Application of the polarizable-ion model on crystals with the corundum structure

This paper reports the results of charge, polarizability and force constant calculations at k? = 0 on crystals with the corundum structure. Published vibrational frequencies are used in the normal coordinate treatment. The calculated electronic parameters are discussed.     

Thermoluminescence kinetics of lithium borate crystals

The paper reports the results of experimental study and modeling of thermally stimulated recombination processes in undoped crystals of lithium borates Li₂B₄O₇ and LiB₃O₅. For both crystals in the unified experimental conditions, we have measured the glow curves of thermally stimulated luminescence, the spectra and the temperature dependencies of intensity of the steady-state luminescence over the temperature range of 90–500 K under X-rays excitation. In the framework of the unified model, we carried out the calculations of the recombination processes and interpretation of the results with the involvement of all the available experimental data on undoped lithium borate crystals.     

Thermal conductivity of single crystals with a fluorite structure: Cadmium fluoride

The thermal conductivity of Ca, Sr, Ba, and Cd difluoride single crystals and the CdF₂ samples doped by 3 mol % NdF₃, 15 mol % HoF₃, and 10 mol % ErF₃ has been studied using the method of steady longitudinal heat flow in the temperature range 50–300 K. The thermal conductivity of the matrices of these compounds decreases in the order CaF₂-SrF₂-BaF₂-CdF₂. The temperature dependences of the phonon mean free path for these crystals have been calculated from experimental data and exhibit different behaviors. It has been assumed that the intense phonon scattering observed in the undoped CdF₂ sample is caused by the specific features of the processes of phonon-phonon scattering. The formation of heterovalent solid solutions of cadmium difluoride and rare-earth trifluorides is accompanied by a drastic decrease in the thermal conductivity and a change in its character from that typical of dielectric single crystals to that typical of glassy materials.     

Thermoluminescence of YAG:Nd crystals coloured with x-rays

The spectral distribution of the thermoluminescence (TL) of YAG:Nd crystals coloured by x-ray irradiation at room temperature (RT) and at 80 K has been investigated. The spectral distribution of TL in the uv-, visible and near ir ranges shows that the energy transfer by bound exciton states (BES) to the RE³⁺ ions (Nd³⁺ and Tb³⁺) decreases with increasing temperature. The TL spectrum in the uv range is ascribed to the hole defect centers. Diminution of the energy transferred by the BES to the Nd³⁺ and Tb³⁺ ions is the direct cause for the occurrence of the group of lines ascribed to the Nd³⁺ ions in TL observed at LT, whereas at RT and higher only the groups of Tb lines are observed.     

Thermoluminescence of anion-deficient aluminum oxide single crystals after high-dose irradiation by nanosecond electron pulses

The effect of deep traps filled by a pulsed electron beam on the dosimetric thermoluminescence (TL) peak at 450 K in anion-deficient aluminum oxide single crystals has been investigated. After filling deep traps, the dosimetric TL peak becomes nonelementary and is characterized by a complex dependence of the TL intensity on the crystal annealing temperature with alternating fall and rise portions. The influence of the occupancy of deep centers of different nature and different energy depths on the structure of dosimetric TL peak is analyzed. The suggestion that basically electron traps are depleted in the temperature ranges of 600–750 and 900–1000 K while holes are depleted at T = 780–900 and above 1000 K is substantiated. The possibility of using TL deep traps for high-dose dosimetry of pulsed electron beams is demonstrated.     

Vortex structure in FeTeSe single crystals

Vortex structure in FeTe_0.66Se_0.44 and FeTe_0.6Se_0.4 single crystals with T _c ∼ 11.7 and 14.5 K, respectively, has been studied using the decoration technique. It has been found that in single crystals with the simplest crystalline structure of 11-family iron-containing superconductors (without interlayers), no regular vortex lattice is observed, similar to the case of the previously studied 122 and 1111 families. Using transmission electron microscopy, the dislocation structure with a density of ∼10⁹ cm⁻² has been observed. The problem of pinning in iron-containing superconductor single crystals is discussed.     

Topological magnetic insulators with corundum structure

Topological insulators are new states of quantum matter in which surface states residing in the bulk insulating gap are protected by time-reversal symmetry. When a proper kind of antiferromagnetic long-range order is established in a topological insulator, the system supports axionic excitations. In this Letter, we study theoretically the electronic states in a transition metal oxide of corundum structure, in which both spin-orbit interaction and electron-electron interaction play crucial roles. A tight-binding model analysis predicts that materials with this structure can be strong topological insulators. Because of the electron correlation, an antiferromagnetic order may develop, giving rise to a topological magnetic insulator phase with axionic excitations.     

Domain structure of silicon iron single crystals

The model of a new domain structure arising after the magnetization of silicon iron single crystals in planes of the (110) type at an angle of 0°<Θ<-55° to the axis of easy magnetization is considered. Using this model the angular dependence of the domain-structure characteristics is established; it agrees closely with direct observations. On magnetizing a single crystal in the angular range 55° <Θ≤ 90° to the easy axis, layers with a uniform resultant magnetization parallel to the [001] direction are formed.     

Electronic structure of CsCdCl3 single crystals

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 53, No. 6, pp. 988–991, December, 1990.     

Surface structure of large germanium single crystals

The dependence between structural-defect generation and the growth kinetics of germanium single crystals pulled from melts is investigated via selective chemical etching and the optical, atomic-force and scanning electron microscopy techniques. It is ascertained that the surface microrelief of germanium crystals grown from melts by means of directional crystallization contains protrusions and cavities with spatial periodicities of 5 and 50 μm. The values of the kinetic coefficients are estimated. It is demonstrated that the main part of the crystals is formed according to the normal mechanism with the kinetic coefficients β_k = 2 × 10^?5 m s^?1 K^?1.     

Plastic properties and defect structure of LiF-LiF:Mg layered single crystals at T = 4.2 K

The compressive stress-strain diagrams are obtained for layered single crystals of the LiF-LiF:Mg type with different orientations of the reinforcing layers at T = 4.2 and 300 K. The strength characteristics and specific features of the defect and dislocation structures of the crystals strained in liquid helium are studied and compared with those for the crystals strained at 300 K. It is established that the layered single crystals remain plastic at T = 4.2 K. Under a strain ε > 1%, there arise microcracks and macrocracks responsible for complete fracture of the crystals. It is revealed that long-term (for ten years) storage of the layered single crystals at 300 K substantially affects the magnesium impurity structure in the reinforcing layers and leads to a considerable enhancement of their strength characteristics. This effect is taken into account when analyzing the strength properties of the layered single crystals in the temperature range T = 300–4.2 K. The inference is made that the results obtained in studying the defect microstructure of the model layered single crystals at the initial stage of plastic deformation can be used to predict the strength properties of the composites at T = 4.2 K.     

Thermoluminescence in single crystals of RbBr:OH− and RbBr:Ca2+

Optical absorption, thermoluminescence glow and emission spectra of RbBr:Ca²⁺ and RbBr:OH⁻ have been studied and analysed. It is observed that both Ca²⁺ and OH⁻ ions enhance theF-centre concentration.F _Z1 band in RbBr:Ca²⁺ appears at 1.55 eV. TL glow peak corresponding toF _Z1 centre on analysis gives a trap depth of 0.84 eV. OH⁻ ions in the crystal seem to act as TL ‘killers’. Spectral distribution of emission under the glow peaks shows five bands around 1.5, 1.8, 2.1, 2.5 and 2.9 eV. Probable models of TL mechanism are suggested to explain the observed TL emission bands.     

Thermoluminescence of nominally pure KTaO 3 crystals

After an exposure to ultraviolet light ( u <350 v nm) at 12 v K, weak thermoluminescence of nominally pure KTaO 3 crystals was observed within the temperature region from 13 to 65 v K for the first time. An analysis of the glow curves of integral thermoluminescence revealed five glow peaks with markedly sample-dependent intensities. Three glow peaks near 26, 31, and 58 v K at the heating rate of 0.155 v K/s correspond to thermoluminescence spectrally very similar to broad-band visible photoluminescence. The glow peaks near 26 and 31 v K were assigned to the electron release from photoinduced Ta 4+ -OH m and Ta 4+ - V O centers and the glow peak near 58 v K to the hole release from photoinduced O m centers. The glow peaks near 34 and 41 v K are connected with the structureless emission band peaking near 714 v nm at 15 v K that was observed in the emission spectra of KTaO 3 crystals for the first time.     

Thermoluminescence of pretreated NaCl: Sr crystals

Thermal glow curves of thermally pretreated NaCl:Sr crystals have been recorded after irradiation with ultraviolet light at room temperature. The observed glow curves exhibit a prominent glow peak at 380°K only when the specimen is annealed and quenched from 750°C. This peak is attributed to the dipole clusters which disintegrate after heating the specimen beyond 380°K     

Thermoluminescence of corundum containing anion defects following ultraviolet laser and x irradiation

The thermoluminescence of single crystals of corundum containing anion defects following x-ray and laser excitation is investigated. Its features in the luminescence bands of F, F⁺, and Cr³⁺ centers are studied. Synchronous measurements of the thermoluminescence and thermally stimulated exoelectron emission are performed by the fractional glow technique following x-ray and laser excitation of the samples. It follows from the results obtained that several traps are active in the temperature range of the principal dosimetric peak (400–500 K). The spectral sensitivity curve contains maxima corresponding to absorption bands of F, F⁺, and Al _i ⁺ centers. A possible mechanism for the recombination luminescence of F centers is discussed. It is found that the material exhibits high sensitivity to small doses of ultraviolet laser radiation. Zh. Tekh. Fiz. 67, 72–76 (July 1997)     

Temperature anomalies in the nonlinear magnetoacoustic properties of manganese-zinc spinel single crystals

The absorption of a magnetoelastic wave and the nonlinear resonance interaction of counterpropagating magnetoelastic waves in a manganese-zinc spinel single crystal have been investigated near the conjectured spin-reorientational transition. The convolution signal vanishes near T _tr=291 K; this is explained by the relaxational dynamics of the magnetic mode near the transition. The increase in the strength of the field corresponding to the maximum of the convolution with decreasing temperature is explained by a change in the demagnetizing field. Fiz. Tverd. Tela (St. Petersburg) 39, 652–655 (April 1997)