Relativistic calculation of atomic X-ray transition rates

The fully relativistic transition probabilities in atomic X-ray emission are calculated for a wide range of selected elements of the periodic table, fromZ=21 toZ=93. The treatment includes the effect of the retardation of the radiation field. The atomic model used is the self-consistent relativistic Hartree-Fock, with the Slater approximation for the exchange term. The formulas for EL and ML multipole transition rates are derived, and transition rates are calculated for all transitions to theK-shell,L-subshells, andM-subshells. The contributions of all multipoles, as determined by the angular momentum and parity selection rules, are included in the present work. The calculated values of (Kα ₂/Kα ₁), (Kβ ₁/Kα ₁), (Kβ ₃/Kβ ₁) and (Kβ ₂ Kβ ₁) transition rates are in good agreement with the available experimental data. Rates for the forbidden (M1) 2s _1/2→1s _1/2 transitions are also presented.     

K0.3MoO3和Tl0.3MoO3派尔斯相变临界行为的比热研究

采用连续绝热加热法测量了K_0.3MoO₃和Tl_0.3MoO₃在100—220K温度范围内的比热。发现K_0.3MoO₃和Tl_0.3MoO₃分别在177.5K和172.3K发生派尔斯相变。对派尔斯相变附近比热临界行为的研究表明,K_0.3MoO₃和Tl_0.3MoO₃的临界区域宽度分别为6K和8K,临界行为属于三维XY模型普适类。 关键词：     

Superhard superconducting materials based on diamond and cubic boron nitride

Superhard superconducting samples with a critical temperature of T_c = 12.6 K are obtained when synthetic diamond powders that were preliminarily coated with a niobium film are sintered at a pressure of 7.7 GPa and a temperature of 1973 K. Superhard superconductors with T_c = 9.3 K are obtained when diamond and molybdenum powders are sintered at a pressure of 7.7 GPa and a temperature of 2173 K. Superconducting samples with T_c = 36.1–37.5 K have been obtained in the systems diamond-MgB₂ and cubic boron nitride-MgB₂.     

PMR studies of molecular motions,phase transitions and quantum tunnelling in NH4SnCl3 and N(CH3)4SnCl3

Molecular reorientation and low temperature relaxation effects of NH⁺ ₄ ion and the effect of CH₃ substitution (in place of H) are investigated by proton spin lattice relaxation time (T₁) measurements at 10 MHz in NH₄SnCl₃ and N(CH₃)₄SnCl₃ in the temperature range 4.2 K upto the melting points of the compounds (? 440 K). Phase transitions around 360 K in NH₄SnCl₃ and around 361 and 116K in N(CH₃)₄SnCl₃ have been observed. In NH₄SnCl₃, the high temperature minimum at 330.5 K is attributed to the translational diffusion of the NH⁺ ₄ ions, while the other T₁, minima at 103.5, 60 and 50 K are ascribed to the reorientations of the NH⁺ ₄ ion about the C₂ and C₃ axes. The low temperature minimum at 13.5 K is attributed to rotational tunnelling of the NH⁺ ₄ ions. In N(CH₃)₄SnCl₃, in addition to the high temperature minima at 212.2 and 182.6 K due to N(CH₃)₄ tumbling and CH₃ reorientation, a temperature independent T₁ behaviour between 83 and 31 K is observed, below which T₁ decreases and tends to go through a minimum around 5 K. This low temperature minimum is attributed to rotational tunnelling of the CH₃ groups. The motional parameters and tunnel frequencies are estimated.     

Low temperature elastic constants and Debye temperature of NbO2

The transit times of ultrasonic waves have been measured in single crystal NbO₂ from 295 K down to 1.5 K for quasilongitudinal and shear waves propagating in the [100] direction and down to 160 K for eight other waves. Values are obtained for the C₄₄ elastic constant and for an elastic constant combination which is approximately equal to C₁₁ for temperatures down to 1.5 K and for C₁₁, C₁₂, C₁₃, C₁₆, C₃₃, and C₆₆ down to 160 K. These results are used to deduce 0 K values for the elastic constants and an elastic Debye temperature of 596 ± 7 K at 1.5 K. The acoustic mode heat capacity calculated from the latter is significantly smaller than the heat capacity measured by Wenger and Keesom at low temperatures. Following Wenger and Keesom, the difference is attributed to phasons (excitations involving the phase modulation of charge density waves). An average velocity is deduced for the phasons.     

Raman measurements of the superionic conductor KAg4I5

Raman measurements of the superionic conductor KAg₄I₅ are reported between 77K and 296K and compared to similar data of RbAg₄I₅. The mode frequencies for the two materials are very similar and a mode at 22.7 cm^?1 abruptly and reversibly disappears at the transition temperature, T_c=137 K, as is observed in RbAg₄I₅, at 121 K.     

Mössbauer study of (NH4)2FeCl5·H2O

Mössbauer spectra of the compound (NH₄)₂FeCl₅·H₂O have been studied as a function of temperature. Two phase transitions are observed in the temperature range between 7 K and 9 K. The transition at 9 K is structural and presents an unusually high thermal hysteresis. AroundT=8 K the substance orders magnetically and different Fe³⁺ contributions are present.     

Simulation of the magnetic properties of manganese oxide Pb<Subscript>3</Subscript>Mn<Subscript>7</Subscript>O<Subscript>15</Subscript>

The magnetic susceptibility, heat capacity, and spin-spin correlation functions of manganese oxide Pb₃Mn₇O₁₅ are calculated by the Monte Carlo method. Two critical temperatures are determined: T ₁ ≈ 20 K, above which a modulated structure along the hexagonal axis is formed, and T ₂ ≈ 70 K, at which the long-range magnetic order disappears. The antiferromagnetic exchange interaction constant in a hexagonal plane is estimated to be J ₁ ～ 7 K, and the antiferromagnetic and ferromagnetic exchange interaction constants between hexagonal planes are calculated to be J ₂ ～ 3 K and K ～ 50 K, respectively.     

Exchange constants for a V<Subscript>15</Subscript> magnetic molecular nanocluster

The spin structure and ground-state energies of a V₁₅ magnetic molecular nanocluster are calculated using a modified Lanczos method. The exchange interaction constants for the V₁₅ magnetic nanocluster (J=290 K, J′=60 K, J₁=30 K, J′′=200 K, and J₂=68 K) are determined from the comparison of the calculated and experimental data. The exchange constants obtained differ significantly from those predicated earlier and permit one to describe quantitatively the magnetization of the V₁₅ nanocluster in weak and strong magnetic fields.     

Superconductivity in CeCu2Si2

Resistivity measurements of CeCu₂Si₂ are carried out under pressures p up to 12 kbars. Unlike polycrystalline samples, no traces of superconductivity have been observed in CeCu₂Si₂ at ambient pressure. When pressure is applied, CeCu₂Si₂ monocrystals become superconducting with anomalously large ratio H_c2(0)/T_c (0) = 34 K0e/K and with the derivative dH_c2/dT(T=T_c) = 140 K0e/K     

Polymorphism of anhydrous ferrous sulfate

The Mössbauer spectra of α-FeSO₄ and β-FeSO₄ have been measured in the temperature range 4.2 K ? T ? 30 K. The high temperature (β) phase is accompanied by a weak fraction of a third component. In analogy to CoSO₄, the third component of the three-fold polymorphism in anhydrous ferrous-sulfate is named the γ phase. X-ray powder measurements confirm this phase to be isomorphous to the corresponding γ-CoSO₄ structure. The parameters of the hyperfine interaction at 4.2 K of the three phases are evaluated. The magnetic ordering temperatures are T_N (α) = 24.0 K, T_N(β) = 17.0 ± 1.5 K and T_N(γ) = 19.5 ± 1.0 K.     

Upper critical fields of NbxGa1-x: A binary high temperature superconductor

The upper critical fields, H_C2, of several Nb_xGa_1-x alloys with values of T_c from 13.3 K to 20.2 K have been measured at temperatures from 4.2 to 20.4 K with dc and pulsed magnet fields. For the highest T_c material, H_C2(4.2 K) = 340 kG. The data are consistent with almost complete suppression of Pauli paramagnetic limiting. Comparisons with other high T_c materials, Nb₃Al and NbAlGe are presented.     

Temperature independent leakage current in Au/TbMnO3/Y Ba2Cu3O7−x capacitors

Au/TbMnO₃/Y Ba₂Cu₃O_7?x capacitors were fabricated on LaAlO₃ substrates and electrical characteristics were studied in the temperature range from 300 to 25 K. When temperature is higher than 225 K, Ohmic conduction is observed in the capacitors. Non-linear current–voltage characteristics are exhibited at low temperatures. Moreover, temperature-independent leakage currents are found at temperatures lower than 150 K. These electrical characteristics can be explained in terms of conduction transition originated from a thermal activation between 225 and 150 K, and the temperature-independent leakage currents are governed by the Poole–Frenkel emission and minor trap ionization energies.     

Hall effect of a linear-chain metal: NbSe3

We present measurements of the low-field Hall constant R_H as a function of field and temperature in the linear-chain metal NbSe₃, The data range from 2 K to 200 K in temperature and up to 15 kG in field. At the two phase transitions T₁ (142 K) and T₂ (58 K) R_H shows an abrupt increase in magnitude but no change in sign. Below 30 K R_H becomes strongly field dependent. The zero-field-limit R_H changes from n-type to p-type at 15 K. It is argued that the results are consistent with a two-band model in which the difference in population p - n is equal to 3 × 10¹⁸ cm^-3 below T₂. The hall data are also consistent with a model in which both a loss of carriers and a drastic change in the conductivity anisotropy occur at T₁ and T₂ and lend support to the charge-density-wave hypothesis which has been used to characterize the two transitions.     

Ultrasonic velocity measurements near the 250°K phase transition in BaMnF4

Ultrasonic velocity measurements for longitudinal and shear modes propagating parallel to the crystallographic axes of orthorhombic BaMnF₄ are presented for temperatures between 200°K and 290°K. Sharp anomalies at ～ 250°K are observed for the modes V_bc = V_cb and V_cc.     

基于成分连续变化计算黏度的合金系临界冷速模型

在大块非晶临界冷却速率的非等温转变计算模型基础上提出了基于成分连续变化计算黏度的合金系临界冷速模型. 依据此模型对Zr-Ni-Al-Cu四元合金的临界冷却速率进行了计算并预测了Zr_66.67(Ni_{xAlyCuz)33.33合金系中容易形成非晶的成分范围. 计算值与实验值符合得较好. 计算结果表明，此合金系具有很强的非晶形成能力，特别是在靠近共晶点的中心区域，临界冷却速率小于100 K/s，为容易形成非晶的成分范围. 冷却过程中，在高于1000K温度区间，没有发生明显的结晶现象，而在980 K至870 K温度范围内，结晶分数快速增大，低于870 K时不再发生明显改变. 此外，分析了合金系中Al，Cu，Ni原子摩尔分数的变化对临界冷速的影响. 关键词： 大块非晶 黏度 临界冷却速率 非晶形成能力}     

Magnetic susceptibility and electrical resistivity of electrotransport purified scandium single crystals from ≈ 1 to 300 k

The magnetic susceptibility of electrotransport purified Sc single crystals was measured from 0.7 to 295 K. Both the easy (χ_a) and hard (χ_c) directions were measured using a microcomputer controlled Faraday apparatus. Above 50 K the present results agree with earlier work on a less pure sample, but below 50 K the present results are lower by as much as 4%. In addition the maximum at ≈ 30 K and the minimum at ≈ 6 K are much more pronounced in the purer material, and the susceptibility increases rapidly as the temperature approaches 0 K. Electrical resistivity measurements were made on the same single crystals from 1.6 to 300 K. The resistance ratios were 365 for the a-axis crystal and 274 for the c-axis crystal. At 300 K, ρ_a ≈ 2.5 ρ_c.     

上海光源低温波荡器永磁铁在低温下的磁特性研究

用综合物性测量系统测试了国产Nd₂Fe₁₄B (N50M)永磁铁在低温下(10–300 K)的M-H和M-T. 获得了N50M剩磁B_r和内禀矫顽力H_ci在低温下的变化图, 对其取向度和三维磁化强度进行了分析研究. 结果表明, N50M在80–150 K发生强烈自旋再取向效应, B_r在120–130 K出现峰值, H_ci随温度下降呈线性增加. 在130 K, B_r和H_ci分别比常温(300 K)增加15.6%和220%, 达到1.65 T和3638 kA/m. 在150–300 K, 随温度下降, N50M宏观取向度与外磁场均匀性逐步改善, 但在80–235 K, 微观外磁场均匀性恶化.实验研究发现, 235 K附近, N50M垂直取向方向呈现“剩余磁化强度跳跃”. 研究结果为上海光源Ⅱ期低温波荡器及其他高精度低温永磁仪器与设备的物理设计提供了参考. 关键词： 低温波荡器 2Fe₁₄B永磁铁')" href="#">Nd₂Fe₁₄B永磁铁 低温 磁特性     

H-D exchange on graphite–potassium intercalation compounds

Potassium graphite intercalation compounds are able to activate C–H bonds of hydrocarbons at room temperature. In this paper, the hydrogen–deuterium exchange of CHD₃ in the presence of C₈K, C₂₄K and C₃₆K is described.     

Thermodynamic properties of (NH4)2KGaF6 elpasolite

The heat capacity of (NH₄)₂KGaF₆ elpasolite is measured in the temperature range from 80 to 350 K. A sequence of three phase transitions at T ₁=288.5 K, T ₂=250 K, and T ₃=244.5 K is revealed, and the thermodynamic characteristics of these transitions are determined. The influence of hydrostatic pressure on the phase transition temperature is investigated. The results obtained are discussed within the model of orientational ordering of NH ₄ ⁺ and GaF ₆ ^3? ionic groups.