Lattice Dynamics of Martensitic Transformations Examined by Atomistic Simulations

We have performaed molecular dynamics simulations of Fe(80)Ni(20) alloys using an inter-atomic potential of the EAM-type which allows the simulation of the martensite-austenite transition. We present results, showing the development of an inhomogeneous shear system on a nanoscale during the thermally induced austenitic transition. In addition to this we obtained the phonon dispersion relations of the martensite phase by calculating the dynamical structure factor from our simulation results. On approaching the transition temperature the phonon dispersion shows anomalies which might be connected with the formation of the microstructure during the austenitic transition.Comment: 7 pages, 4 figures, Contribution submitted to the "IV European Symposium on Martensitic Transformations" (ESOMAT 97), July 1-5, 1997, Enschede (The Netherlands

Density functional simulation of small Fe nanoparticles

We calculate from first principles the electronic structure, relaxation and magnetic moments in small Fe particles, applying the numerical local orbitals method in combination with norm-conserving pseudopotentials. The accuracy of the method in describing elastic properties and magnetic phase diagrams is tested by comparing benchmark results for different phases of crystalline iron to those obtained by an all-electron method. Our calculations for the bipyramidal Fe_5 cluster qualitatively and quantitatively confirm previous plane-wave results that predicted a non-collinear magnetic structure. For larger bcc-related (Fe_35) and fcc-related (Fe_38, Fe_43, Fe_62) particles, a larger inward relaxation of outer shells has been found in all cases, accompanied by an increase of local magnetic moments on the surface to beyond 3 mu_B.Comment: 15 pages with 6 embedded postscript figures, updated version, submitted to Eur.Phys.J.

Ab initio study of the interface properties of Fe/GaAs(110)

We have investigated the initial growth of Fe on GaAs(110) by means of density functional theory. In contrast to the conventionally used (001)-surface the (110)-surface does not reconstruct. Therefore, a flat interface and small diffusion can be expected, which makes Fe/GaAs(110) a possible candidate for spintronic applications. Since experimentally, the actual quality of the interface seems to depend on the growth conditions, e.g., on the flux rate, we simulate the effect of different flux rates by different Fe coverages of the semiconductor surface. Systems with low coverages are highly diffusive. With increasing amount of Fe, i.e., higher flux rates, a flat interface becomes more stable. The magnetic structure strongly depends on the Fe coverage but no quenching of the magnetic moments is observed in our calculations.Comment: 9 pages, 8 figure

Phonon-induced and phonon-free superconductivity in correlated systems : Eliashberg equations for the two-dimensional Hubbard model

The problem of phonon-induced and phonon-free superconductivity in the two-dimensional Hubbard model has been addressed. We have generalized the Eliashberg equations to account for both on-site and intersite pairing and consider the electron–electron and electron-phonon channel on an equal footing. This approach allows for the discussion of pairing and depairing properties of the local repulsive interaction. We demonstrate the possibility of cooperation between electron-phonon and electron–electron interaction in the stabilization of the d-wave superconductivity, in particular close to the experimental value of optimal doping ( ≃ 0.15). We have also discussed the problem of phonon-induced superconductivity in the two-dimensional Hubbard model close to the metal-insulator transition. Here, the Coulomb correlations have been incorporated within the Hubbard I approximation whereas the superconductivity is treated by the Eliashberg scheme. The results support the view that a d-wave component dominates in the gap function