Giant nonlinear absorption in the NiO antiferromagnet

A study of the spectrum of nonlinear two-photon and two-step absorption in NiO single crystals, carried out in the energy region ?ω₁ + ?ω₂ = 2.45–4.575 eV, showed it to have a complex shape and consist of very strong peaks (from 0.05 to 2.7 cm/MW). Within the energy interval 2.45–3.3 eV, the spectrum is due to d-d transitions in the Ni²⁺ ion. The band gap width was determined to be E _g =3.466 eV. The spectral features seen above this energy originate from interband transitions from three valence subbands to the conduction band bottom.     

First-principles study of the H and F centers in LiYF4

We have investigated the electronic structures of the LiYF₄ containing interstitial fluorine atoms and F center (a fluorine ion vacancy trapping an electron) using first-principles density functional theory. It is found that the interstitial fluorine atoms in two different interstitial positions would combine with its nearest neighbor two or three formal lattice fluorine ions forming fluorine molecular ions or by different ways, which would cause the 260 nm absorption band. Simultaneously, our study indicates that one electronic state appears in the forbidden band of the perfect LiYF₄ crystal resulting from the F center in the LiYF₄ crystal. And the energy difference of this electronic state and the bottom of the conduction band is 3.74 eV, corresponding to the 331 nm absorption band. It is predicted that the 330 nm absorption band could arise from the F center in LiYF₄ crystals.     

Growth,fabrication, and testing of bismuth tri-iodide semiconductor radiation detectors

Bismuth tri-iodide (BiI₃) is an attractive material for high energy resolution radiation detectors. For the purpose of this research, detectors were fabricated using single crystals grown from ultra-pure BiI₃ powder; synthesized by the Physical Vapor Transport (PVT) technique. This technique yielded powder with total impurity level of 7.9 ppm. Efforts were also made to purify commercial BiI₃ powder using a custom-built Traveling Zone Refining (TZR) system. Initial trial runs were successful in reducing the total impurity level of the commercial powder from 200 ppm to less than 50 ppm. Using the modified vertical Bridgman technique and a customized sharp tip ampoule, a large BiI₃ single crystal was grown. The crystal had a surface area of 2.2 cm² and a thickness of 0.8 cm, which corresponds to a volume of 1.78 cm³. Radiation detectors were fabricated and then tested by measuring their electrical characteristics and radiation response. An alpha particle spectrum (using a ²⁴¹Am α-source) was recorded at room temperature with a BiI₃ detector 0.09 cm thick and with a surface area of 0.16 cm². The electron mobility was estimated to be 433 ± 79 cm²/V.     

Absorption edge and photoconductivity of CuPbAsS3 Single crystals

Absorption curves of CuPbAsS₃ have been measured in the temperature range 90÷300 K. Their analysis leads to conclusion that the absorption edge of CuPbAsS₃ is due to allowed indirect electronic transitions between the valence and conduction band extrema. The forbidden-gap width, its temperature dependence and energies of three interacting phonons have been determined. The temperature shift of the photoconductivity band is in agreement with that of the forbidden gap in CuPbAsS₃.Leninovo nám. 565, Pardubice, Czechoslovakia.The authors are indebted to Dr. Vl. Syneek and Mrs V. Míková from the Institute of Solid State Physics, Czechosl. Acad. Sci., Prague for advice and help in orientation of prepared crystals.     

Location of the energy levels of the rare-earth ions in BaF<Subscript>2</Subscript> and CdF<Subscript>2</Subscript>

The location of the energy levels of rare-earth (RE) elements in the energy band diagram of BaF₂ and CdF₂ crystals is determined. The role of RE ³⁺ and RE ²⁺ ions in the capture of charge carriers, luminescence, and the formation of radiation defects is evaluated. It is shown that the substantial difference in the luminescence properties of BaF₂: RE and CdF₂: RE is associated with the location of the excited energy levels in the band diagram of the crystals.     

Europium luminescence in fluorite upon high-energy excitation

The reflection and luminescence excitation spectra of CaF₂ crystals containing europium ions in divalent (Eu²⁺) and trivalent (Eu³⁺) states were measured in the range from 4 to 16 eV. It was established that, in CaF₂ : Eu³⁺ crystals, luminescence of Eu³⁺ ions (the f-f transitions) is effectively excited both in the charge-transfer band (at ～8 eV) and in the region of the 4f–5d transitions (at ～10 eV) but is virtually not excited in the fundamental region of the crystal (at an energy higher than 10.5 eV). Luminescence of Eu²⁺ ions (the 427-nm band) in CaF₂ : Eu³⁺ is effectively excited in the fundamental region of the crystal; i.e., luminescence of divalent europium ions occurs through the trapping mechanism. Emission of Eu²⁺ ions in CaF₂ : Eu²⁺ crystals is characterized by the excitation band at an energy of 5.6 eV (the 4f → 5d,t _2g transitions), as well as by the exciton and interband luminescence excitations. The results obtained and data available in the literature are used to construct the energy level diagram with the basic electron transitions in the CaF₂ : Eu crystals.     

Effective Hamiltonian of Bloch electrons in a homogeneous electrical field. Two-band approximation

The method proposed earlier [1] for determining the renormalized dispersion law _q ^F () from Bloch electrons in a homogeneous electrical field is developed in a two-band approximation. For a broadband semiconductor with a narrow forbidden band, a form of _q ^F () valid in arbitrary fields is determined. The electronic transition for a certain critical field intensity determined by the forbidden band width is predicted.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 80–85, July, 1984.The author is grateful to A. S. Kotosonov, V. L. Bonch-Bruevich, and the participants of his seminar, S. D. Beneslavskii and A. G. Mironov, for fruitful discussion of the results obtained.     

First-principles study of color centers in PbMoO4 crystals

Electronic structures of PbMoO₄ crystals containing Mn ion impurities located at Pb²⁺ sites are studied within the framework of the fully relativistic self-consistent Direc-Slater theory, using a numerically discrete variational (DV-Xα) method. The calculated results show that Mn²⁺ ions have donor energy levers in the forbidden band, which may correspond to the yellowish color absorption band of PbMoO₄ as-grown in air. The new-formed Mn³⁺ ions have acceptor energy levers in the forbidden band, which may relate to the photochromic effect in PbMoO₄ crystal.     

Electrical properties of n-type CdSe single crystals prepared under a nitrogen pressure

Single crystals of n-type CdSe were prepared from the melt at a pressure of about 100 atmospheres of nitrogen. The results of andR _H measurements as a function of temperature are presented for the region of 160–1500°K. The activation energy of the donors and the width of the forbidden band as determined from the temperature dependence are _D=(0.15±0.02) eV andE _g=(1.88±0.03) eV respectively. For comparison the results of measurements carried out on crystals prepared in a vacuum are also mentioned.The authors wish to thank Professor Dr. E. Klier and Dr. R. Kuel for valuable comments on this work.     

Origin bands of electronically forbidden spectra made allowed by centrifugal distortion

Watson’s theory of pure rotational spectra in nonpolar molecules is adapted to show that, if appropriate symmetry conditions are met, centrifugal distortion can lead to the appearance of the forbidden 0-0 origin band in an electronically forbidden spectrum. The intensity of such bands will be low; in the case of the benzene ùB_2u-¹A_1g system the maximum intensity of the forbidden origin is estimated to be about 10⁻⁷ of that of the 6¹₀ band in the main vibronic spectrum. The prospects for the experimental detection of such weak bands are discussed.     

Radiation and thermally induced effects in YAlO3:Mn crystals

We have studied effects induced by γ-radiation and temperature in Mn-doped YAlO₃ crystals. The studies have been performed by means of optical spectroscopy that include measuring of optical absorption changes induced by γ-radiation and elevated temperature as well as thermally stimulated luminescence (TSL). It has been shown that under γ-irradiation of YAlO₃:Mn crystals, along with the ionization of Mn_Al⁴⁺ ions (Mn_Al⁴⁺→Mn_Al⁵⁺+e⁻), some additional coloration processes take place. This additional coloration is characterized by a wide intense band centered at 26,000- that is ascribed to color centers intrinsic to YAlO₃ lattice. This coloration is removed by the way of crystal warming at , while the coloration caused by Mn_Al⁵⁺ ions is removed at higher temperature . The observed TSL glow of irradiated crystals reveals three peaks near 360, 400 and that correspond to three types of traps. Parameters of the traps have been determined. The TSL emission corresponds to intra-center luminescence of Mn_Al⁴⁺ and Mn_Y²⁺ ions. The possible ionization and trapping mechanisms in YAlO₃:Mn crystals are discussed.     

Luminescence centers generated by chromium ions in potassium-aluminum (indium) diphosphates

Crystals of potassium-aluminum (indium) diphosphates KMeP₂O₇ (Me = Al, In) doped by chromium ions are synthesized for the first time, and their main spectral luminescence properties are described. The luminescence of these crystals is studied in the temperature range 4.2–300 K under excitation by light with a wavelength of 270–650 nm. The luminescence spectrum has two wide bands in the ranges 450–650 nm (“green” photoluminescence) and 700–800 nm (“red” photoluminescence). The green photoluminescence band is assumed to be a superposition of the intrinsic radiation of the matrix and the radiation of molecular centers generated by chromium ions that are surrounded by four oxygen atoms. At low temperatures, the red photoluminescence band consists of a wide unstructured band and narrow lines on its short-wavelength wing and is associated with the radiation of Cr³⁺ ions that replace Al³⁺ matrix ions and are in an octahedral oxygen environment. Parameters of the electron energy level diagram of the Cr³⁺ ion in the matrices of the investigated crystals are determined.     

Magneto-optical spectroscopy and optical detection of EPR spectra of Yb<Superscript>3+</Superscript> paramagnetic centers with cubic symmetry in single crystals <Emphasis Type="Italic">Me</Emphasis>F<Subscript>2</Subscript> (<Emphasis Type="Italic">Me</Emphasis> = Cd,Ca, Pb)

Cubic paramagnetic centers formed by Yb³⁺ impurity ions in fluorite-type crystals MeF₂ (Me = Cd, Ca, Pb) have been investigated using electron paramagnetic resonance, magnetic circular dichroism, magnetic circular polarization of luminescence, Zeeman splitting of optical absorption and luminescence lines, and optical detection of electron paramagnetic resonance. The g factors of the ²Γ₇ state in the excited multiplet ² F _5/2 of Yb³⁺ ions in Me F₂ crystals, the hyperfine interaction constant ¹⁷¹ A (¹⁷¹Yb) for the excited multiplet ² F _5/2 in the CaF₂ crystal, and the energies and symmetry properties of all energy levels of Yb³⁺ ions in MeF₂ crystals are determined. The crystal-field parameters for the crystals under investigation are calculated.     

Defect structure of hydrogen-reduced BaTiO3 with additions of Fe,Co and Ni

A discussion is presented of the possibilities of trapping an electron set free by hydrogen reduction in BaTiO₃ crystals without and with additions of Fe, Co and Ni. It is proved with the aid of the absorption spectra of reduced BaTiO₃ measured in the visible region, and of a theoretical calculation of the energy gain accompanying electron trapping that the preferred reduction is that of Fe, Co and Ni ions to divalent ones. The minimum hydrogen reduction temperature at which a band at 2·5 to 4 is initiated, belonging — as our results indicate — to Ti³⁺ in a certain configuration of the defect oxygen octahedron, was calculated from chemical analysis data of BaTiO₃ monocrystals and also verified experimentally.     

Raman scattering,luminescence, and absorption edge under hydrostatic pressures of layered BiI3 and SbI3

Raman scattering from BiI₃ and SbI₃ crystals has been studied at hydrostatic pressures up to 2 GPa. The wavenumbers monotonically increased with pressure for all modes in BiI₃. Although both crystals belong to the same space group, the pressure coefficients of SbI₃ were significantly different from those of BiI₃. At pressure P₀ = 0.92 ± 0.01 GPa, the pressure coefficients of all modes changed. Above P₀, all modes in SbI₃ had positive pressure coefficients as they have in BiI₃. By comparing this unusual behavior of the Raman lines in SbI₃ around P₀ with the results in BiI₃ and comparing the two lattice structures, we suggest that the type of bonding of SbI₃ changes to become more ionic and hence more similar to the bonding of BiI₃. This change in structure is supported by changes in the bandwidth of the self‐trapped exciton (STE) luminescence and the slope of the absorption edge photon energy versus pressure. Copyright © 2007 John Wiley & Sons, Ltd.     

Electrolytic coloration of air-grown sodium fluoride crystals

Air-grown sodium fluoride crystals were colored electrolytically by using a pointed cathode at various temperatures and electric field strengths, which should mainly benefit appropriate coloration temperatures and electric field strengths. , F, M, N₁, N₂ color centers and O²⁻-F⁺ complexes were produced in the colored crystals. Current-time curves for the electrolytic colorations were given, and activation energy for the V color center migration was determined. The formation of the color centers was explained.