Aldo Romero

Intermetallic Li-Al compounds are on the one hand key materials for light-weight engineering, and on the other hand, they have been proposed for high-capacity electrodes for Li batteries. We determine from first-principles the phase diagram of Li-Al binary crystals using the minima hopping structural prediction method. Beside reproducing the experimentally reported phases (LiAl, Li3Al2, Li9Al4, LiAl3, and Li2Al), we unveil a structural variety larger than expected by discovering six unreported binary phases likely to be thermodynamically stable. Finally, we discuss the behavior of the elastic constants and of the electric potential profile of all Li-Al stable compounds as a function of their stoichiometry.

ABSTRACT Molecular-dynamics simulations of H diffusion in Nb are performed for a system consisting of 432 Nb atoms and 8 H atoms at two different temperatures: T=450 and 580 K. For the interatomic interactions we use a description proposed by Finnis, Sinclair, and Gillan. We compare our results with quasielastic-neutron-scattering data and our model reproduces quite well both the distinct deviation from simple jump-diffusion behavior and the ``anomalous'' Debye-Waller factor. To reveal the details of the H motion the residence-time distribution at the stable sites (T sites) as well as the correlation character among consecutive ``jumps'' are evaluated. We find that the residence-time distribution is composed of two distinct contributions; one narrow component with a short residence time of the order 35 fs, and one broad component with roughly exponential decay. The narrow component corresponds to that the H atom moves rapidly among two or more sites belonging to what has been called a 4T configuration. The typical decay time of the broad component is found to be of the order of 160 fs and 300 fs in the time intervals 60<t<300 fs and 300<t<600 fs, respectively, which should be compared with the mean residence time derived from the diffusion constant, τres=a20/48Ds=324 fs. We also find substantial contributions of second-nearest-neighboring jumps, but the division between nearest- and second-nearest-neighboring jumps is ambiguous. The diffusive and the vibrational motion of the H atom cannot be clearly separated and the time spent and the spatial excursion performed in the ``jump phase'' are not negligible.

ABSTRACT The use of nano-scale zero valent iron (nZVI) in the removal of Pb2+ from aqueous solution is studied as a function of pH, evaluating the structural and surface changes of the nZVI, as well as its magnetic properties after the sorption process. The kinetics of the removal process of Pb2+ by nZVI was studied as a function of pH, where the pseudo second order (PSO) kinetics model reproduces the experimental results, with an initial adsorption (h) at pH 4 almost three times greater than at pH 6.0. From the sorption isotherm results, we find an enhanced sorption capacity as pH is increased. The Langmuir model is in good agreement with our experimental data, where a maximum sorption capacity is found to be 50.31 +/- 236 and 32.85 +/- 3.87 (mg g(-1)) at pH 6 and 4, respectively, suggesting the formation of new active sites on the external surface of the nZVI. The XRD results, obtained from the samples after removal of Pb2+, show new mineral phases of nZVI, which can be attributed to Fe and Pb polycrystals. These results obtained from the samples, before and after Pb2+ sorption at different pH values, indicate that these values conditioned the surface and morphological changes observed in nZVI, even modifying their magnetic properties.

Presented is a study of uncompensated magnetization in antiferromagnets performed by SQUID and VSM magnetometry, and polarized neutron reflectometry. This uncompensated magnetization is observed and studied as a function of temperature and cooling conditions. Uncompensated magnetization is found close to the top surface of the antiferromagnet (interface with the ferromagnet) and also in the “bulk” of the antiferromagnetic film. It

Detailed knowledge of magnetization reversal and possible magnetic configurations in magnetic nanostructures is essential for high-density magnetic memory. Magnetism of nanostructured magnets, of size comparable to or smaller than ferromagnetic domain size, offers a great potential for new physics. We present results of a systematic study of magnetic reversal in sub-100 nm nanodots, and how this reversal can be affected

We present a detailed study of the chemical reaction A + B â 0 in two dimensions for different initial distributions of reactants as characterized by the structure function. We argue that the long-wavelength components of the initial fluctuations determine the long-time delay of the reactant density. Numerical simulations of the coarse grained reaction-diffusion dynamics agree with analytical predictions. 27

ABSTRACT Phonons vibrational frequencies for Hg1-xCdxSe (x <= 0.5) crystalline alloys were theoretical and experimentally studied. Two simple expressions that summarize the optical modes behavior with composition were derived. Such that the behavior of the vibrational properties of II-VI and III-V semiconductors at the G point can be expressed as function of the product of the ionic charges (Z(1)Z(2)) and the nearest neighbor distance, d. A good agreement has been found between the observed and calculated values of the vibrational frequencies V-LO for the ternary Hg1-xCdxSe alloys. Furthermore, our calculations suggest a non typical behavior of the acoustic vibrational modes similar to the one reported for HgSe. Besides, we also report the theoretical dependence of the phonon dispersion relation as function of composition.

... Aldo H. Romero Department of Physics, University of California, San Diego, La Jolla, California 92093-0354 Department of Chemistry and Biochemistry ... URL: http://link.aps.org/doi/10.1103/ PhysRevB.58.15904. DOI: 10.1103/PhysRevB.58.15904. PACS: 87.15.He, 68.35.Ja. ...

We report a novel magnetic phenomenon consisting of the formation of helical spin configurations during the magnetization of densely packed ferromagnetic nanowires encapsulated inside carbon nanotubes. We studied the hysteresis loops when the magnetic fields are applied parallel and perpendicular to the nanotubes axes. We also performed theoretical calculations on aligned nanowire arrays that clearly indicate the creation of helical spin vortices in the hysteresis loops. The latter are caused by the presence of strong dipolar interactions among neighboring wires.

Phonon linewidths can exhibit a large variation when either pressure or isotopic masses are changed. These effects yield detailed information about the mechanisms responsible for linewidths and lifetimes, e.g., anharmonicity or isotopic disorder. We report Raman measurements of the linewidth of the upper E2 phonons of ZnO crystals with several isotopic compositions and their dependence on pressure. Changes by a factor of 12 are observed at a given temperature. Comparison with calculated densities of one-phonon states, responsible for isotope scattering, and of two-phonon states, responsible for anharmonic decay, yields a consistent picture of these phenomena. Isotopic disorder broadening by 7 cm(-1) is found in samples with mixed 16O-18O content, whereas the anharmonic processes involve decay into sums and differences of two phonons.