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

Chemical Raman Enhancement of Organic Adsorbates on Metal Surfaces

Using a combination of first-principles theory and experiments, we provide a quantitative explanation for chemical contributions to surface-enhanced Raman spectroscopy for a well-studied organic molecule, benzene thiol, chemisorbed on planar Au(111) surfaces. With density functional theory calculations of the static Raman tensor, we demonstrate and quantify a strong mode-dependent modification of benzene thiol Raman spectra by Au substrates. Raman active modes with the largest enhancements result from stronger contributions from Au to their electron-vibron coupling, as quantified through a deformation potential, a well-defined property of each vibrational mode. A straightforward and general analysis is introduced that allows extraction of chemical enhancement from experiments for specific vibrational modes; measured values are in excellent agreement with our calculations.Comment: 5 pages, 4 figures and Supplementary material included as ancillary fil

Lattice dynamics and structural stability of ordered Fe3Ni, Fe3Pd and Fe3Pt alloys

We investigate the binding surface along the Bain path and phonon dispersion relations for the cubic phase of the ferromagnetic binary alloys Fe3X (X = Ni, Pd, Pt) for L12 and DO22 ordered phases from first principles by means of density functional theory. The phonon dispersion relations exhibit a softening of the transverse acoustic mode at the M-point in the L12-phase in accordance with experiments for ordered Fe3Pt. This instability can be associated with a rotational movement of the Fe-atoms around the Ni-group element in the neighboring layers and is accompanied by an extensive reconstruction of the Fermi surface. In addition, we find an incomplete softening in [111] direction which is strongest for Fe3 Ni. We conclude that besides the valence electron density also the specific Fe-content and the masses of the alloying partners should be considered as parameters for the design of Fe-based functional magnetic materials.Comment: Revised version, accepted for publication in Physical Review

Abstract. Role of shuffles and atomic disorder in Ni-Mn-Ga

We report results of ab-initio calculations of the ferromagnetic Heusler alloy Ni2MnGa. Particular emphasis is placed on the stability of the low temperature tetragonal structure with c/a = 0.94. This structure cannot be derived from the parent L21 structure by a simple homogeneous strain associated with the soft elastic constant C'. In order to stabilise the tetragonal phase, one has to take into account shuffles of atoms, which form a wave-like pattern of atomic displacements with a well defined period (modulation). While the modulation is related to the soft acoustic [110]-TA2 phonon mode observed in Ni2MnGa, we obtain additional atomic shuffles, which are related to acoustic-optical coupling of the phonons in Ni2MnGa. In addition, we have simulated an off-stoichiometric system, in which 25 % of Mn atoms are replaced by Ni. The energy of this structure also exhibits a local minimum at c/a = 0.94. This allows us to conclude that both shuffles and atomic disorder stabilise the c/a = 0.94 structure. In both cases the stability seems to be associated with a dip in the minority-spin density of states at the Fermi level, being related to the formation of hybrid states of Ni-d and Ga-p minority-spin orbitals