Dark conductivity in copper-doped lithium niobate crystals

The impact of protons and copper on the dark conductivity of copper-doped photorefractive lithium niobate crystals is investigated by observation of the dark decay of holograms. The dark conductivity is significantly smaller than that of iron-doped lithium niobate. A non-mono-exponential hologram decay is observed, which needs to be considered for multiplexing of holograms. Received: 25 March 2002 / Published online: 2 May 2002     

Luminescence of KBr crystals doped with copper

Radiation produced in copper-doped KBr crystals with a maximum near 2.49 eV was investigated. Luminescence with a maximum at 2.49 eV is caused by excitation processes at luminescence centers, the structure of which includes interstitial positively charged univalent copper ions bound to divacancies and bromine ions. The copper-doped solid solution (K,Na)Br was used for observation of the transformation of luminescence centers and for increasing their concentration.S. M. Kirova State Pedagogical Institute, Pskov. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 79–82, September, 1993.     

Preparation and electrical properties of copper-doped cadmium telluride single crystals

Preparation and the electrical properties of copper-doped cadmium telluride single crystals prepared by directional freezing and by diffusion of copper from a layer of Cu₂ Te are described in this paper. The dependence of electrical properties on the preparation mode is followed. In the enclosed infra-red spectra it is proved that samples suitable for optical measurements can be prepared by both described techniques.     

Mutually Pumped Phase Conjugation of Three Incoherent Beams with a Novel Photorefractive Crystal

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Paramagnetic copper centers in ferroelectric lead germanate with halogens

The electron paramagnetic resonance spectrum of triclinic bivalent copper centers in copper-doped Pb₅Ge₃O₁₁ single crystals has been investigated. The spectrum intensity increases after thermal treatment in a chlorine- or fluorine-containing atmosphere. The parameters of the electron Zeeman, hyperfine, and nuclear quadrupole interactions have been determined. The model of the observed centers has been proposed.     

On the berry phase in quantum oscillations observed in 3D topological insulators

It is demonstrated that the copper-doped high-quality Bi Se single crystals with a high density of bulk charge carriers are certainly 3D topological insulators. The analysis of quantum Shubnikov?de Haas (SdH) oscillations reveals that these materials simultaneously exhibit two types of such oscillations determined by the Landau levels related to both the 3D and 2D Fermi surfaces.     

Photoluminescence of Copper-Doped Lithium Niobate Crystals

The photoluminescence (PL) of copper-doped lithium niobate single crystals is studied using different UV–Vis light-emitting diodes and a pulse-periodic laser with a wavelength of 266 nm as excitation radiation sources. With the resonance excitation from a 527-nm light-emitting diode, the intensity of PL increases sharply (by two orders of magnitude). When using a 467-nm light-emitting diode for excitation, the PL spectrum is characterized by the presence of multiphonon lines in the range of 520–620 nm.     

Ionic conductivity and dielectric measurements in single crystal βAgI

Ionic conductivity measurements have been made on pure, copper-doped and cadmium-doped single crystals. Dielectric measurements in the frequency range 30Hz–100Hz showed that there was no anomalously to be (0.64 ± 0.02) eV and migration energies for silver ion intersitials and vacancies in the c direction to be (0.41 ± 0.02) eV and (0.50 ± 0.02) eV respectively. ESR measurements have shown that copper exists as Cu⁺ in these crystals. Dielectric measurements in the frequency range (OHz–100KHz showed that there was no anomalously high value for ? as reported earlier.     

Characterization of pure and copper-doped iron tartrate crystals grown in silica gel

Single crystal growth of pure and copper-doped iron tartrate crystals bearing composition Cu _x Fe_(1???x)C₄H₄O₆·nH₂O, where x = 0, 0.07, 0.06, 0.05, 0.04, 0.03, is achieved using gel technique. The elemental analysis has been done using energy-dispersive X-ray analysis (EDAX) spectrum. The characterization studies such as Fourier transform infrared spectroscopy (FTIR), powder X-ray diffraction (XRD), magnetic analysis and thermal analysis have been done for crystals with x = 0 for pure iron tartrate and with x = 0.05 for copper-mixed iron tartrate crystals. A detailed comparison has been made between pure and doped crystals.     

High-temperature-recorded index gratings in periodically poled lithium niobate

Holographic index gratings based on a zero-electric-field photorefractive effect are recorded at high temperatures in copper-doped periodically poled lithium niobate crystals. The interplay between the domain structure and the index grating is studied: the fundamental grating with spatial frequency K is strongly suppressed. Pronounced sideband gratings with K(s)=K+sG appear, where G is the domain grating vector and s is an integer number. After development, an additional grating based on the electro-optic effect shows up. In contrast with the previously mentioned gratings, this grating allows anisotropic diffraction.     

Copper-induced diamond formation in amorphous carbon

Diamond formation in hydrogenated amorphous-carbon films containing ultradisperse copper has been studied by measuring IR absorption at two-phonon diamond frequencies. The anomalously high two-phonon absorption observed in the experiments has permitted improving the sensitivity of the method. Mechanisms of the two-phonon absorption enhancement are discussed in terms of the theory of a medium containing nanosized copper-doped graphite fragments. It is shown that the observed enhancement of two-phonon absorption is caused by electrical induction of the local fields induced by irradiation in diamond nanocrystals acting on adjoining copper-doped graphite fragments. Fiz. Tverd. Tela (St. Petersburg) 41, 1863–1866 (October 1999)     

Formation of defects in CdS and CdS:Cu single-crystals irradiated by E = 46 MeV protons

The results of an experimental study of the spectral dependence of the adsorption coefficient, reflection exciton spectra, luminescence, photoconductivity (in the wavelength range from 0.45 to 1 m), and dark conductivity of pure and copper-doped CdS single-crystals, irradiated by E=46 MeV protons, are presented. It is shown that irradiation produces characteristic defects both in the cadmium sublattice and in the sulfur sublattice, but for one and the same irradiation dosages the defect concentration in CdS:Cu is many times higher than the concentration of defects in the pure crystals. A significant difference is observed in the concentrations of defects in these samples only when they are irradiated by high-energy particles which strongly ionize the lattice of the crystals. It is proposed that there exists an ionization mechanism for the transfer of a significant part of the energy of the moving protons to Cu atoms, which form a large number of defects (more than 1 per copper atom).Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 62–66, May, 1987.     

Effects of the structural state of a W — Re solid solution on the Fe-57 Mössbauer spectrum

Mössbauer spectra for iron-57 are reported for alloys containing up to 27 at.% rhenium. The cobalt-57 was introduced by diffusion annealing as the parent isotope for the iron-57. Diffusion annealing at 1080°C for 16 hr causes the cobalt to diffuse mainly along lattice defects (dislocations and grain boundaries). These defects give rise to regions whose electronic structure differs from that of the matrix. The precise type of structure and the number of such regions are complex functions of the alloy composition and structural state. Results are given on the iron-57 spectra for tungsten, rhenium, the σ phase, and an alloy in the two-phase region.     

Magnetic resonance of exchange-coupled copper complexes in perovskite-structure crystals: The potassium tantalate and cuprate superconductors

This paper reports on parallel EPR studies of high-temperature superconductors based on the cuprate perovskites RBa₂Cu₃O_6+x (R=Y, Gd, Nd) and of KTaO₃: Cu, which also has a perovskite structure. EPR measurements performed on copper-doped KTaO₃ crystals revealed Cu²⁺-Cu²⁺ copper pair centers. The copper ions making up pairs are assumed to occupy adjacent tantalum sites. The pair centers are chains consisting of two equivalent Cu²⁺ ions and three oxygen vacancies aligned in the 〈100〉 direction. The crucial point in the model proposed is the presence of an oxygen vacancy sandwiched between two Cu²⁺ ions, whereas the outer vacancies do not necessarily occupy neighboring sites. In this structure, complete charge compensation is achieved. Ferromagnetic exchange coupling takes place between the two copper ions. An investigation of the exchange and superhyperfine interactions of copper centers in crystalline potassium tantalate has permitted the estimation of the respective interactions in crystals of the cuprate superconductors which exhibit magnetic resonance signals due to exchange-coupled copper clusters in the case of oxygen deficiency.     

New nanoheterostructures based on astralenes: Spectral characteristics and some application aspects

Original compositions in the form of suspensions of astralenes with luminescing solubilizers in various solvents, including liquid crystals (LCs), are developed. Using an LC composition as an example, the possibility of extending the dynamic range of optical limiting due to the joint action of two-photon absorption in the LC matrix and stimulated scattering by carbon nanoparticles is demonstrated. The spectral and luminescent characteristics of such suspensions are determined. The role played by the photochemical factor in the photodynamics of optical limiting in carbon heteronanostructures is discussed.     

Field-induced anisotropy in concentrated systems of rigid particles and macromolecules

This paper presents experimental results on the use of spectroscopic optical polarimetry to study structure in dense systems of rigid particles and rigid polymer liquid crystals. These measurements probe microstructural anisotropy induced by the application of electric fields in the case of dense suspensions of rigid spheres, or flow fields in the case of polymer liquid crystals. It is demonstrated that conservative linear dichroism can measure moments of the particle pair distribution function in dense suspensions. In liquid crystals, the dichroism is a result of field-induced anisotropy in the defect structure of the material.     

Preparation, characterization and study of optical properties of ZnS nanophosphor

In this work the preparation, characterization and photoluminescence studies of pure and copper-doped ZnS nanophosphors are reported, which are prepared by using solid-state reaction technique at a temperature of 100 °C. The as-obtained samples were characterized by X-ray diffraction (XRD) and UV-VIS Reflectance spectroscopy. The XRD analysis confirms the formation of cubic phase of undoped as well as Cu²⁺-doped ZnS nanoparticles. Furthermore it shows that the average size of pure as well as copper-doped samples ranges from 15 to 50 nm. The room-temperature PL spectra of the undoped ZnS sample showed two main peaks centered at around 421 and 450 nm, which are the characteristic emissions of interstitial zinc and sulfur vacancies, respectively. The PL of the doped sample showed a broad-band emission spectrum centered at 465 nm accompanied with shoulders at around 425, 450 and 510 nm, which are the characteristic emission peaks of interstitial zinc, sulfur vacancies and Cu²⁺ ions, respectively. Our experimental results indicate that the PL spectrum confirms the presence of Cu²⁺ ions in the ZnS nanoparticles as expected.     

Room temperature ferromagnetism in Cu-doped ZnO synthesized from CuO and ZnO nanoparticles

AC susceptibility and ferromagnetic resonance (FMR) measurements indicate that ZnO doped with Cu by a simple sintering process starting from nanoparticles of ZnO and CuO is ferromagnetic above room temperature. FMR measurements above room temperature indicate the ordering temperature to be above 520 K. The observation supports the recent theoretical calculations of Huang et al. which predict ferromagnetism in copper-doped ZnO.     

Photo-thermal ionization in a copper-doped germanium detector

The spectral response of a copper-doped germanium photo-conductive detector has been examined. Two series of sharp photo-thermal neutral acceptor excitations are observed; one is identified with copper impurities while the other is of unknown origin.     

Preparation of colloidal films with different degrees of disorder from monodisperse spherical silica particles

The process of coagulation of suspensions of monodisperse spherical silica particles driven by appropriate coagulants (NH₄Cl, HCl, C₁₆H₃₃N(CH₃)₃Br) has been studied. Films of photonic crystals and photonic glasses have been grown by the sedimentation method. The possibility of controlling the degree of structural disorder in colloidal films by properly varying the resistance of the water suspension of particles to aggregation has been demonstrated.