Blue luminescence in Tm-doped KGd(WO4)2 single crystals

Up-conversion blue emissions of trivalent thulium ions in monoclinic KGd(WO₄)₂ single crystals at 454 and 479 nm are reported for a single pump laser source at 688 nm. We grew thulium-doped KGd(WO₄)₂ single crystals at several concentrations from 0.1% to 10%. We recorded a polarized optical absorption spectrum for the ³F₂+³F₃ energy levels of thulium at room temperature and low temperature (6 K). From the low temperature emission spectra we determined the splitting of the ³H₆ ground state. The blue emissions are characterized as a function of the dopant concentration and temperature from 10 K to room temperature. To our knowledge, this is the first time that sequential two-photon excitation process (STEP) generated blue emissions in thulium-doped single crystals with a single excitation wavelength.     

A nuclear magnetic resonance study of the structural properties and molecular motions of Li2KH(SO4)2 and LiKSO4 single crystals

The structural properties and relaxation mechanisms of Li₂KH(SO₄)₂ crystals were determined using the temperature dependences of NMR spectra and the spin-lattice relaxation times (T₁) of their ¹H, ⁷Li, and ³⁹K nuclei. The results obtained were compared with the previously reported physical properties of LiKSO₄ crystals. The substitution of the potassium ions with protons in the LiKSO₄ crystals were variations in the phase transition temperatures, and the non-appearance of ferroelastic properties. The ⁷Li T₁ for the Li₂KH(SO₄)₂ crystals was much shorter than the ⁷Li T₁ for the LiKSO₄ crystals, and these findings indicate that the presence of the protons in Li₂KH(SO₄)₂ causes the Li ions to move with greater freedom.     

MgF2单晶的THz光谱研究

利用THz时域光谱技术对MgF₂晶体(样品1)和MgF₂：Co晶体(样品2)在0.5—2.5 THz的吸收特性进行了研究.在0.5—2.5 THz波段，样品1吸收系数α(ν)随频率ν增加而增大，最大值为24 cm^-1.样品2的吸收系数比样品1大得多，Co掺杂使晶格吸收带边向低频移动，而且样品2在1.9 THz有吸收峰，吸收系数达到70 cm^-1，由此求出F^--Co²⁺离子键伸缩振动的键力常数K为3.40×10^-2　N/cm.这一结果表明，THz光谱分析有可能成为研究晶体化学键的一种重要手段.利用光学常数之间的关系计算了两个样品在0.5—2.5 THz的介电函数的实部ε₁(ν)，得到样品1的ε₁(ν)值在4.67至4.73之间，样品2的ε₁(ν)值在4.62至5.01之间. 关键词： THz辐射 光谱 2晶体')" href="#">MgF₂晶体     

A cross-sectional scanning tunneling microscopy study of IrO2 rutile single crystals

Rutile iridium dioxide (IrO₂) surfaces were studied by cross-sectional scanning tunneling microscopy (XSTM). Atomically flat surfaces prepared by in situ ultra high vacuum cleaving of crystalline platelets of thicknesses <50 μm were successfully demonstrated. In addition to (1 1 0) surface, several vicinal planes, e.g., (1 2 0), (1 3 0) and , were also examined. The cleaved planes are close to bulk-terminated surfaces with predominant [0 0 1]-oriented bridge oxygen rows. Unlike TiO₂, bright oxygen rows are imaged and oxygen defects appear as dim species. Our studies show that XSTM is a viable technique to study oxide surfaces that are otherwise difficult to prepare.     

Cu2ZnSnSe4 thin films prepared by selenization of co-electroplated Cu-Zn-Sn precursors

A novel technique for growth of high quality Cu₂ZnSnSe₄ (CZTSe) thin films is reported in our work. The CZTSe thin films were fabricated onto Mo layers by co-electroplating Cu-Zn-Sn precursors followed by annealing in the selenium vapors at the substrate temperature of 550 °C. The morphology and structure of CZTSe thin films were characterized using scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), X-ray diffraction (XRD) and Raman scattering spectrum, respectively. The results revealed that the single phase was in the CZTSe thin films, and the other impurities such as ZnSe and Cu₂SnSe₃ were not existed though they were difficult to distinguish both from EDS and XRD.     

XPS investigation of diffusion of two-layer ZrO2/SiO2 and SiO2/ZrO2 sol–gel films

Two-layer ZrO₂/SiO₂ and SiO₂/ZrO₂ films were deposited on K9 glass substrates by sol–gel dip coating method. X-ray photoelectron spectroscopy (XPS) technique was used to investigate the diffusion of ZrO₂/SiO₂ and SiO₂/ZrO₂ films. To explain the difference of diffusion between ZrO₂/SiO₂ and SiO₂/ZrO₂ films, porous ratio and surface morphology of monolayer SiO₂ and ZrO₂ films were analyzed by using ellipsometry and atomic force microscopy (AFM). We found that for the ZrO₂/SiO₂ films there was a diffusion layer with a certain thickness and the atomic concentrations of Si and Zr changed rapidly; for the SiO₂/ZrO₂ films, the atomic concentrations of Si and Zr changed relatively slowly, and the ZrO₂ layer had diffused through the entire SiO₂ layer. The difference of diffusion between ZrO₂/SiO₂ and SiO₂/ZrO₂ films was influenced by the microstructure of SiO₂ and ZrO₂.     

Study of the switching phenomena of TlGaS2 single crystals

Single crystals of TlGaS₂ were prepared by a special modified Bridgman technique and used to investigate the switching phenomena. The particular interest shown in switching studies of p-type TlGaS₂ compound is associated with the possibility of its uses as an effective switching and memory elements in electronic devices. The switching effect observed in such crystal shows a memory character. Using a crystal holder and cryostat we measured the switching phenomenon at different ambient conditions such as temperature, light illumination as well as sample thickness. Pronounced parameters for switching for sample of thickness 0.17 cm were determined from the experimental data such as threshold voltage V_th = 400 V, threshold current I_th = 37 μA, holding voltage V_h = 350 V, holding current I_h = 42.3 × 10⁻⁴ A, threshold power P_th = 1.48 × 10⁻² W, threshold field E_th = 196.429 V/cm as well as the ratio between the resistance in the off state R_OFF to the resistance in the conducting state R_ON as 130.253. The factors affecting these parameters have also been investigated.     

强冲击压缩下LiF，Al2O3和LiTaO3单晶的透光性

在二级轻气炮上，用高速电子相机扫描照相技术和改进的Mallory实验装置，对z切LiF，Al₂O₃(蓝宝石)和LiTaO₃单晶材料的冲击透光性进行了对比测量，并用黑密度计提取出动态图像定量化的光强对比度变化曲线.结果表明，LiF单晶在102 GPa压强下能够保持长时间的初始透光性不变，与公认的LiF具有优良的高压下透明性的认识一致.LiTaO₃单晶在实验压力(139GPa)下变成基本不透明.而Al_{关键词： 2}O₃')" href="#">Al₂O₃ LiF 3')" href="#">LiTaO₃ 光学透明性     

Magnetization anomaly and anisotropy of Bi2Sr2CaCu2−xNixO8 single crystals

We have observed the anomalous magnetization of Bi₂Sr₂CaCu_2−xNi_xO₈ (x = 0 and 0.02) single crystals. Anisotropy decreases with iodine intercalation although it expands the space between CuO₂ layers. Iodine intercalation seems to suppress the magnetization anomaly for Ni = 1% crystals, but not for Ni = 1% substituted crystals. We have discussed these results in terms of the increase of anisotropy by Ni substitution and the dimensional crossover of flux lines. Effects of both oxygen concentration and substitution of a magnetic element for the Cu site on the anisotropy of Bi₂Sr₂CaCu₂O₈ crystals show the same tendency as the case of the YBa₂Cu₃O₇ superconductor.     

Effects of divalent impurities on the fluctuation conductivity of YBa2Cu3O7 single crystals

We have studied experimentally the in-plane fluctuation conductivity near the superconducting transition in single crystal samples of YBa₂Cu₃O₇, Y_0.98Ca_0.02Ba₂Cu₃O₇, YBa_1.9Sr_0.1Cu₃O₇ and YBa₂Cu_2.97Zn_0.03O₇. In order to test the stability of the observed fluctuation regimes, low magnetic fields were applied perpendicular to the Cu-O₂ atomic planes. When the transition is approached from above we first observe a three-dimensional (3D) Gaussian regime then a crossover to a genuine critical region where the exponent is consistent with the predictions of the 3D-XY-E universality class. Decreasing further the temperature towards T_c, our results systematically reveal the occurrence of a regime beyond 3D-XY characterized by a very small critical exponent. We propose that this regime is precursory to a weak first-order superconducting transition driven by antiferromagnetic excitations related to the pseudogap phenomenon. The dilution of divalent impurities in YBa₂Cu₃O₇ does not affect the stability of the fluctuation regime beyond 3D-XY and in the case of Ca doping a further approach towards the first-order behaviour is observed.     

Synthesis and photocatalytic oxidation of different organic dyes by using Mn2O3/TiO2 solid solution and visible light

Mn₂O₃/TiO₂ solid solution was prepared from two different oxides, manganese oxide (from KMnO₄ and ethanol) and TiO₂, these samples were characterized by BET, XRD, EDAX, SEM, FT-IR, ESR, XPS and UV–vis absorption spectroscopy. Photocatalytic activities of Mn₂O₃/TiO₂ powder was investigated by photooxidation of different dyes like Rhodamine B, thymol blue, methyl orange and Bromocresol green under visible light (300-W Xe lamp; λ > 420 nm). The results show that the alloy of TiO₂ with 1 mol% of Mn₂O₃ (MNT1) exhibit photocatalytic activity 3–5 times higher than that of P25 TiO₂ for oxidation of various dyes (RB, TB, MO and BG). The average particle size and crystallite size of MNT1 were found to be 100 nm and 12 nm measured from SEM and XRD, respectively. The EPR spectra of the Mn₂O₃/TiO₂ samples is a sharp five-line Mn(III) component centered on g_eff = 1.99.     

Pb2，PdPb2分子的势能函数

采用Gaussian 98程序,运用B3LYP方法,对Pd和Pb原子采用收缩价基组LANL2DZ,对Pb₂和PdPb₂分子的微观结构进行了理论计算. 由于Pb₂分子离解后一个Pb原子处于基态,另一个Pb原子处于激发态,采用最小二乘法拟合Pb₂分子的势能函数,选用的函数形式为Murrell-Sorbie势能函数加上开关函数. 使用多体展式理论导出了势函数中的参数进而给出PdPb₂分子基态势函数的解析表达式,其势能面准确地复现了PdPb₂分子的两个稳定构型(C_2V和C_∞v)及其能量关系. 关键词： 2')" href="#">Pb₂ 2')" href="#">PdPb₂ 势能函数     

On the ferromagnetism of AlFe2B2

Based on extensive Mossbauer effect (ME) and magnetization measurements, the orthorhombic AlFe₂B₂ was characterized as a ferromagnet (FM) because this character is evident as an onset of a FM transition at T_c=320 K and characteristic magnetizations isotherms below T_c. At liquid helium temperatures, the magnetization saturates to μ_sat≈1μ_B per Fe atom; a value which is half the one reported for the iron metal indicating a relatively more filled 3d band. The ME analysis revealed a hyperfine field H(0) of 88(2) kOe, an isomer shift (relative to Fe) of 0.50(2) mm/s, and a quadrupole parameter of 0.02 mm/s: all parameters extrapolated to zero Kelvin. The itinerant character of the magnetic moment will be discussed.     

Magnetocrystalline anisotropy in RAu2Ge2 (R = La, Ce and Pr) single crystals

Anisotropic magnetic properties of single crystalline RAu₂Ge₂ (R=La, Ce and Pr) compounds are reported. LaAu₂Ge₂ exhibits a Pauli-paramagnetic behaviour whereas CeAu₂Ge₂ and PrAu₂Ge₂ show an antiferromagnetic ordering with Nèel temperatures T_N = 13.5 and 9 K, respectively. The anisotropic magnetic response of Ce and Pr compounds establishes [0 0 1] as the easy axis of magnetization and a sharp spin-flip type metamagnetic transition is observed in the magnetic isotherms with H // [0 0 1]. The transport and magnetotransport behaviour of these compounds, in particular LaAu₂Ge₂, indicate an anisotropic Fermi surface. The magnetoresistivity of CeAu₂Ge₂ apparently reveals the presence of a residual Kondo interaction. A crystal electric field analysis of the anisotropic susceptibility in conjunction with the experimentally inferred Schottky heat capacity enables us to propose a crystal electric field level scheme for Ce and Pr compounds. For CeAu₂Ge₂ our values are in excellent agreement with the previous reports on neutron diffraction. The heat capacity data in LaAu₂Ge₂ show clearly the existence of Einstein contribution to the heat capacity.     

Hydrostatic pressure study of the structural phase transitions and superconductivity in single crystals of (Ba1−xKx)Fe2As2 (x=0 and 0.45) and CaFe2As2

We studied the effect of hydrostatic pressure (P) on the structural phase transitions and superconductivity in the ternary and pseudo-ternary iron arsenides CaFe₂As₂, BaFe₂As₂, and (Ba_0.55K_0.45)Fe₂As₂, by means of measurements of electrical resistivity (ρ) in the 1.8-300 K temperature (T) range, pressures up to 20 kbar, and magnetic fields up to 9 T. CaFe₂As₂ and BaFe₂As₂ (lightly doped with Sn) display structural phase transitions near 170 and 85 K, respectively, and do not exhibit superconductivity in ambient pressure, while K-doped (Ba_0.55K_0.45)Fe₂As₂ is superconducting for T<30 K. The effect of pressure on BaFe₂As₂ is to shift the onset of the crystallographic transformation down in temperature at the rate of ~−1.04 K/kbar, while shifting the whole ρ(T) curves downward, whereas its effect on superconducting (Ba_0.55K_0.45)Fe₂As₂ is to shift the onset of superconductivity to lower temperatures at the rate of ~−0.21 K/kbar. The effect of pressure on CaFe₂As₂ is first to suppress the crystallographic transformation and induce superconductivity with onset near 12 K very rapidly, i.e., for P<5 kbar. However, higher pressures bring about another phase transformation characterized by reduced-resistivity, and the suppression of superconductivity, confining superconductivity to a narrow pressure dome centered near 5 kbar. Upper critical field (H_c2) data in (Ba_0.55K_0.45)Fe₂As₂ and CaFe₂As₂ are discussed.     

Optically pumped submillimeter laser lines from CH2F2 and CD2Cl2 using a12C18O2 CW laser

Thirty-nine new submillimetre laser lines in CH₂F₂ and twelve in CD₂Cl₂ have been obtained in a Fabry-Perot FIR resonator by optically pumping with a CW¹²C¹⁸O₂ laser. The wavelength range obtained for CH₂F₂ is 126m to 1091m and for CD₂Cl₂ 212m to 774m. The wavelength measurements are accurate to within 5.10^–3. The relative polarisations of the pump laser and the FIR laser output were also determined. Tentative assignments of the IR and FIR transitions were made using existing microwave data.     

Emission features of Ho ion in Nb2O5, Ta2O5 and La2O3 mixed Li2O-ZrO2-SiO2 glasses

Li₂O-ZrO₂-SiO₂: Ho³⁺ glasses mixed with three interesting d-block elemental oxides, viz., Nb₂O₅, Ta₂O₅ and La₂O₃, were prepared. Optical absorption and photoluminescence spectra of these glasses have been recorded at room temperature. The luminescence spectra of Nb₂O₅ and Ta₂O₅ mixed Li₂O-ZrO₂-SiO₂ glasses (free of Ho³⁺ ions) have also exhibited broad emission band in the blue region. This band is attributed to radiative recombination of self-trapped excitons (STEs) localized on substitutionally positioned octahedral Ta⁵⁺ and Nb⁵⁺ ions in the glass network. The Judd-Ofelt theory was successfully applied to characterize Ho³⁺ spectra of all the three glasses. From this theory various radiative properties, like transition probability A, branching ratio β_r and the radiative lifetime τ_r, for ⁵S₂ emission levels in the spectra of these glasses have been evaluated. The radiative lifetime for ⁵S₂ level of Ho³⁺ ions has also been measured and quantum efficiencies were estimated. Among the three glasses studied the La₂O₃ mixed glass exhibited the highest quantum efficiency. The reasons for such higher value have been discussed based on the relationship between the structural modifications taking place around the Ho³⁺ ions.     

CO2 laser assisted removal of UO2 and ThO2 particulates from metal surface

Pulsed laser assisted removal of uranium dioxide and thorium dioxide particulates from stainless steel surface have been studied using a TEA CO₂ laser. Decontamination efficiency is measured as a function of laser fluence and number of pulses. Threshold fluence for the removal of UO₂ particulates has been found to be lower than that required for the removal ThO₂ particulates. Usage of a ZnSe substrate, that is transparent to the laser wavelength used here, enabled us to decouple the cleaning effect arising out of absorption in the particulates from that in the substrate and has contributed towards understanding the mechanism responsible for cleaning. The experimental observations are also corroborated by simple theoretical calculations.     

Al2O3-Y2O3-ZrO2三相复合陶瓷的介电谱研究

采用传统的固相反应法,在1400–1500 ℃下烧结,制备得到Al₂O₃-Y₂O₃-ZrO₂三相复合陶瓷.样品的结构、形貌和电性能分别用X射线衍射(XRD)、扫描电子显微镜(SEM)及介电谱表征.XRD表明此三相复合体系无其他杂相,加入Y₂O₃及ZrO₂后使得Al₂O₃成瓷温度降低;SEM表明此体系晶粒直径为200–500 nm,并且样品随烧结温度的升高而变得更加致密,晶界更加清晰;介电损耗谱中出现峰值弛豫现象,根据Cole-Cole复阻抗谱得出其为非德拜弛豫. 关键词： 2O₃-Y₂O₃-ZrO₂三相陶瓷')" href="#">Al₂O₃-Y₂O₃-ZrO₂三相陶瓷 介电弛豫 阻抗谱 热导率     

Influence of reducing annealing on the luminescent properties of Li2B4O7:Cu single crystals

Li₂B₄O₇:Cu single-crystal annealing in the reducing environment results in a sharp drop of intensity of thermoluminescence (TSL) and X-ray luminescence (XL) peaks. It was suggested that during annealing in the reducing environment, an oxygen vacancy is produced in the LTB structure at the bridging oxygen atom site. Oxygen vacancy charge compensation takes place at the expense of reduction of the doping Cu⁺ ion to the Cu⁰ state. In this case the A⁰-type thermoluminescence center formation mechanism due to irradiation becomes impossible.