Potential relief on the surface of polarized ferroelectric electrets from an analysis of the anomalous electron emission spectra

A technique is proposed for determining the specific features in the potential distribution over a free surface of polarized ferroelectric electrets from analyzing the anomalous electron emission spectra. This technique is applied to examine the potential distribution over the surface of a lead magnoniobate single crystal and ferroelectric ceramics. Reasoning from the results obtained for the lead magnoniobate crystal, the inference is drawn that the degree of perfection of surface layers of ferroelectric single crystals can be controlled using the anomalous electron emission spectra.     

Polarized Raman and photoluminescence studies of a sub‐micron sized hexagonal AlGaN crystallite for structural and optical properties

The polarized Raman spectroscopy is capable of giving confirmation regarding the crystalline phase as well as the crystallographic orientation of the sample. In this context, apart from crystallographic X‐ray and electron diffraction tools, polarized Raman spectroscopy and corresponding spectral imaging can be a promising crystallographic tool for determining both crystalline phase and orientation. Sub‐micron sized hexagonal AlGaN crystallites are grown by a simple atmospheric pressure chemical vapor deposition technique using the self catalytic vapor–solid process under N‐rich condition. The crystallites are used for the polarized Raman spectra in different crystalline orientations along with spectral imaging studies. The results obtained from the polarized Raman spectral studies show single crystalline nature of sub‐micron sized hexagonal AlGaN crystallites. Optical properties of the crystallites for different crystalline orientations are also studied using polarized photoluminescence measurements. The influence of internal crystal field to the photoluminescence spectra is proposed to explain the distinctive observation of splitting of emission intensity reported, for the first time, in case of c‐plane oriented single crystalline AlGaN crystallite as compared with that of m‐plane oriented crystallite. Copyright © 2016 John Wiley & Sons, Ltd.     

Charge distribution in surface layers of polarized electret ceramics according to electron spectroscopy measurements

The anomalous electron emission (AEE) from the negative surface of a polarized SrTiO₃ ceramic sample is studied both experimentally and theoretically. The form of the low-energy edge and the fine structure of AEE spectra are interpreted within the “brick wall” model. The distribution of electric potential over the charged surface is calculated using Green’s functions under the assumption of p-type conduction of intergrain boundaries. It is demonstrated that the AEE spectra of SrTiO₃ ceramic surface mainly reflect the charge distribution in the polarized electret SrTiO₃ sample.     

Electron emission in ferroelectrics for different values of the coercive field

Results are presented of experimental studies of electron emission from single crystals of triglycine sulfate, nominally pure and doped with Cr³⁺. The relation between the parameters of the processes of switching and electron emission from ferroelectrics by a coercive field and the threshold field for the onset of emission is investigated. It is shown that the temperature and concentration dependences of the threshold field can be explained by the corresponding dependences of the coercive field. Fiz. Tverd. Tela (St. Petersburg) 41, 1675–1678 (September 1999)     

Methods of calculating the band structure and low-energy secondary electron spectroscopy of iridium

A theoretical interpretation is put forward for the fine structure of the secondary electron emission spectra of Ir normal to the (111) surface and the total current spectrum of an Ir polycrystal. The calculations took into account the energy dependence of the broadening of the energy band levels, the electron-electron and electron-plasmon contributions to the nonequilibrium electron distribution function, and the isotropic component of the current from the electrons scattered at the surface. It is shown that the fine structure of the secondary electron emission spectrum and the total current spectrum is mainly attributable to the electron structure of the final states into which the electrons enter or from which they are emitted so that the characteristics of the band configuration in the energy band structure can be reconstructed directly from the experimental data. This method can be used to separate bulk effects from surface effects in the secondary electron emission and total current spectra. It is confirmed that the fine structure of the secondary electron emission and total current spectra depends on the geometric structure and the degree of ordering of the crystals. A reduction in the intensity of the fine structure serves as a measure of the defect structure in the surface region of the sample which can be successfully used to monitor the surface state during treatment. Zh. Tekh. Fiz. 69, 94–96 (June 1999)     

Synchrotron reflectivity spectra and the optical properties of CuGaS2 crystal

For a CuGaS₂ crystal, the polarized spectra of complete sets of optical functions in the energy range of 0 to 24 eV are obtained for the first time. The calculations are performed using synchrotron reflectivity spectra for E ∥ c and E ⊥ c polarizations in the energy ranges of 0 to 24 eV and of 1 to 5 eV by means of computer programs using the Kramers-Kronig relations. The main features of the spectra of permittivity, characteristic electron loss, and other optical functions are determined. The band maximum energies for the volume and surface plasmons are obtained, and their independence from the type of polarization is found.     

The polarization of radiation in single crystals in the semiclassical approach

The radiation emission spectra of polarized photons emitted from charged particles in single oriented crystals are obtained in Bayer-Katkov-Strakhovenko semiclassical approach. The results of numerical calculations are presented in the region of small angles of incidence for which the coherent theory fails but the magnetic bremsstrahlung region is not yet achieved. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 3, 171–175 (10 February 1999)     

Polarized Infrared and Raman Spectra of 2(3H)-Benzoxazolone

Abstract

The infrared spectra of 2(3H)-benzoxazolone have been measured from 4000 to 180 cm^?1. The polarized spectra of single crystals have been also obtained. The Raman spectra of polycrystalline samples and solutions have been recorded and the polarization of many lines has been determined. On the basis of the i.r. dichroism, Raman spectra and correlative arguments most of the fundamental vibrations have been assigned.     

Polarized spectral analysis of Er3+ ions in biaxial Bi2(MoO4)3 crystal

An Er³⁺:Bi₂(MoO₄)₃ single crystal has been grown by the Czochralski technique. The Stark sublevels of the ⁴I_15/2 and ⁴I_13/2 multiplets of Er³⁺ ions in the crystal were determined. The polarized absorption spectra, polarized fluorescence spectra, and fluorescence decay curve of the crystal were measured at room temperature and the relevant spectroscopic parameters, including the Judd–Ofelt intensity parameters, spontaneous emission probability, fluorescence branching ratio, radiative lifetime, and stimulated emission cross section, were estimated. The effect of re-absorption on the spectroscopic parameters was discussed. When the crystal was excited at 977 nm, up-conversion green fluorescence was observed and discussed.     

The processes occurring in the surface region of CdS single crystals under electron bombardment,photoexcitation, and gas adsorption and their role in the 80 °K-exciton photoluminescence

Investigations have been carried out in the effect of 3–5 keV electron irradiation, intense photoexcitation, and gas adsorption on photoluminescent spectra of CdS single crystal platelets. CdS platelets subjected to electron bombardment were found to be very active for gas adsorption. The bound exciton emission is influenced by oxygen. A detailed study of the effect of the electron bombardment on the photoluminescence spectra is presented. The results indicate that correlation can be made between the behaviour of the electron bombardment-induced bound exciton emission and the electron processes occurring in the surface region of CdS single crystals. The changes in the photoluminescent spectra caused by the electron bombardment followed by the room temperature photoexcitation, have been interpreted in terms of surface photochemical effects.     

Anomalous electron emission from lithium niobate and lithium tantalate single crystals

Anomalous electron emission from the surface of LiNbO₃ and LiTaO₃ single crystals excited by soft x radiation has been discovered and is investigated. The absence of anomalous emission from the face of these single crystals is established. The experimental results confirm the theoretical conclusions that the anomalous emission is caused by the presence of a maximum in the distribution of the potential in the near-surface layer. The dependence of the lifetime τ of the anomalous emission on the dielectric constant ɛ of the ferroelectric is calculated. Fiz. Tverd. Tela (St. Petersburg) 39, 679–682 (April 1997)     

Abnormal blueshift of UV emission in single-crystalline ZnO nanowires

Single-crystalline ZnO nanowires (NWs) were synthesized by a facile vapor transport method. The good orientation and high crystal quality were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and high-resolution transmission electron microscope (HRTEM) measurements. Excitation-power-dependence photoluminescence spectra of ZnO NWs show that the UV emission displayed an evident blueshift with increasing excitation power and the corresponding energy shift might be as large as 10 meV. This anomalous phenomenon correlates to the band bending level caused by the surface built-in electric field due to the existence of substantial oxygen vacancies. By increasing the excitation power, the enhanced neutralization effect near the surface will reduce the built-in electric field and lead to a reduction of band bending which triggers the blueshift of the UV emission.     

Spectroscopic analysis of Pr3+:CaYAlO4 crystal

The polarized absorption, emission spectra and decay time measurements of Pr³⁺-doped CaYAlO₄ single crystal have been performed at room temperature. Based on the Judd–Ofelt theory, the spectroscopic parameters \(\Upomega_{t} (t = 2,4,6)\) , radiative transition probabilities, radiative lifetimes and branching ratios were obtained. The stimulated emission cross-section, fluorescence lifetimes and the quantum efficiency of the promising laser transition were also calculated and compared with other reported crystals. The results show that Pr³⁺:CaYAlO₄ is a promising candidate for visible solid-state laser emission.     

Deformation Luminescence in gamma-irradiated KCl single crystals

Abstract

Deformation Luminescence(DL) is studied on KCl crystal colored by γ-irradiation. The spectra analysis of DL and thermoluminescence reveals that F-center plays a role as electron donor through the interaction with moving dislocation and V₂-center is a probable luminescence center. A theory is presented for understanding the deformation rate and temperature dependences of DL intensity.     

Pure anomalous Hall effect in nonmagnetic zinc-blende semiconductors

We report the experimental observation of the pure anomalous Hall effect (AHE) in nonmagnetic zinc-blende semiconductors without application of the external magnetic fields. The AHE without any contribution from the ordinary Hall current originates from nonequilibrium magnetization induced by spin-polarized electrons generated by the circularly polarized light (σ). We measure the pure AHE as a function of the external bias, crystal temperature and pumping σ-photon energy. The results of their dependences are discussed.     

An EPR study of Er3+ impurities in RbTiOPO4 single crystals

An electron paramagnetic resonance (EPR) study of Er³⁺ ions in single crystals of RbTiOPO₄ (RTP) is presented. The EPR spectra show the existence of six different Er³⁺ centres. The g-matrix has been determined for these centres from the analysis of the angular dependences of the spectrum in three planes of the crystal. The study supports that erbium can enter the Rb⁺ and Ti⁴⁺ low-symmetry sites of RTP. This conclusion differs from those for KTP:Er³⁺ and RTP codoped with Nb and Er. The different occupancies found for Er in these various crystals is suggested to be due to the differences in Er concentration.     

Polarized Raman mapping study of the microheterogeneity in 0.67PbMg1/3 Nb2/3O3‐0.33PbTiO3 single crystal

Polarized micro‐Raman spectroscopy was carried out on the (001) face of a 0.67PbMg_1/3Nb_2/3O₃‐33%PbTiO₃ (PMN‐33%PT) single crystal. The Raman images revealed the spatial variations of the intensity of the Raman bands, suggesting that the structure in the PMN‐33%PT single crystal varied from one micro‐area to another. When changing the polarization direction of the incident light with respect to the selected crystalline axes, the intensities of the Raman modes varied periodically. According to the Raman selection rules (RSRs), the angular dependences of the Raman modes indicated that the PMN‐33%PT single crystal is in the monoclinic phase. Furthermore, the color patterns in the Raman images were associated with the coexistence of the M_A‐ and M_C‐type monoclinic phases in the PMN‐33%PT single crystal. Our results provide useful information for understanding the microheterogeneity of the relaxor PMN‐xPT single crystals with compositions near the morphotropic phase boundary region. Copyright © 2010 John Wiley & Sons, Ltd.     

The bandstructure of Pd(110) above the fermi level

The band-structure of Pd(110) was studied by k-resolved inverse photoemission (IPE). The data are interpreted on the basis of a relativistic band-structure calculation. Apart from direct bulk band transitions, weak surface umklapp and surface emission features are observed. Four different surface state bands could be detected in the gaps at X and Y. An anomalous temperature behaviour is found for a transition into the d-bands close to the Fermi level. Finally, structures resulting from LEED reflections are identified in the IPE spectra and their characteristics are discussed.     

Electromagnetic radiation of MeV electrons in thick aligned single crystals

Abstract

Earlier published theoretical models for MeV electrons are generalized in this paper. Different theoretical predictions for planar channeling and accompanying electromagnetic radiation in thick crystals are presented. A comparative analysis of theoretical and experimental spectra of photon radiation is given. Thickness dependences of channeling quantum state populations, radiation line broadening and photon flux intensities are obtained. The existence of planar electron channeling at 54 MeV in a silicon single crystal with a thickness of several millimetres is shown.     

Anomalous Structure In The Reflection Spectrum Of The Tcnq Molecular Crystal

The polarized normal incidence specular reflection spectra for both principal directions in the [010] face of the TCNQ single crystal have been obtained from 18kK to 37kK. A band of high metallic-like reflection is observed in this region with an unique anomalous structure in the high energy, two-particle, region of the band. The results are discussed in terms of the classical and semi-classical Drude-Lorentz theory.