24 research outputs found
Specific heat and magnetic structure of GdT2X2 compounds (T=Cu,Ni,X=Sn,Sb)
The magnetic specific heat of GdÎi 2 Sn2 , GdÎi2 Sb2 and GdCu2 Sb2
shows a λ-type anomaly at TN with a noticeable reduction of the discontinuity
compared to that expected for a simple antiferromagnet. It can be
associated with the existence of amplitude modulated magnetic structure
just below TN. Above TN a weak magnetic contribution is observed, evidencing
the short-range ordering. From neutron diffraction studies it can be also
concluded that investigated compounds have not simple antiferromagnetic
structures. For GdÎi2Sn2 the propagation vector Q = (0.3, 1/2, 1/2) was
found with the magnetic moments along the [1,0,0] axis
Ab Initio Molecular Dynamics Simulation of Liquid Ga_xAs_{1-x} Alloys
We report the results of ab initio molecular dynamics simulations of liquid
Ga_xAs_{1-x} alloys at five different concentrations, at a temperature of 1600
K, just above the melting point of GaAs. The liquid is predicted to be metallic
at all concentrations between x = 0.2 and x = 0.8, with a weak resistivity
maximum near x = 0.5, consistent with the Faber-Ziman expression. The
electronic density of states is finite at the Fermi energy for all
concentrations; there is, however, a significant pseudogap especially in the
As-rich samples. The Ga-rich density of states more closely resembles that of a
free-electron metal. The partial structure factors show only a weak indication
of chemical short-range order. There is also some residue of the covalent
bonding found in the solid, which shows up in the bond-angle distribution
functions of the liquid state. Finally, the atomic diffusion coefficients at
1600K are calculated to be 2.1 \times 10^{-4} cm^2/sec for Ga ions in
Ga_{0.8}As_{0.2} and 1.7 \times 10^{-4} cm^2/sec for As ions in
Ga_{0.2}As_{0.8}.Comment: 29 pages, 10 eps figures, accepted for publication in Phys. Rev.
Random local strain effects in the relaxor ferroelectric BaTi<sub>1â<i>x</i> </sub>Zr<sub> <i>x</i> </sub>O<sub>3</sub>: experimental and theoretical investigation
We report an investigation of the local structure in homovalent-substituted BaTi1âx Zr x O3 relaxors by a combination of experimental and theoretical methods, namely neutron total scattering, X-ray absorption spectroscopy, and supercell ab-initio calculations. It is shown that unlike Zr atoms, Ti atoms are largely displaced in their octahedra, and are thus associated with strong local dipole moments. Besides, we give evidence that the difference in the size of Ti4+ and Zr4+ cations leads to a significant size mismatch of the Ti-O6 and Zr-O6 octahedra. When they link to form the perovskite structure of BaTi1âx Zr x O3, the O6 octahedra undergo slight distortions in order to accommodate their different sizes. It is shown that they are compressed in the direction of Zr neighbors, and expanded in the direction of Ti neighbors. The polar Ti displacements, which are sensitive to the octahedral distortions, then become constrained in their orientation according to the local Zr/Ti distribution. Such constraints impede a perfect alignment of all the Ti displacements as existing in the classic ferroelectric BaTiO3. Our results shed light on the structural mechanisms that lead to disordered Ti displacements in BaTi1âx Zr x O3 relaxors, and probably in other BaTiO3-based relaxors with homovalent substitution
The structure of amorphous Si:H using steady state and pulsed neutron sources
SIGLEAvailable from British Library Document Supply Centre- DSC:9091.9(MPD-NBS--345) / BLDSC - British Library Document Supply CentreGBUnited Kingdo
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Spherical momentum distribution of the protons in hexagonal ice from modeling of inelastic neutron scattering data
The spherical momentum distribution of the protons in ice is extracted from a high resolution deep inelastic neutron scattering experiment. Following a recent path integral Car-Parrinello molecular dynamics study, data were successfully interpreted in terms of an anisotropic
Gaussian model, with a statistical accuracy comparable to that of the
model independent scheme used previously, but providing more detailed
information on the three dimensional potential energy surface
experienced by the proton. A recently proposed theoretical concept is
also employed to directly calculate the mean force from the experimental
neutron Compton profile, and to evaluate the accuracy required to
unambiguously resolve and extract the effective proton potential from the experimental dat