Magnetic structure and anisotropy of YFe 6 Ga 6 and HoFe 6 Ga 6

The magnetic structure of RFe₆Ga₆ intermetallic compounds with R = Y, Ho have been determined by neutron powder diffraction, ⁵⁷Fe M?ssbauer spectroscopy, AC susceptibility, TGA (Thermo-Gravimetric Analysis) and magnetization measurements. Both compounds crystallize in the tetragonal ThMn₁₂ structure (space group I4/mmm) with the magnetic structure of YFe₆Ga₆ consisting of a simple ferromagnetic alignment of Fe moments in the basal plane with a Curie temperature of 475(5) K. Gallium atoms are found to fully occupy the 8i site, with Fe and Ga atoms equally distributed over the 8j site, whilst Fe atoms fully occupy the 8f site. The average Fe moments are 1.68(10) and 1.46(10) at 15 and 293 K, respectively. The average room temperature Fe magnetic moments determined by neutron diffraction are in overall agreement with the average Fe moment deduced from M?ssbauer spectroscopy and bulk magnetization measurements on this compound. The magnetic anisotropy of the compound HoFe₆Ga₆ is also planar in the temperature range 6-290 K, with Ho magnetic moments of 9.28(20) and 2.50(20) at 6 K and 290 K, respectively, coupled anti-ferromagnetically to the Fe sublattice and a Curie temperature of 460(10) K. The magneto-crystalline anisotropies of both compounds are comparable at low temperatures. Received 8 March 2001 and Received in final form 18 June 2001     

Hyperfine coupling constants of muonium-substituted cyclohexadienyl radicals in the gas phase: C6H6Mu, C6D6Mu, C6F6Mu

Muon spin rotation (μSR) and avoided level crossing resonance (ALCR) have been used to determine the hyperfine coupling constants (hfcs) of the muonium-substituted cyclohexadienyl radicals C₆H₆Mu, C₆D₆Mu and C₆F₆Mu in the gas phase, at pressures ～1 and 15 atm and temperatures in the range 40–80°C. Equivalent studies of polyatomic free radicals in gases, by electron spin resonance (ESR) spectroscopy, are generally not possible in this pressure range. The present gas phase results support the findings of earlier studies of cyclohexadienyl radicals in the condensed phase, by both μSR and ESR. Minor but not insignificant (～1%) effects on the hfcs are observed, which can be qualitatively understood for such nonpolar media in terms of their differing polarizabilities. This is the first time that comparisons of this nature have been possible between different phases at the same temperatures. These μSR/ALCR gas-phase results provide a valuable benchmark for computational studies on radicals, free from possible effects of solvent or matrix environments.     

Crystal fields in PrB6 and NdB6

The electron-phonon relaxation times of the crystal-field levels of the metastable state⁵D₄ of Tb³⁺ were measured between 2 and 4.2K. The relaxation of the lowest pseudodoublet (E=1.25 cm^–1) shows an Orbach process via higher crystal-field levels. The other levels show direct relaxation on a nanosecond time scale. The higher crystal-field levels were used to look for propagation of high-energy (4THz) nonequilibrium phonons by phonon-induced fluorescence. No ballistic propagation of high-energy phonons could be found.     

Intercalant-driven superconductivity in YbC6 and CaC6

Recently discovered superconductivity in YbC6 and CaC6, at temperatures substantially higher than previously known for intercalated graphites, raises several new questions. (1) Is the mechanism considerably different from that of previously known intercalated graphites? (2) If superconductivity is conventional, what are the relevant phonons? (3) Given the extreme similarity between YbC6 and CaC6, why are their critical temperatures so different? We address these questions on the basis of first-principles calculations and conclude that coupling with intercalant phonons is likely to be the main force for superconductivity in YbC6 and CaC6, but not in alkaline-intercalated compounds, and explain the difference in T(c) by the "isotope effect" due to the difference in Yb and Ca atomic masses.     

CARS spectroscopy of SF6 and UF6

CARS spectra of thev ₁-band of SF₆ and UF₆ performed under (static) cell conditions and under supersonic jet expansion have been recorded. The cooling behavior in supersonic jet expansions has been demonstrated for both gases.Work supported by BMFT-funds     

Magnetic-field-induced spin-reorientational transition in TbMn6Sn6

Magnetic state of the intermetallic TbMn₆Sn₆ compound was studied by neutron diffraction analysis in external magnetic fields of H=0, 4, and 5 kOe. Magnetic moments and their orientations were determined for the Tb and Mn sublattices. Magnetic fields higher than 4 kOe were shown to induce spin-reorientational transition at room temperature from the c axis to the basal plane.     

6-S-Alkyl-6-thiohexonolactones and 6-S-Alkyl-6-thiohexitols: Efficient synthesis and new smectic liquid crystal phases

The mesophasic properties of S-alkylthiohexonolactones (d-galactono- and d-mannono-) and the corresponding itol derivatives with general formula Su-SR (R = C₁₁H₂₃) have been investigated. The observation that the synthesized compounds of this series has been studied using crossed polarized optical microscopy. This study involves the discovery of new chiral Smectic A liquid crystal phase through direct comparison with the defective texture of known Smectic A phase of available commercially biphenyls 8CB and 10CB.     

Band topologies of hexaborides CaB_6 and EuB_6

<正>In last decades,topological materials[1-3]have attracted lots of research interest.Topological Kondo insulator(TKI)[4],as an exotic quantum state,has been proposed theoretically in two hexaborides Sm B6and Yb B6,in which there is a d/f(p)band inversion at momentum point X[5].There are lots of experimental work supporting the TKI state and metallic surface state in typical Kondo insulator Sm B6[6-10].However,more and more experimental results demonstrated that Yb B6is trivial insulator at ambient pressure[11].Under high pressure,the band gap of Yb B6will close and the d/p band inversion can happen under pressure about 15 GPa[12].     

Adatom-vacancy mechanisms for the C6)/Al111-(6 x 6) reconstruction

The irreversible (6x6) reconstruction of the C(60)/Al(111) system from the (2sqrt[3]x2sqrt[3])R30 degrees phase is studied by first-principles techniques. We find that C60 binds optimally to the surface if an Al vacancy is created directly underneath. The removed Al atoms form a (6x6) array of ad-dimers in the interstices below the C60 overlayer, to which they strongly bind. This spontaneous local process, rather than the compression state of the unreconstructed C60 overlayer, explains why one molecule out of three protrudes from the surface upon reconstruction.     

Experimentally determined transition probabilities of the systems 6p2→6p6d, 6p7s, 6p8s of neutral lead

Branching ratio measurements have been performed for all of the observed transitions from eight levels in neutral lead (6p²→6p6d, 6p²→ 6p7s, 6p²→6p8s) excited in a hollow cathode discharge in the 200–730 nm spectral range. From the relative emission intensity data, a set of absolute transition probabilities is derived for 28 lines, including lines for which transition probabilities have not been previously published. The absolute scales are obtained by averaging the measured lifetimes of the ³P⁰_1,2, ¹P_1⁰(6p7s), ³F^0_2,3, ³D^0_1,2(6p6d) levels.     

Laser-catalyzed production of ultracold molecules: the 6Li+6Li7Li-->homega6Li-6Li+7Li reaction

We show that by using laser catalysis, we can employ translationally cold (Tr approximately 1.75 K) collisions to produce ultracold (0.01 mKhomega(6Li6Li7Li)*(1(4)A')-->homega6Li6Li+7Li reaction in the collinear approximation. Ultracold 6Li6Li product molecules are shown to be produced at an extraordinary yield of up to 99.97%, using moderate laser intensities of I=100 kW/cm(2)-10 MW/cm2.     

Volume electrical discharge in Ne/SF6/C6H14 and pure SF6

The electrical and spatial characteristics of a volume electrical discharge are studied as functions of the ratio of partial pressures of the mixture components at varying initial voltage. Early in the discharge, hot (plasma) cathode spots are observed with diffusive plasma flares developing above them. Overlapping of the flares produces a volume plasma column. The column homogeneity depends on the diffusive-flare density, and the length of the volume-discharge stage is determined by the time it takes for one or more flares to transform and give rise to the channel stage. Estimates of the ionization, attachment, and ion recombination rates are used to show that the electron density in plasma channels is two orders of magnitude lower than that of positive and negative ions forming a stagnant stiff skeleton of the plasma. Institute of High Current Electronics of the Siberian Branch of the Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 43–49, August, 1999.     

SF_6气体及其衍生物的红外光谱分析

SF6气体大量应用于气体绝缘组合电器(GIS)中。通过化学方法检测SF6气体及其衍生物是检测GIS设备故障的一种重要方法。该文采用红外光谱技术分析了三种情况下的GIS设备内SF6气体组成情况,得出S2F10气体含量可以用来分析GIS故障原因是否为强火花或者电弧放电引起;CF4气体可以用来表征GIS设备内绝缘气体的绝缘状态。另外,研究表明,GIS设备在线运行时,绝缘气体气室密封失效、造成气体泄漏,引入新的气体杂质的问题比较严重。所以在进行GIS设备维护工作时,绝缘气室的密封维护工作十分重要。文章在研究GIS设备绝缘气体SF6红外谱图的基础之上提出了建立基于红外光谱技术的GIS设备故障诊断专家系统的意见。