42 research outputs found
On the effect of heterovalent substitutions in ruthenocuprates
We discuss the properties of superconducting derivatives of the RuSr2GdCu2O8
(1212-type) ruthenocuprate, for which heterovalent doping has been achieved
through partial substitution of Cu ions into the RuO2 planes
(Ru1-xSr2GdCu2+xO8-d, 0<x<0.75, Tcmax=72 K for x=0.3-0.4) and Ce ions into the
Gd sites (RuSr2Gd1-yCeyCu2O8, 0<y<0.1). The measurements of XANES, thermopower,
and magnetization under external pressure reveal an underdoped character of all
compounds. Muon spin rotation experiments indicate the presence of magnetic
order at low temperatures (Tm=14-2 K for x=0.1-0.4). Properties of these two
series lead us to the qualitative phase diagram for differently doped 1212-type
ruthenocuprates. The difference in temperature of magnetic ordering found for
superconducting and non-superconducting RuSr2GdCu2O8 is discussed in the
context of the properties of substituted compounds. The high pressure oxygen
conditions required for synthesis of Ru1-xSr2RECu2+xO8-d, have been extended to
synthesis of a Ru1-xSr2Eu2-yCeyCu2+xO10-d series. The Cu->Ru doping achieved in
these phases is found to decrease the temperature for magnetic ordering as well
the volume fraction of the magnetic phase.Comment: Proceedings of the 3rd Polish-US Workshop on Magnetism and
Superconductivity of Advanced Materials, July 14-19, 2002, Ladek Zdroj
(Poland) to appear in Physica
Structural analysis, magnetic and transport properties of the (Ru1-xCox)Sr2GdCu2O8 system
The effects of Co substitution on structural and superconducting properties
of RuSr2GdCu2O8 compound have been studied. Rietveld refinements of the X-ray
diffraction patterns indicate that the cobalt ion progressively replaces
ruthenium sites. This replacement induces significant changes on the crystal
structure and on the magnetic and superconducting properties. The effects Co
substitution on the superconducting behaviour, and more particulary on the
changes induce by the hole doping mechanism, were investigated in
(Ru1-xCox)Sr2GdCu2O8 by a "bond valence sum" analysis with Co content from x=
0.0 to x = 0.2. The weak ferromagnetic transition at Tm= 138.2 K is shifted to
lower temperature, and suppressed at higher Co content. From the
crystallographic point of view the Ru-O(1)-Cu bond angle, associated to the
rotation of the RuO6 octahedra, around the c-axis remain essetially constant
when Ru is substituted by Co. Furthermore, increasing Co content has the effect
to increase the weak ferromagnetic moment, which may be interpreted as the main
responsible for breaking the delicate balance between magnetic and
superconducting ordering.Comment: 21 pages, 8 figure
Synthesis effects on the magnetic and superconducting properties of RuSr2GdCu2O8
A systematic study on the synthesis of the Ru-1212 compound by preparing a
series of samples that were annealed at increasing temperatures and then
quenched has been performed. It results that the optimal temperature for the
annealing lies around 1060-1065 C; a further temperature increase worsens the
phase formation. Structural order is very important and the subsequent grinding
and annealing improves it. Even if from the structural point of view the
samples appear substantially similar, the physical characterization highlight
great differences both in the electrical and magnetic properties related to
intrinsic properties of the phase as well as to the connection between the
grains as inferred from the resistive and the Curie Weiss behaviour at high
temperature as well as in the visibility of ZFC anf FC magnetic signals.Comment: 17 pages, 12 figures. Proc. Int. Workshop " Ruthenate and
rutheno-cuprate materials: theory and experiments", Vietri, October 2001. To
be published on LNP Series, Springer Verlag, Berlin, C. Noce, A. Vecchione,
M. Cuoco, A. Romano Eds, 200
Study of the crystal structure, superconducting and magnetic properties of Ru1-xFexSr2GdCu2O8
Samples of the Ru1-xFexSr2GdCu2O8 system with x = 0, 0.025, 0.05, 0.075, 0.1
and 0.2, were prepared and their structural, superconducting and magnetic
properties were studied. Rietveld refinement of the X-ray diffraction patterns
show that the Fe substitution occurs in both Ru and Cu sites. An increase of Fe
concentration produces no significant changes in the bond angle Ru-O(3)-Ru,
which is a measure of the rotation of the RuO6 octahedra around the c-axis, and
also in the bond angle Ru-O(1)-Cu, which is a measure of the canting of the
RuO6 octahedra. On the other hand, the bond angle Cu-O(2)-Cu, which is a
measure of the buckling of the CuO2 layer, has a slight tendency to decrease
with the increase of the Fe content. We found thet both ferromagnetic and
superconducting transition temperatures are reduced with the increase of Fe
concentration. Analisys related to the decay of the superconducting and
ferromagnetic states is presented.Comment: 9 pages, 7 figure
Infrared optical properties of Pr2CuO4
The ab-plane reflectance of a Pr2CuO4 single crystal has been measured over a
wide frequency range at a variety of temperatures, and the optical properties
determined from a Kramers-Kronig analysis. Above ~ 250 K, the low frequency
conductivity increases quickly with temperature; the resistivity follows the
form e^(E_a/k_BT), where E_a ~ 0.17 eV is much less than the inferred optical
gap of ~ 1.2 eV. Transport measurements show that at low temperature the
resistivity deviates from activated behavior and follows the form
e^[(T_0/T)^1/4], indicating that the dc transport in this material is due to
variable-range hopping between localized states in the gap. The four
infrared-active Eu modes dominate the infrared optical properties. Below ~ 200
K, a striking new feature appears near the low-frequency Eu mode, and there is
additional new fine structure at high frequency. A normal coordinate analysis
has been performed and the detailed nature of the zone-center vibrations
determined. Only the low-frequency Eu mode has a significant Pr-Cu interaction.
Several possible mechanisms related to the antiferromagnetism in this material
are proposed to explain the sudden appearance of this and other new spectral
features at low temperature.Comment: 11 pages, 7 embedded EPS figures, REVTeX
Integration of Expressed Sequence Tag Data Flanking Predicted RNA Secondary Structures Facilitates Novel Non-Coding RNA Discovery
Many computational methods have been used to predict novel non-coding RNAs (ncRNAs), but none, to our knowledge, have explicitly investigated the impact of integrating existing cDNA-based Expressed Sequence Tag (EST) data that flank structural RNA predictions. To determine whether flanking EST data can assist in microRNA (miRNA) prediction, we identified genomic sites encoding putative miRNAs by combining functional RNA predictions with flanking ESTs data in a model consistent with miRNAs undergoing cleavage during maturation. In both human and mouse genomes, we observed that the inclusion of flanking ESTs adjacent to and not overlapping predicted miRNAs significantly improved the performance of various methods of miRNA prediction, including direct high-throughput sequencing of small RNA libraries. We analyzed the expression of hundreds of miRNAs predicted to be expressed during myogenic differentiation using a customized microarray and identified several known and predicted myogenic miRNA hairpins. Our results indicate that integrating ESTs flanking structural RNA predictions improves the quality of cleaved miRNA predictions and suggest that this strategy can be used to predict other non-coding RNAs undergoing cleavage during maturation
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Properties of cast CF-8 stainless-steel weldments at cryogenic temperatures
ISABELLE is a 400 x 400 GeV proton-proton colliding beam accelerator now under construction at Brookhaven National Laboratory. The beams will be guided and focused by superconducting magnets. A total of 722 dipole beam bending magnets and 280 quadrupole beam focusing magnets are required. Centrifugally cast CF-8 stainless steel tubes were selected to provide a rigid support and to house the superconducting magnet assembly. The selection of this material for the support tubes is discussed by Dew-Hughes and Lee. Their study indicates that the presence of delta ferrite strengthens the material but causes a decrease in ductility if the ferrite content is greater than 10%. Brown and Tobler found that the fracture toughness is also decreased as the delta ferrite content is increased
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Brookhaven National Laboratory Reports
Report issued by the Brookhaven National Laboratory discussing corrosion behaviors of materials in bismuth. As stated in the introduction, "this report will present the details of the corrosion testing and the results obtained to date on materials tested under static conditions in bismuth media" (p. 1). This report includes tables, illustrations, and photographs