61 research outputs found
Neutron Scattering Study of Crystal Field Energy Levels and Field Dependence of the Magnetic Order in Superconducting HoNi2B2C
Elastic and inelastic neutron scattering measurements have been carried out
to investigate the magnetic properties of superconducting (Tc~8K) HoNi2B2C. The
inelastic measurements reveal that the lowest two crystal field transitions out
of the ground state occurat 11.28(3) and 16.00(2) meV, while the transition of
4.70(9) meV between these two levels is observed at elevated temperatures. The
temperature dependence of the intensities of these transitions is consistent
with both the ground state and these higher levels being magnetic doublets. The
system becomes magnetically long range ordered below 8K, and since this
ordering energy kTN ~ 0.69meV << 11.28meV the magnetic properties in the
ordered phase are dominated by the ground-state spin dynamics only. The low
temperature structure, which coexists with superconductivity, consists of
ferromagnetic sheets of Ho{3+ moments in the a-b plane, with the sheets coupled
antiferromagnetically along the c-axis. The magnetic state that initially forms
on cooling, however, is dominated by an incommensurate spiral antiferromagnetic
state along the c-axis, with wave vector qc ~0.054 A-1, in which these
ferromagnetic sheets are canted from their low temperature antiparallel
configuration by ~17 deg. The intensity for this spiral state reaches a maximum
near the reentrant superconducting transition at ~5K; the spiral state then
collapses at lower temperature in favor of the commensurate antiferromagnetic
state. We have investigated the field dependence of the magnetic order at and
above this reentrant superconducting transition. Initially the field rotates
the powder particles to align the a-b plane along the field direction,
demonstrating that the moments strongly prefer to lie within this plane due to
the crystal field anisotropy. Upon subsequently increasing the field atComment: RevTex, 7 pages, 11 figures (available upon request); Physica
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Feasibility study for automation of the Central Laboratories, Water Resources Division, U.S. Geological Survey
This study of the feasibility of further automating the Central Laboratories deals specifically with the combined laboratory operations in Salt Lake City, Utah, and Denver, Colorado and is prepared with the understanding that such a system will also be implemented at the Central Laboratories in Atlanta, Georgia, and Albany, New York. The goals of automation are defined in terms of the mission of a water analysis laboratory, propose alternative computer systems for meeting such goals, and evaluate these alternatives in terms of cost effectiveness and other specified criteria. It is found that further automation will be beneficial and an in-house system that incorporates dual minicomputers is recommended: one for time-shared data acquisition, processing, and control; the second for data management. High-use analytical instruments are placed on-line to the time-shared minicomputer, with a terminal at each instrument and backup data storage on magnetic tape. A third, standby computer is switched in manually should the time-shared computer go down. Field-proven, modular hardware and software are chosen. Also recommended is the incorporation of the highly developed, computer-integrated instruments that are commercially available for determining petrochemicals and other organic substances, and are essential to the Laboratories' mission. (auth
Synthesis and characterization of Ba3(Pb1-xBix)2O7
The synthesis and initial characterization of a layered perovskite-based lead-bismuth oxide are reported. The phase, Ba3(Pb1-xBix)2O7, for 0≤x≤0.5, is the n=2 member of the Ruddlesden-Popper series An+1BnO3n+1. It can be synthesized only under very narrowly defined conditions. Despite the analogy to the well-known three-dimensional perovskite superconductor BaPb0.75Bi0.25O3, layered Ba3Pb2O7 does not become superconducting (down to 1.8 K) on doping with Bi.
Cs8.5W15O48 and CsW2O6: Members of a New Homologous Series of Cesium Tungsten Oxides
The crystal structures of two new reduced cesium tungsten oxides are reported. Along with the previously reported compound Cs6W11O36, they represent several members of a homologous series of layer compounds between the hexagonal tungsten bronze and pyrochlore structure types. The series formula is [A] Cs2nW4n–1O12n, where n – 1 is the number of pyrochlore layers.
Superconductivity at 28 K in a cuprate with a niobium oxide intermediary layer
A new copper oxide based superconductor is reported based on the ordered interleaving of NbO6 or TaO6 octahedra between the apices of copper oxide pyramids. Good bulk superconductivity is observed for only a narrow range of temperatures, annealing times and cooling rates. The relatively low Tc of the prototype compound Sr2Nd1.5Ce0.5NbCu2O10–δ, may be due to the presence of the insulating NbO6 layers or the wide (Nd, Ce)2O2 layers which separate the CuO2 planes
Magnetic and electrical properties of La2–xSrxNiO4±δ
The magnetic properties and resistivities of the La2–xSrxNiO4 solid solution have been investigated with particular emphasis on conductive materials near LaSrNiO4. Near that composition, weakly temperature-dependent magnetic susceptibilities, and resistivities, similar in magnitude to those in the isostructural metallic (La,Sr)2CuO4 compounds, are found. No superconductivity was found for the highly conductive compositions down to temperatures as low as 30 mK.
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