Specific heat and magnetic structure of GdT2X2 compounds (T=Cu,Ni,X=Sn,Sb)

The magnetic specific heat of GdΝi 2 Sn2 , GdΝi2 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 GdΝi2Sn2 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_1−<i>x</i>Zr_<i>x</i>O₃: experimental and theoretical investigation

We report an investigation of the local structure in homovalent-substituted BaTi1−x 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 BaTi1−x 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 BaTi1−x 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

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