A critical discussion of calculated modulated structures, Fermi surface nesting and phonon softening in magnetic shape memory alloys Ni2_2Mn(Ga, Ge, Al) and Co2_2Mn(Ga, Ge)

A series of first principles calculations have been carried out in order to discuss electronic structure, phonon dynamics, structural instabilities and the nature of martensitic transformations of the Heusler alloys Ni$_2$Mn(Ga, Ge, Al) and Co$_2$Mn(Ga, Ge). The calculations show that besides electronic pecularities like Fermi--surface nesting, hybridizing optical and acoustic phonon modes are important for the stabilization of the modulated martensitic structures.Comment: 3 pages, 4 figures, JEMS-200

Revealing Interaction Of Organic Adsorbates With Semiconductor Surfaces Using Chemically Enhanced Raman

Surface enhanced Raman spectroscopy (SERS) is frequently associated with chemical enhancement (CE), which is an effect of the chemical coupling between reporting molecules and surfaces. While SERS technique is mainly attributed to the studies of metallic surfaces, chemical coupling must be present on semiconductor surfaces as well. Here, we examine binding of trans-1,2-two(4-pyridyl) ethylene (BPE) to various crystallographic facets of PbSe semiconductor. The calculated off-resonant Raman spectra vary significantly on different crystallographic facets of PbSe, correlating with the electronic structure of each type of semiconductor surface. We distinguish situations when the charge transfer is present and when it is not, which raises the question about what exactly should be called the chemical enhancement . We attempt to clarify this situation by introducing the concept of the charge-transfer and charge-transfer-less chemical enhancement. We also demonstrate a transition between these two regimes, which exhibits a nonlinear behavior of the vibrational coupling and a significantly stronger contribution to the Raman intensity

Ab-initio study of structure and dynamics properties of crystalline ice

We investigated the structural and dynamical properties of a tetrahedrally coordinated crystalline ice from first principles based on density functional theory within the generalized gradient approximation with the projected augmented wave method. First, we report the structural behaviour of ice at finite temperatures based on the analysis of radial distribution functions obtained by molecular dynamics simulations. The results show how the ordering of the hydrogen bonding breaks down in the tetrahedral network of ice with entropy increase in agreement with the neutron diffraction data. We also calculated the phonon spectra of ice in a 3x1x1 supercell by using the direct method. So far, due to the direct method used in this calculation, the phonon spectra is obtained without taking into account the effect of polarization arising from dipole-dipole interactions of water molecules which is expected to yield the splitting of longitudinal and transverse optic modes at the Gamma-point. The calculated longitudinal acoustic velocities from the initial slopes of the acoustic mode is in a reasonable agreement with the neutron scatering data. The analysis of the vibrational density of states shows the existence of a boson peak at low energy of translational region a characteristic common to amorphous systems.Comment: International symposium on structure and dynamics of heterogeneous system SDHS'0

First-principles investigation of phonon softenings and lattice instabilities in the shape-memory system Ni2MnGa

Ferromagnetic Ni2MnGa has unique magnetoelastic properties. These are investigated by detailed computational studies of the phonon-dispersion curves for the non-modulated cubic L21 and tetragonal structures. For the L21 structure, a complete softening of the transverse-acoustic mode has been found around the wave vector q=[1/3,1/3,0](2π/a). The softening of this TA2 phonon mode leads to the premartensitic modulated superstructure observed experimentally. Further phonon anomalies, related to other structural transformations in Ni2MnGa, have also been found and examined. These anomalies appear to be due to the coupling of particular acoustic-phonon modes and optical modes derived from Ni.Peer reviewe

Structural, electronic and magnetic properties of SrRuO3_3 under epitaxial strain

Using density functional theory within the local spin density approximation, structural, electronic and magnetic properties of SRO are investigated. We examine the magnitude of the orthorhombic distortion in the ground state and also the effects of applying epitaxial constraints, whereby the influence of large (in the range of $\pm 4$) in-plane strain resulting from coherent epitaxy, for both [001] and [110] oriented films, have been isolated and investigated. The overall pattern of the structural relaxations reveal coherent distortions of the oxygen octahedra network, which determine stability of the magnetic moment on the Ru ion. The structural and magnetic parameters exhibit substantial changes allowing us to discuss the role of symmetry and possibilities of magneto-structural tuning of \SRO-based thin film structures.Comment: 11 page

Transferable Pair Potentials for CdS and ZnS Crystals

A set of interatomic pair potentials is developed for CdS and ZnS crystals. We show that a simple energy function, which has been used to describe the properties of CdSe [J. Chem. Phys. 116, 258 (2002)], can be parametrized to accurately describe the lattice and elastic constants, and phonon dispersion relations of bulk CdS and ZnS in the wurtzite and rocksalt crystal structures. The predicted coexistence pressure of the wurtzite and rocksalt structures, as well as the equation of state are in good agreement with experimental observations. These new pair potentials enable the study of a wide range of processes in bulk and nanocrystalline II-VI semiconductor materials