The anisotropy of X-ray emission spectra for 2H-MoS2 single crystals

The angular dependence of the polarised X-ray S Kβ₁ and Mo Lβ_{2, 15} spectra of single-crystal 2H-MoS₂ has been studied. A discussion of the experimental data obtained is performed on the basis of a local symmetry analysis of atoms.     

ESR spectra of Eu centers in defect Ga2S3 single crystals

In this paper, the author presents the results of measurements of the low-temperature and angular dependences of the ESR spectra of Eu²⁺ centers in defect Ga₂S₃ single crystals in the temperature range 8–29 K and for 0–180° orientations of the static magnetic field. The electron structure of impurity ¹⁵¹Eu atoms in Ga₂S₃:Eu single crystals has been studied by using the ESR method at different doping proportions of Eu atoms. Ga₂S₃ single crystals were grown from the melt using the Bridgman method. The Eu concentration was determined by atomic absorption analysis and X–ray fluorescence analysis (XRFA). By investigation on the ESR spectra, the author has first determined the values of charge states for Eu, which have turned out to be a Eu²⁺(4f⁷) ion with spin S=7/2, g=4.18±0.02 and concentration of the states of Eu N=6.3×10¹⁴ cm⁻³.     

Electroluminescence in LiNbO3 single crystals

The light emission from LiNbO₃ single crystals caused by the applied a.c. high voltage is studied. It is interpreted as electroluminescence due to impact processes of field accelerated electrons. Two bands can be distinguished in the emission spectrum: one larger with the maximum at 660nm and one smaller at about 500nm.     

X-ray and thermoluminescence of LiKB4O7 single crystals

The thermoluminescent (TL) and X-ray luminescent (XL) spectra of undoped LiKB₄O₇ (LKBO) single crystals had been investigated in the temperature range 80-300 K. It was found that in LKBO crystals, there are two intensive TL peaks at 112 and 132 K. The only one band emission spectra of sharply defined Gaussian shape, confirming the same mechanism of XL and TL by the radiation annihilation of the strongly localized self-trapped excitons (STE), had been observed in the TL and XL spectra. The possible models of these localization centers STE have been discussed.     

K-shell excitation spectra of CO,N2 and O2

Electron energy loss spectra of CO, N₂ and O₂ have been recorded in the regions of carbon, nitrogen and oxygen K-shell excitation and ionisation. These results are compared to previous energy loss, photoabsorption and theoretical studies of the same spectral regions. Several inconsistencies in the published spectra are clarified in the present work. Comparisons with recent calculations of the K-shell continua of these molecules are presented. Vibrational structure in the K → π * transitions of CO (C 1s) and N₂ (N 1s) has been resolved in high-resolution studies (< 0.1 eV FWHM) of these species.     

Infrared spectra of hydrogen impurities in BaTiO3 crystals

The IR spectra in the 2.8 μm region of the OH^? impurities in thick single domain crystals have been measured for different polarization geometries and various temperatures. Simple calculations of the Coulomb interactions between hydrogen and lattice ions show that these interactions are much more important than the hydrogen bonding ones for determining the positions of the hydrogen ions. The appearance of only one strong non-dichroic OH^? stretching line is due to an almost equal population of the proton axial sites. The side bands are interpreted as combinations of the stretching mode with a low-frequency translational mode. The large broadening of the main line just above T_c supports the model which attributes a considerable order—disorder character to the ferroelectric phase transition.     

Magnetization and magnetic susceptibility of single crystals of HoTiO3 and ErTiO3

Bulk magnetization measurements have been performed on single crystals of HoTiO₃ and ErTiO₃. The easy direction of magnetization is along the b-axis for HoTiO₃ and along the c-axis for ErTiO₃ with respect to the Pbnm chemical cell. These findings are in qualitative agreement with the results of two independent powder neutron diffraction studies. The saturation magnetic moments at 4.2 K along the easy axes are 7.3(2)μ_B per formula unit for HoTiO₃ and 6.9(2)μ_B for ErTiO₃. In addition, single crystal susceptibility data were analyzed using the theory of Boutron to yield values for the Heisenberg exchange coupling and the second-order crystal field terms B⁰₂ and B²₂.     

The electronic absorption spectra of [(CH3)3NH] MnCl3.2H2O single crystals

The electronic absorption spectra of [(CH₃)₃NH] MnCl₃.2H₂O single crystals are reported in the 15,000–45,000 cm^-1 region. In addition to the normally studied sextet → quartet transitions, special attention has been paid to the sextet → doublet transitions. Crystal field parameters evaluated (including the Trees' correction factor) to fit the observed spectra are B = 800, C = 2900, Dq = 680, and α = 76 cm^-1.     

Superconducting energy gap in MgCNi3 single crystals

Even though MgCNi₃ contains large molar volume of magnetic Ni atoms it clearly shows superconductivity. The origin of superconductivity in this material has not yet been clarified. There is a lot of discrepancy in experimental results and physical interpretation where even unconventional pairing or a two-band model have been proposed for the system. All that is based on results obtained on polycrystalline samples. Here we present the first point-contact spectroscopy and ac-calorimetry measurements on the MgCNi₃ single crystals of very good quality. Measurements have been performed in the temperature range from 1.5 to 10 K and in magnetic fields up to 8 T. The temperature dependence of the energy gap of the system is presented and compared to the BCS model.     

Investigation of emission spectra and lifetimes of Cs2NaErCl6 single crystals

The fluorescence spectra and lifetimes of low lying states of Er³⁺ in Cs₂NaErCl₆ single crystals have been investigated at room temperature and at 77 K. The assignments of the ⁴F_{$\text{9}\text{2}$}-⁴I_{$\text{15}\text{2}$} and ⁴I_{$\text{11}\text{2}$}-⁴I_{$\text{15}\text{2}$} phonon-assisted transitions have been proposed. The fluorescence lifetimes of low lying fluorescing levels have been estimated.     

New measurements of the emission spectra of Sm2+ in KMgF3 and NaMgF3 crystals

The fluorescence of divalent samarium in KMgF₃ and NaMgF₃ crystals is investigated. The emission is observed to originate from transitions between the ⁵D_J, and ⁷F_J multiplets of the 4?⁶ configuration. More precisely, the lowest ⁵D_J level, ⁵D₀, appears to be the most efficient emitting level in the temperature range 4–300K. Contrary to what has been reported elsewhere, the Sm²⁺ fluorescence in both crystals does not exhibit any broad band emission even at room temperature. The great number of lines in the ⁵D₀→⁷F_J patterns gives evidence of the multiple-center origin of the fluorescence.     

Photoluminescence spectra of rare earth doped CaGa2S4 single crystals

Six kind CaGa₂S₄ single crystals doped with different rare earth (RE) elements are grown by the horizontal Bridgman method, and their photoluminescence (PL) spectra are measured in the temperature range from 10 to 300 K. The PL spectra of Ce or Eu doped crystals have broad line shapes due to the phonon assisted 4f-5d transitions. On the other hand, those of Pr³⁺, Tb³⁺, Er³⁺ or Tm³⁺ doped samples show narrow ones owing to the 4f-4f transitions. The assignments of the electronic levels are made in reference to the reported data of RE 4f multiplets observed in same materials.     

Surface characterization of In4Se3 single crystals

Surface properties of indium subselenide (In₄Se₃) were studied. It was confirmed, that the superstructure of this crystal is characterized by nanowire-like cylindrical clusters with diameter dimensions of about 20 nm and stairs along the a-axis up to 5 nm, depending on the cleavage conditions.     

Effect of pressure on Raman spectra of SnS2 single crystals

The 2H polytype of a SnS₂ layered crystal has been studied using Raman spectroscopy at pressures of up to 5 GPa in a diamond anvil cell. The Raman frequency of the intralayer mode increases linearly with increasing pressure at baric coefficients of 5.2 cm⁻¹/GPa for P<3 GPa and 3.4 cm⁻¹/GPa for P>3 GPa. This change in the baric coefficient for Raman scattering and the available data on X-ray measurements of the compressibility of 2H-SnS₂up to 10 GPa suggest that the crystal structure undergoes a transformation at about 3 GPa.     

Ferroelectric phase transition in Ga2Te3 single crystals

Measurements of the electrical conductivity and Hall effect were carried out in a wide temperature range (200-500 K) for Ga₂Te₃ crystals. The crystals were grown in single crystalline form by making a modification of the travelling heater method technique. The measurements revealed unusual observations in the electric conductivity and Hall mobility indicating the presence of some type of phase transitions at about 430 K. So, ferroelectric behavior was examined for confirming the presence of second-order (ferroelectric) phase transition. An energy gap of 1.21 eV and depth of the impurity center of 0.11 eV were found.     

UV-induced two-photon absorption in BiB3O6 single crystals

We show that BiB₃O₆ (BiBO) crystals, well known for their excellent second harmonic generation (SHG) properties, may also be of interest for third-order optical phenomena, particularly for two-photon absorption (TPA). Photoinduced TPA measurements were performed under illumination of excimer Xe–F laser (λ = 217 nm) as a photoinducing (pumping) beam. It created a thin surface layer (about 85 nm) that was a source of the observed photoinduced TPA. Raman shifted Nd-YAG laser radiation (λ = 1.9 μm) as well as its second and fourth harmonics (λ = 950 and λ = 475 nm, respectively) were used as fundamental (probing) beams of the TPA. The highest values of the TPA β coefficient were achieved for a polarization of the pumping light directed along crystallographic axis b. Quantum chemical simulations indicate on substantial contribution of UV-induced electron–phonon anharmonicity to the observed TPA. The obtained values of TPA coefficients indicate a possibility of using BiBO crystals as UV-operated optical limiters in a wide spectral range.     

Luminescence of undoped β-Ga2O3 single crystals excited by picosecond X-ray and sub-picosecond UV pulses

Undoped β-Ga₂O₃ single crystals were grown using the floating zone technique under a pressure of 2 atm oxygen. Luminescence spectra of the crystals were measured with steady-state X-ray (<15 keV) and UV (258 nm, 4.8 eV) sources. The X-ray excitation produced a spectrum with a peak at 390 nm (3.2 eV) whereas the UV excited spectrum had a peak at 430 nm (2.9 eV). The luminescence rise and decay were also examined by using picosecond X-ray and sub-picosecond UV pulses. It was found that the X-ray pulse excitation gave a slower rise and a faster decay of the luminescence compared with the UV pulse excitation. These results suggest that X-ray excitation generates high energy electrons, building up luminescent states until those electrons lose their kinetic energies, giving rise to the formation of local hot spots in the gallium oxide crystals.     

Cs nuclear magnetic resonance study in CsZnCl3 single crystals of perovskite ABX3 type

CsZnCl₃ single crystals were grown by the slow evaporation method, and the spin-lattice relaxation rates and resonance lines of the ¹³³Cs nuclei in the resulting crystals were investigated using FT NMR spectrometry. The temperature dependence of the relaxation rate of the ¹³³Cs nuclei in the CsZnCl₃ crystals was found to be continuous near T_C (=366 K), and was not affected by this phase transition. Our results for CsZnCl₃ are compared with those obtained previously for other CsBCl₃ (B=Mn, Cu, and Cd) perovskite crystals. The Cs relaxation time of CsCdCl₃ is longer than that of CsMnCl₃. The differences between the atomic weights of Mn, Cu, Zn, and Cd are responsible for the differences between the spin-lattice relaxation times of these single crystals. The influence of paramagnetic ions is also important in these crystals. The differences between the spin-lattice relaxation times of these crystals could also be due to differences between the electron structures of their metal ions, in particular the structures of the d electrons.     

La施主掺杂SrTiO_3单晶的阻变性能研究

以La施主掺杂SrTiO_3(La STO)单晶为样品,制备了Pt/LaSTO/In结构存储器件.通过一系列电学测试,发现该器件具有稳定的多级阻变现象,最大开关比为10~4;高低阻电流-电压关系曲线的拟合分析表明,高阻时存在界面势垒,而低阻时满足电子隧穿模型特性.电子顺磁共振研究表明LaSTO单晶内存在带正电的空穴缺陷中心.综合分析证明器件的高低阻之间的转变由界面空位缺陷导致的电子俘获与去俘获引起.此外发现光照会对LaSTO单晶的阻值产生影响.该实验结果为LaSTO单晶在阻变存储器件中的应用提供了理论和技术指导.     

Change of the optical constants of Bi2Te3 single crystals

New structures in the reflectivity spectrum of Bi₂Te₃ are found and assigned to proper critical transitions.