296 research outputs found
Synthesis of H<sub>x</sub>Li<sub>1-x</sub>LaTiO<sub>4</sub> from quantitative solid-state reactions at room temperature
The layered perovskite HLaTiO4 reacts stoichiometrically with LiOH·H2O at room temperature to give targeted compositions in the series HxLi1-xLaTiO4. Remarkably, the Li+ and H+ ions are quantitatively exchanged in the solid state and this allows stoichiometric control of ion exchange for the first time in this important series of compounds
A novel route to phase formation of cobalt oxyhydrates using KMnO4 as an oxidizing agent
We have first succeefully synthesized the sodium cobalt oxyhydrate
superconductors using KMnO4 as a de-intercalating and oxidizing agent. It is a
novel route to form the superconductive phase of NaxCoO2.yH2O without resorting
to the commonly used Br2/CH3CN solution. The role of the KMnO4 is to
de-intercalate the Na+ from the parent compound Na0.7CoO2 and oxidize the Co
ion as a result. The higher molar ratio of KMnO4 relative to the sodium content
tends to remove more Na+ from the parent compound and results in a slight
expansion of the c-axis in the unit cell. The superconducting transition
temperature is 4.6-3.8 K for samples treated by the aqueous KMnO4 solution with
the molar ratio of KMnO4 relative to the sodium content in the range of 0.3 and
2.29.Comment: 10 pages, 3 figure
Unconventional Superconductivity and Nearly Ferromagnetic Spin Fluctuations in NaCoOHO
Co NQR studies were performed in recently discovered superconductor
NaCoOHO to investigate physical properties in the
superconducting (SC) and normal states. Two samples from the same NaCoO
were examined, SC bilayer-hydrate sample with K and non-SC
monolayer-hydrate sample. From the measurement of nuclear-spin lattice
relaxation rate in the SC sample, it was found that the coherence peak
is absent just below and that is proportional to temperature far
below . These results, which are in qualitatively agreement with the
previous result by Fujimoto {\it et al.}, suggest strongly that unconventional
superconductivity is realized in this compound. In the normal state,
of the SC sample shows gradual increase below 100K down to , whereas
of the non-SC sample shows the Korringa behavior in this temperature
range. From the comparison between and in the SC
sample, the increase of is attributed to nearly ferromagnetic
fluctuations. These remarkable findings suggest that the SC sample possesses
nearly ferromagnetic fluctuations, which are possibly related with the
unconventional superconductivity in this compound. The implication of this
finding is discussed.Comment: 4 pages, 5 figures. submitted to J. Phys. Soc. Jp
Discovery of Li2(Pd,Pt)3B superconductors
Critical temperature Tc of the Li2(Pd1-xPtx)3B was reported to be 7-8K for
x=0 and 2.2-2.8K for x=1. In this article we present our preliminary results on
behavior of magnetization-temperature curves with starting composition of Pd-B
precursor, y-Li concentration in LiyPd3B and post-annealing of the Pd-end
compound. Results suggest that to maximize Tc ratio Pd:B should be close to
3:1, while y-Li has to be optimum. The lowest Tc for LiyPd3B was 4.4-4.6K,
while post-annealings at 560 deg. C allowed enhancement of Tc up to 8.2-8.4K.
Compositions Li2Z3B with Z=Ni, Ru, Rh, Re, Ag are not superconducting down to
1.8K. Exception is composition with Re showing superconductivity due to Re3B
compound. All samples were prepared by arc melting.Comment: 6 pages, 5 figs. presented at M2S, 200
Nanostructured Nickel Phosphide as an Electrocatalyst for the Hydrogen Evolution Reaction
Nanoparticles of nickel phosphide (Ni_2P) have been investigated for electrocatalytic activity and stability for the hydrogen evolution reaction (HER) in acidic solutions, under which proton exchange membrane-based electrolysis is operational. The catalytically active Ni_2P nanoparticles were hollow and faceted to expose a high density of the Ni_2P(001) surface, which has previously been predicted based on theory to be an active HER catalyst. The Ni2P nanoparticles had among the highest HER activity of any non-noble metal electrocatalyst reported to date, producing H_2(g) with nearly quantitative faradaic yield, while also affording stability in aqueous acidic media
Electrocatalytic hydrogen evolution using amorphous tungsten phosphide nanoparticles
Amorphous tungsten phosphide (WP), which has been synthesized as colloidal nanoparticles with an average diameter of 3 nm, has been identified as a new electrocatalyst for the hydrogen-evolution reaction (HER) in acidic aqueous solutions. WP/Ti electrodes produced current densities of −10 mA cm^(−2) and −20 mA cm^(−2) at overpotentials of only −120 mV and −140 mV, respectively, in 0.50 M H_2SO_4(aq)
Orbital order in NaTiO : A first principles study
The debate over the orbital order in layered triangular lattice system
NaTiO has been rekindled by the recent experiments of McQueen, et al.
\cite{cava} on NaVO ({\em Phys. Rev. Lett.} {\bf 101}, 166402 (2008)). In
view of this, the nature of orbital ordering, in both high and low temperature
states, is studied using an ab-initio electronic structure calculation. The
orbital order observed in our calculations in the low temperature structure of
NaTiO is consistent with the predictions of McQueen, et. al. An LDA plus
dynamical mean-field calculation shows considerable transfer of spectral weight
from the Fermi level but no metal- insulator transition, confirming the poor
metallic behaviour observed in transport measurements.Comment: 6 pages, 6 figure
Variational Monte Carlo Studies of Pairing Symmetry for the t-J Model on a Triangular Lattice
As a model of a novel superconductor Na_xCoO_2\cdotyH_2O, a single-band t-J
model on a triangular lattice is studied, using a variational Monte Carlo
method. We calculate the energies of various superconducting (SC) states,
changing the doping rate \delta and sign of t for small J/|t|. Symmetries of s,
d, and d+id (p+ip and f) waves are taken up as candidates for singlet (triplet)
pairing. In addition, the possibility of Nagaoka ferromagnetism and
inhomogeneous phases is considered. It is revealed that, among the SC states,
the d+id wave always has the lowest energy, which result supports previous
mean-field studies. There is no possibility of triplet pairing, although the
f-wave state becomes stable against a normal state in a special case
(\delta=0.5 and t<0). For t<0, the complete ferromagnetic state is dominant in
a wide range of \delta and J/|t|, which covers the realistic parameter region
of superconductivity.Comment: 10 pages, 13 figure
Bulk Electronic structure of NaCoO.1.3HO
High-energy (h = 5.95 keV) synchrotron Photoemission spectroscopy (PES)
is used to study bulk electronic structure of NaCoO.1.3HO,
the layered superconductor. In contrast to 3-dimensional doped Co oxides, Co
core level spectra show well-separated Co and Co ions.
Cluster calculations suggest low spin Co and Co character, and a
moderate on-site Coulomb correlation energy U3-5.5 eV. Photon
dependent valence band PES identifies Co and O derived
states, in near agreement with band structure calculations.Comment: 4 pages 4 figures Revised text added referenc
Multi-orbital analysis on the Superconductivity in Na_{x}CoO_{2} \cdot y H_{2}O
We preform a multi-orbital analysis on the novel superconductivity in
Na_{x}CoO_{2} \cdot yH_{2}O. We construct a three-orbital model which
reproduces the band structure expected from the LDA calculation. The effective
interaction leading to the pairing is estimated by means of the perturbation
theory. It is shown that the spin triplet superconductivity is stabilized in
the wide parameter region. This is basically owing to the ferromagnetic
character of spin fluctuation. The p-wave and f-wave superconductivity are
nearly degenerate. The former is realized when the Hund's rule coupling is
large, and vice versa. In a part of the parameter space, the d-wave
superconductivity is also stabilized. We point out that the orbital degeneracy
plays an essential role for these results through the wave function of
quasi-particles. The nearly degeneracy of p-wave and f-wave superconductivity
is explained by analysing the orbital character of each Fermi surface. We
discuss the validity of some reduced models. While the single band Hubbard
model reproducing the Fermi surface is qualitatively inappropriate, we find an
effective two-orbital model appropriate for studying the superconductivity. We
investigate the vertex corrections higher than the third order on the basis of
the two-orbital model. It is shown that the vertex correction induces the
screening effect but does not affect on the qualitative results.Comment: To appear in J. Phys. Soc. Jpn. 74 (2005) No.
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