Evolution of magnetism in U(Ni 1- xPd x) Si single crystals

Single crystals of U(Ni_1-xPd_x)₂Si₂ with x = 0.05, 0.09 and 0.135 have been grown. Magnetization and electrical resistivity measurements were performed in a wide range of temperatures and magnetic fields in order to study stability of magnetic phases in the solid solutions between UNi₂Si₂ and UPd₂Si₂ with a special emphasis on the type of ground state. In UPd₂Si₂ the simple AFI-type antiferromagnetic structure of U moments is observed at low temperatures. UNi₂Si₂ adopts the uncompensated AF structure (UAF) with the + + - stacking of U moments along the c-axis and consequently this compound exhibits a spontaneous magnetization corresponding to 1/3 of the U moment. The substitution of Pd for Ni leads to a rapid decay of the spontaneous magnetization. The evolution of magnetization and electrical resistivity behavior with Pd doping is tentatively attributed to the coexistence of the AF-I and UAF phases in the ground state of U(Ni_0.91Pd_0.09)₂Si₂ and U(Ni_0.865Pd_0.135)₂Si₂. In this scenario, the volume fraction of the AF-I phase rapidly grows with Pd doping on account of the UAF. At lowest temperatures an irreversible transition to the UAF phase is observed when a sufficiently high magnetic field is applied along the c-axis. Received 28 March 2002 / Received in final form 8 August 2002 Published online 19 December 2002 RID="a" ID="a"e-mail: sech@mag.mff.cuni.cz     

On the chemical states of nitrogen on iron surfaces reacted with ammonia and sodium cyanide,studied by X-ray photoelectron spectroscopy

The reaction products of iron reacted with ammonia gas and molten sodium cyanide are studied using X-ray photoelectron spectroscopy. Ammonia gas dissociates on iron surfaces to form a nitride and a loosely bound NH₃-like species with N 1sB.E.'s at 397.2 and 399.8 eV, respectively. On iron specimens dipped into molten NaCN three species are found at 398.6, 397.8 and 397.2 eV. The 398.6-eV peak is ascribed to adsorbed ?CN, and the 397.8-eV peak to carbonitride formed by the reaction of the cyanide with iron. Decreasing binding energies are found for adsorbed ?CN ligand, carbonitride compound and nitride, which seem to correspond to an apparent order of stronger interaction of nitrogen with iron. An iron sample was scratched with a file and the surface species of nitrogen were observed below 400°C. The nitrogen atoms in the sample diffuse and concentrate on the surface, depending on the nitrogen content in the bulk. Desorption patterns of nitrogen from a heated iron surface are examined with a view to the determination of gaseous constituents in metal samples.     

Thermodynamic properties in the approach to the quantum critical point of the spin-ladder material Na2Co2(C2O4)3(H2O)2

Magnetic susceptibility and heat capacity measurements as a function of temperature on a single-crystal sample of a spin-ladder material, Na2Co2(C2O4)3(H2O)2, are reported. Principal susceptibilities, parallel and perpendicular to the ladder direction, respectively, show broad maxima around 22 and 17 K. Both susceptibilities decay exponentially down to about 5 K and thereafter they are essentially independent of temperature. These findings amount to a signature of a quantum phase transition from a spin-liquid to Néel ordered state previously predicted theoretically. No anomaly is found in the heat capacity around the transition temperature.     

Improving the spectral resolution of wedged etalons and linear variable filters with incidence angle

Compact spectrometers fabricated by placing a wedged etalon or a linear variable filter in front of a linear photodiode array typically do not exhibit the spectral resolution of a parallel filter of the same finesse. A theoretical analysis of the beam emerging from a wedged etalon is shown to predict a comalike aberration, explaining the loss in resolution. This analysis also predicts a focused beam waist whose axial position depends on the incidence angle, indicating that resolution can be improved by placing this waist on the detector. These results are experimentally confirmed in a multicavity linear variable filter to achieve 0.09 nm spectral resolution at 1550 nm.     

Photoassociation spectroscopy of laser-cooled ytterbium atoms

We report the photoassociation spectroscopy of laser-cooled ytterbium atoms in an optical trap. We observed more than 90 photoassociation resonances of vibrational levels in the (1)Sigma(+)(u) state, including 80 consecutive series, up to 490 GHz detuning with respect to the atomic resonance. From the resonance frequencies we derived the atomic radiative lifetime of the (6s6p) 1P1 state to be 5.464+/-0.005 ns, which is about 2 orders of magnitude improvement over previous results. We also observed line broadening of resonances, which is ascribed to the predissociation to the triplet states, and estimated the transition probability to be 0.2. Furthermore, we observed the decrease of the photoassociation signal intensity, from which the scattering length is estimated to be equal to or less than 3 nm.     

Synthesis and structural characterisation of a cationic trinuclear organobismuth complex with an unprecedented coordination mode of hydrotris(2-mercaptoimidazolyl)borate ligands

A new coordination mode of hydrotris(2-mercaptoimidazolyl)borate ligands was found in the reaction of [Na(Tm(tBu))] with Me(2)BiCl to form a trinuclear organobismuth complex [(Me(2)Bi)(3)(Tm(tBu))(2)](+)[Me(2)BiCl(2)](-)(Tm(tBu)= hydrotris(2-mercapto-1-tert-butylimidazolyl)borate).     

Synthesis of the P-cluster inorganic core of nitrogenases

The [8Fe-7S] core of the P-clusters in nitrogenases is unique among the known [Fe-S] clusters which are essential to electron-transfer processes in nature. The [8Fe-7S] cluster has been thought unstable and to exist only in protein environments. We found that this unusual [8Fe-7S] structure can be self-assembled from the reaction of Fe(II) bis-amide, tetramethylthiourea, 2,4,6-triisopropylbenzenethiol, and elemental sulfur in a specific mole ratio. The structure of the complex isolated therefrom closely resembles that of the reduced form (PN) of the P-clusters, while the 6Fe(II)2Fe(III) oxidation state was manifested by the M?ssbauer study.     

Author Index

Authors Index

Author Index     

Analysis of sialo-N-glycans in glycoproteins as 1-phenyl-3-methyl-5-pyrazolone derivatives by capillary electrophoresis

A method for the analysis of the sialo-N-glycans in glycoproteins was established by the electrokinetic chromatography mode of capillary electrophoresis (CE) in sodium dodecyl sulfate (SDS) micelles as 1-phenyl-3-methyl-5-pyrazolone (PMP) derivatives, using sialo-N-glycans in fetuin as a model. Six major and some minor peaks were observed for the N-glycans in fetuin, which were well separated from each other using 50 mM phosphate buffer, pH 6.0, containing SDS to a concentration of 30 mM in an uncoated fused-silica capillary, and these peaks were assigned to sialo-N-glycans having either of the biantennary or β1-3/β1-4 linked galactose-containing complex type triantennary N-glycans as the basic structures, by an indirect method based on the assignment of the peaks in high-performance liquid chromatography separated in parallel with CE and peak collation between these two separation methods. The attaching position of the sialic acid residue was determined using the linkage preference of neuraminidase isozymes. The established system is considered to be useful for routine analysis of microheterogeneity of the carbohydrate moiety of this model glycoprotein from the following reasons: (1) the derivatization with PMP proceeds quantitatively under mild conditions without causing release of the sialic acid residue, (2) the derivatives can be sensitively detected by UV absorption, (3) the procedure is simple, rapid and reproducible. Preliminary results of N-glycan analysis for several other glycoproteins under these conditions are also presented.     

Scaling algorithms for the calculation of solar radiative fluxes

We derived new scaling formulae based on the method of successive orders of scattering to calculate solar radiative flux. In this report, we demonstrate a multiple scaling method, in which we introduce scaling factors for each scattering order independently. The formula of radiative transfer by the method of successive orders of scattering cannot be solved rapidly except in the case of optically thin atmospheres. Then we further derived a double scaling method, which scales the ordinary radiative transfer equation by two scaling factors. We applied the double scaling method to two-stream and four-stream approximations of the discrete ordinates method. Comparing the results of the double scaling method with those of the delta-M method, we found that the double scaling method improved the accuracy of radiative fluxes at large solar zenith angles, especially in the optically thin region, and that in the region where multiple scattering dominates, its accuracy was comparable to that of the delta-M method. Once we determined the scaling factors appropriately, the double scaling method calculated radiative fluxes as rapidly as the delta-M method in the two-stream and four-stream approximations. This method, therefore, is useful for accurate computation of solar radiative fluxes in general circulation models.