photo-induced electron transfer from a bivalent acceptor lanthanide (Eu, Sm, and Yb) onto a trivalent donor lanthanide (Nd, Sm, Dy, Tm, and Yb) and the inverse thermally activated transfer are studied in barium fluoride crystals. At room temperature, photoinduced electron transfer is accompanied by oncoming displacement of the interstitial charge-compensating fluoride ion. On photoquenching at low temperatures, a bivalent donor lanthanide remains next to the interstitial fluoride ion, which causes a redshift of its 4f–5d absorption bands. The shift increases as the lanthanide size decreases (as in the series Nd, Sm, Dy, Tm, and Yb). A detailed analysis of the mechanisms of photo and thermal electron transfers between the different lanthanide centers in BaF₂ crystals is provided.

The procedure of calculation of the spectral line shape in optical spectra of rare-earth ions in crystals with the inclusion of random deformations of an elastically anisotropic crystal lattice caused by point defects is developed. The distribution function of components of the random strain tensor in the case of a low defect concentration is obtained as the generalized six-dimensional Lorentz distribution. The distribution function parameters are represented by the integral functional of the strain tensor components on a sphere of unit radius containing an isotropic point defect in its center. The numerical calculations of the strain tensors induced by point defects and the parameters of the distribution functions of random strains in LiLuF₄ and LaAlO₃ crystals have been performed. The calculated envelope with the doublet structure corresponding to the Γ₂(³H₄) → Γ₃₄(³H₅) singlet–doublet transition in the absorption spectrum of Pr³⁺ ions in the LiLuF₄ crystal agrees well with the data of the measurements.

The analysis of the unified series of single-phase Zn_2 – 2xMn_2xSiO₄ samples (x ≤ 0.2) has provided the possibility to determine the optimal dopant concentration x = 0.13 for the maximum luminescence intensity. It has been established that the dominating mechanism of concentration luminescence quenching and excitation energy dissipation is the oxidation of some Mn²⁺ activating ions and the growth of defectness in the luminophore due to this process Phosphors.

Cubic Y₂Ti₂O₇ single crystals doped with Er³⁺ or Yb³⁺ ions have been studied by the methods of electron spin resonance (ESR) and selective laser spectroscopy. ESR spectra exhibit signals from rare-earth ions that substitute for yttrium ions in sites with local trigonal symmetry. The g tensor components are determined. The results of optical investigations indicate that impurity centers of several types are formed; the sublevel energies of the ground and excited multiplets of these centers are found. Among the great variety of detected optical centers, the centers that dominate in the formation of ESR spectra are discriminated. An analysis of the experimental data using the exchange-charge model have made it possible to determine the sets of parameters of the crystal field for Er³⁺ and Yb³⁺ ions substituting Y³⁺ ions in regular crystallographic sites in pyrochlore Y₂Ti₂O₇.

The optical properties of BaBrI crystals doped with cerium ions are studied. The possible types of cerium-containing luminescence centres are established. Using the obtained experimental data for trivalent rare-earth elements, the Dorenbos diagram is constructed. The possible energy transfer mechanisms are proposed on the basis of results of studying the luminescence spectra measured with X-ray excitation.

The fluorescence intensities ratio (FIR) method is applied for measuring the temperature inside the luminescent LiY_0.8Yb_0.2F₄:Tm³⁺ crystal. The obtained results are used for monitoring of the sample temperature in the experiment when the exciting radiation waist is shifted by the focusing lens inside the sample. It is shown that a considerable change in the luminescence intensity of the LiY_0.8Yb_0.2F₄:Tm³⁺ crystal (0.2%) is not accompanied by a significant change in the temperature. This verifies the hypothesis about the appearance of amplified spontaneous luminescence (ASL) of Yb³⁺ ions during intense laser pumping in these crystals.

A brief review is presented of application of the high-sensitive laser polarimetry for detecting small changes—either regular, or stochastic—of the medium magnetization. Principles and results of application of the spectroscopy of spin fluctuations to atomic and semiconductor systems are described. New possibilities of studying the radiospectroscopic and optical properties of spin systems using the methods of noise spectroscopy are considered. Prospects for expanding the circle of objects of the spin noise spectroscopy at the expense of impurity dielectrics are discussed.

The regression analysis of the rise kinetics of up-conversion luminescence of the LiY_0.8Yb_0.2F₄:Tm³⁺ (0.2 at %) crystal is performed. The kinetics curve is obtained with rectangular pulsed excitation by radiation from a laser diode (IR LD) with a wavelength of λ_p = 933 nm. The most important—in these experimental conditions—mechanisms of the energy transfer from Yb³⁺ ions to Tm³⁺ ions are established, which are responsible for the transitions between the ground ³H₆ and excited ³F₄, ³H₄, ¹G₄, ¹D₂, and ¹I₆ terms of the Tm³⁺ ions. The durations of the relevant energy transfer processes are determined. It is shown that the energy transfer between rare earth ions in the LiY_0.8Yb_0.2F₄:Tm³⁺ (0.2 at %) crystal occurs through the dipole–dipole interactions.

Cubic garnets activated by neodymium and holmium, Li₆CaLa₂Nb₂O₁₂:Nd³⁺,Ho³⁺, were obtained by solid-state method. The main regularities of changes in the luminescence properties of Li₆C-aLa₂Nb₂O₁₂:Nd³⁺,Ho³⁺ solid solutions in the visible, near-IR, and short-wave IR ranges under 808 nm laser diode radiation are determined. The energy transfer mechanism between active centers, involving the participation of Nd³⁺ ions as sensitizers for the luminescence of Ho³⁺ ions, is proposed. Low phonon energy, high luminescence intensity in the range of 2.0–3.0 μm, and weak up-conversion luminescence in the region of 450–780 nm make it possible to consider the cubic lithium–lanthanum niobates Li₆CaLa₂Nb₂O₁₂:Nd³⁺,Ho³⁺ as promising phosphors of the short-wave IR range.

The use of the simplest Heisenberg or Dzyaloshinskii–Moriya-type spin Hamiltonians traditional for the ground orbital nondegenerate states does not allow the features of the exchange and exchange-relativistic couplings for the excited states of 3d and 4f ions in crystals to be correctly described. We have considered a generalized Hamiltonian of the exchange and superexchange couplings, which makes it possible to take into account the effects of orbital (quasi)degeneracy and the exchange mechanism of excitation transfer using a single approach. A new mechanism of the spin–other orbit exchange-relativistic couplings is discussed, in particular, the spin–orbital analog of the Dzyaloshinskii–Moriya interaction and its manifestation in the circular magnetooptics of weak ferromagnets.

The absorption spectra of cobalt-doped anatase TiO₂ nanopowders in the infrared and visible ranges have been studied after various oxidative and reductive treatments. The annealing leads to the appearance of the Drude-like contribution in the infrared region and significant change in the absorption in an visible range due to the formation of defects of the oxygen-vacancy-type and Ti³⁺ ions. The observed additional contribution in the absorption spectra of TiO₂:Co nanopowders as compared to the spectra of undoped powders is ascribed to the d–d transitions in ions Co²⁺. The change in the absorption related to cobalt after annealing is explained by a change in the local environment of Co ions from the octahedral to the tetrahedral environment.

A photoluminescence (PL) study of Cu(In,Ga)Se₂ (CIGSe) single crystals, (grown by the vertical Bridgman technique) with the [Ga]/[Ga + In] ratio of 7 and 12% and the [Cu]/[In + Ga] ratio greater than unity, as measured by energy dispersive spectroscopy, is presented. Analysis of the excitation intensity and temperature dependence of the PL spectra suggested the excitonic nature of the observed near-band-edge emissions peaks. Free and bound excitons in CIGSe single crystals with both 7 and 12% Ga content are clearly observed, analyzed and identified. An activation energy of 19 meV is determined for the free exciton in the PL spectra of the sample with 12% Ga. The presence of the excitons demonstrated a high structural quality of the material.

Nanophosphors in the amorphous state are first obtained via pulsed electron beam evaporation of targets made of polycrystalline phosphors with the compositions Ca₂M₈(SiO₄)₆O₂:Eu (M = Y and Gd) and Ca₂La₈(GeO₄)₆O₂:Eu with the structure of oxyapatite. Reduction of ions Eu³⁺ → Eu²⁺ in the electron beam is found. Modification of the Raman scattering (RS) spectra of the samples in the case of a decrease in the size of the particles from bulk to a nanosized state is found. The change in the forbidden band width E_g of the samples in the case of transition from a bulk powder to an NP is considered. The spectral and luminescence characteristics of the samples in the polycrystalline and nanoamorphous states are studied. It is shown that, in the case of transition to nanosamples, the ligand field around Eu²⁺ changes. This may be due to the violation of the translational symmetry in the NP. The bond between the 4f and 5d electrons weakens. Degeneracy of the ²e_g level appears. Presumably, the reduction of ions Eu³⁺ → Eu²⁺ in the electron beam due to the breaking of the Si(Ge)–O bond in the process of evaporation of the samples and capture of the released electron by the Eu³⁺ ions is found.