Magnetization and susceptibility of ferrofluids

A second-order Taylor series expansion of the free energy functional provides analytical expressions for the magnetic field dependence of the free energy and of the magnetization of ferrofluids, here modelled by dipolar Yukawa interaction potentials. The corresponding hard core dipolar Yukawa reference fluid is studied within the framework of the mean spherical approximation. Our findings for the magnetic and phase equilibrium properties are in quantitative agreement with previously published and new Monte Carlo simulation data.Comment: 8 pages including 4 figure

The Structure of Barium in the hcp Phase Under High Pressure

Recent experimental results on two hcp phases of barium under high pressure show interesting variation of the lattice parameters. They are here interpreted in terms of electronic structure calculation by using the LMTO method and generalized pseudopotential theory (GPT) with a NFE-TBB approach. In phase II the dramatic drop in c/a is an instability analogous to that in the group II metals but with the transfer of s to d electrons playing a crucial role in Ba. Meanwhile in phase V, the instability decrease a lot due to the core repulsion at very high pressure. PACS numbers: 62.50+p, 61.66Bi, 71.15.Ap, 71.15Hx, 71.15LaComment: 29 pages, 8 figure

Structure and Stability of an Amorphous Metal

Using molecular dynamics simulations, with a realistic many-body embedded-atom potential, and a novel method to characterize local order, we study the structure of pure nickel during the rapid quench of the liquid and in the resulting glass. In contrast with previous simulations with pair potentials, we find more crystalline order and fewer icosahedra for slower quenching rates, resulting in a glass less stable against crystallization. It is shown that there is not a specific amorphous structure, only the arrest of the transition from liquid to crystal, resulting in small crystalline clusters immersed in an amorphous matrix with the same structure of the liquid.Comment: 4 pages, 4 ps figs., to appear in Phys. Rev. Let

Structure and Dynamics of Liquid Iron under Earth's Core Conditions

First-principles molecular dynamics simulations based on density-functional theory and the projector augmented wave (PAW) technique have been used to study the structural and dynamical properties of liquid iron under Earth's core conditions. As evidence for the accuracy of the techniques, we present PAW results for a range of solid-state properties of low- and high-pressure iron, and compare them with experimental values and the results of other first-principles calculations. In the liquid-state simulations, we address particular effort to the study of finite-size effects, Brillouin-zone sampling and other sources of technical error. Results for the radial distribution function, the diffusion coefficient and the shear viscosity are presented for a wide range of thermodynamic states relevant to the Earth's core. Throughout this range, liquid iron is a close-packed simple liquid with a diffusion coefficient and viscosity similar to those of typical simple liquids under ambient conditions.Comment: 13 pages, 8 figure

Structural study of an amorphous NiZr2 alloy by anomalous wide angle X-ray scattering and Reverse Monte Carlo simulations

The local atomic structure of an amorphous NiZr2 alloy was investigated using the anomalous wide-angle x-ray scattering (AWAXS), differential anomalous scattering (DAS) and reverse Monte Carlo (RMC) simulations techniques. The AWAXS measurements were performed at eight different incident photon energies, including some close to the Ni and Zr K edges. From the measurements eight total structure factor S(K,E) were derived. Using the AWAXS data four differential structure factors DSFi(K,Em,En) were derived, two about the Ni and Zr edges. The partial structure factors SNi-Ni(K), SNi-Zr(K) and SZr-Zr(K) were estimated by using two different methods. First, the S(K,E) and DSFi(K,Em,En) factors were combined and used in a matrix inversion process. Second, three S(K,E) factors were used as input data in the RMC technique. The coordination numbers and interatomic distances for the first neighbors extracted from the partial structure factors obtained by these two methods show a good agreement. By using the three-dimensional structure derived from the RMC simulations, the bond-angle distributions were calculated and they suggest the presence of distorted triangular-faced polyhedral units in the amorphous NiZr2 structure. We have used the Warren chemical short-range order parameter to evaluate the chemical short-range order for the amorphous NiZr2 alloy and for the NiZr2 compound. The calculated values show that the chemical short-range order found in these two materials is similar to that found in a solid solution.Comment: Submitted to Phys. Rev. B, 8 figure

Epitaxial overgrowth of II-VI compounds on patterned substrates

The selected area epitaxial overgrowth of narrow gap HgTe as well as wide gap CdTe and ZnTe on CdTe/GaAs substrates, which had been structured by dry etching techniques, has been investigated. A plasma etching process using a barrel reactor with CH$_4$-CH$_2$ gases has been employed to prepare stripes with a width of about 1 μm with anisotropic as well as isotropic etching profiles. It has been found, that the selected area HgTe overgrowth takes place with a high local selectivity to the low index planes of the patterned surface. In contrast, the selected area overgrowth of the wide gap CdTe and ZnTe is controlled by anisotropic growth kinetics provided that the substrate temperature is not lower than 220°C and the starting surface consists of well developed low index crystallographic planes

Mare Nostrum: geomed report - 2

GEOMED is a project for the determination of the geoid and of the Sea Surface Topography in the Mediterranean Sea. The existing set of data available on the Mediterranean in already quite significant and it is going to be improved in these years because of the new altimetric data coming from ERS1 and the TOPEX-POSEIDON missions. In the execution of the project we use, mainly, radar altimetric data (f rom SEASAT, GEOSAT and ESR1 missions) gravimetrie data, bathymetric models, as well as land gravi ty data set and DTMs of surrounding regions: these data sets are generally poor on the southern side of the Mediterranean. More geophysical data, like tidal and Moho depth models, will be user too. The partieularity of this projeet is in that the dynamics of the Mediterranean, as a elosed sea, eannot be easily modelled, so that the time dependent part of the altimetric signal has to be modelled empirically together wi th the radial orbi terror; on the other hand, sinee the satellite ares are very short, this choice is well justified. The separation between the stationary Sea Surface Topography and the geoid can be done in such an area using both data sets, the altimetric and the gravimetric one, whieh is available eontrary to the situation in open oceanic areas. This volume, the second of the MARE NOSTRUM Series, eolleet the works done by many groups involved in the GEOMED organization, in the last year, as informed in the GEOMED Meeting held in Madrid at october 1992, and will be followed by others with new eontributions.Peer reviewe