473 research outputs found
Crystal growth, structural studies and superconducting properties of beta-pyrochlore KOs2O6
Single crystals of KOs2O6 have been grown in a sealed quartz ampoule.
Detailed single crystal X-ray diffraction studies at room temperature show
Bragg peaks that violate Fd-3m symmetry. With a comparative structure
refinement the structure is identified as non-centrosymmetric (F-43m). Compared
to the ideal beta-pyrochlore lattice (Fd-3m), both Os tetrahedral and O
octahedral network exhibit breathing mode like volume changes accompanied by
strong anisotropic character of the K channels. The crystals show metallic
conductivity and a sharp transition to the superconducting state at Tc = 9.65
K. Superconducting properties have been investigated by magnetization
measurements performed in a temperature range from 2 to 12 K and in magnetic
fields from 0 to 60 kOe. The temperature dependence of the upper critical field
Hc2(T) has been determined and the initial slope (dHc2/dT)Tc = -33.3 kOe/K has
been obtained near Tc. The upper critical field at zero temperature was
estimated to be Hc2(0) \cong 230 kOe, which is a value close to the Pauli
paramagnetic limiting field Hp(0)\cong 250 kOe. Then, the Ginzburg-Landau (GL)
coherence length xi GL(0) \approx 3.8 nm was calculated, and the Maki parameter
alpha \approx \sqrt 2 was obtained, suggesting the possibility that KOs2O6
might behave unconventionally at low temperatures and high magnetic fields
Local spin and charge properties of beta-Ag0.33V2O5 studied by 51V NMR
Local spin and charge properties were studied on beta-Ag0.33V2O5, a
pressure-induced superconductor, at ambient pressure using 51V-NMR and
zero-field-resonance (ZFR) techniques. Three inequivalent Vi sites (i=1, 2, and
3) were identified from 51V-NMR spectra and the principal axes of the
electric-field-gradient (EFG) tensor were determined in a metallic phase and
the following charge-ordering phase. We found from the EFG analysis that the V1
sites are in a similar local environment to the V3 sites. This was also
observed in ZFR spectra as pairs of signals closely located with each other.
These results are well explained by a charge-sharing model where a 3d1 electron
is shared within a rung in both V1-V3 and V2-V2 two-leg ladders.Comment: 12pages, 16figure
Magnetic, thermodynamic, and electrical transport properties of the noncentrosymmetric B20 germanides MnGe and CoGe
We present magnetization, specific heat, resistivity, and Hall effect
measurements on the cubic B20 phase of MnGe and CoGe and compare to
measurements of isostructural FeGe and electronic structure calculations. In
MnGe, we observe a transition to a magnetic state at K as identified
by a sharp peak in the ac magnetic susceptibility, as well as second phase
transition at lower temperature that becomes apparent only at finite magnetic
field. We discover two phase transitions in the specific heat at temperatures
much below the Curie temperature one of which we associate with changes to the
magnetic structure. A magnetic field reduces the temperature of this transition
which corresponds closely to the sharp peak observed in the ac susceptibility
at fields above 5 kOe. The second of these transitions is not affected by the
application of field and has no signature in the magnetic properties or our
crystal structure parameters. Transport measurements indicate that MnGe is
metal with a negative magnetoresistance similar to that seen in isostructural
FeGe and MnSi. Hall effect measurements reveal a carrier concentration of about
0.5 carriers per formula unit also similar to that found in FeGe and MnSi. CoGe
is shown to be a low carrier density metal with a very small, nearly
temperature independent diamagnetic susceptibility.Comment: 16 pages 23 figure
Developments in the Ni–Nb–Zr amorphous alloy membranes
Most of the global H2 production is derived from hydrocarbon-based fuels, and efficient H2/CO2 separation is necessary to deliver a high-purity H2 product. Hydrogen-selective alloy membranes are emerging as a viable alternative to traditional pressure swing adsorption processes as a means for H2/CO2 separation. These membranes can be formed from a wide range of alloys, and those based on Pd are the closest to commercial deployment. The high cost of Pd (USD *31,000 kg-1) is driving the development of less-expensive alternatives, including inexpensive amorphous (Ni60Nb40)100-xZrx alloys. Amorphous alloy membranes can be fabricated directly from the molten state into continuous ribbons via melt spinning and depending on the composition can exhibit relatively high hydrogen permeability between 473 and 673 K. Here we review recent developments in these low-cost membrane materials, especially with respect to permeation behavior, electrical transport properties, and understanding of local atomic order. To further understand the nature of these solids, atom probe tomography has been performed, revealing amorphous Nb-rich and Zr-rich clusters embedded in majority Ni matrix whose compositions deviated from the nominal overall composition of the membrane
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