Alexey T. Zayak

Modelling the phase diagram of magnetic shape memory Heusler alloys

We have modelled the phase diagram of magnetic shape memory alloys of the Heusler type by using the phenomenological Ginzburg–Landau theory. When fixing the parameters by realistic values taken from experiment we are able to reproduce most details of, for example, the phase diagram of Ni2+xMn1−xGa in the (T, x) plane. We present the results of ab initio calculations of the electronic and phonon properties of several ferromagnetic Heusler alloys, which allow one to characterize the structural changes associated with the martensitic instability leading to the modulated and tetragonal phases. From the ab initio investigations emerges a complex pattern of the interplay of magic valence electron per atom numbers (Hume–Rothery rules for magnetic ternary alloys), Fermi surface nesting and phonon instability. As the main result, we find that the driving force for structural transformations is considerably enhanced by the extremely low lying optical modes of Ni in the Ni-based Heusler alloys, which interfere with the acoustical modes enhancing phonon softening of the TA2 mode. In contrast, the ferromagnetic Co-based Heusler alloys show no tendency for phonon softening.

Chemical Raman Enhancement of Organic Adsorbates on Metal Surfaces

Using first-principles theory and experiments, chemical contributions to surface-enhanced Raman spectroscopy for a well-studied organic molecule, benzene thiol, chemisorbed on planar Au(111) surfaces are explained and quantified. Density functional theory calculations of the static Raman tensor demonstrate 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 straightforward and general analysis is introduced to extract chemical enhancement from experiments for specific vibrational modes; measured values are in excellent agreement with our calculations.

Anomalous vibrational effects in non-magnetic and magnetic Heusler alloys

First-principles calculations are used in order to investigate phonon anomalies in non-magnetic and magnetic Heusler alloys. Phonon dispersions for several systems in their cubic L2

First-principles investigations of homogeneous lattice-distortive strain and shuffles in Ni2MnGa

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e∕a dependence of the lattice instability of cubic Heusler alloys from first principles

structure were obtained along the [110] direction. We consider compounds which exhibit phonon instabilities and compare them with their stable counterparts. The analysis of the electronic structure allows us to identify the characteristic features leading to structural instabilities. The phonon dispersions of the unstable compounds show that, while the acoustic modes tend to soften, the optical modes disperse in a way which is significantly different from that of the stable structures. The optical modes that appear to disperse at anomalously low frequencies are Raman active, which is considered an indication of a stronger polarizability of the unstable systems. We show that phonon instability of the TA

Structural, electronic, and magnetic properties of Sr Ru O 3 under epitaxial strain

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Shape Memory Alloys: A Summary of Recent Achievements

mode in Heusler alloys is driven by interaction(repulsion) with the low energy optical vibrations. The optical modes show their unusual behavior due to covalent interactions which are additional bonding features incommensurate with the dominating metallicity in Heusler compounds.

Transferable pair potentials for CdS and ZnS crystals.

A series of first-principles calculations were performed for ferromagnetic Ni2MnGa using density functional theory and PAW potentials. Theoretically, a tetragonal crystal structure homogeneous lattice-distortive strain is stabilized around c/a = 0.94 with respect to the L21 structure when, in addition, modulation shuffles with a period of five atomic planes are taken into account. This is in agreement with the observed structures in experimental works. The modulation appears to be critically important for stability of the tetragonal structure with c/a < 1. Here, we report a new feature which is related to the optimum amplitudes of the modulation in different atomic planes. Related to this are systematic changes in the minority spin density of states near the Fermi surface, like in the formalism of a pseudo-gap.

Harnessing Chemical Raman Enhancement for Understanding Organic Adsorbate Binding on Metal Surfaces.

First-principles calculations are used to investigate chemical trends in the structural instability of the high-symmetry cubic structure for a selected set of ferromagnetic Heusler compounds. The role of the valence-electron-to-atom ratio e∕a is investigated both by comparing compounds of different compositions, and by artificially changing the electron density in the first-principles calculations. Systematic trends in the interatomic force constant, previously found to be an indicator of the structural instability, are identified and discussed.

A Ginzburg-Landau Theory For Ni-Mn-Ga

Using density functional theory within the local spin density approximation, the structural, electronic, and magnetic properties of