The temperature evolution of the electronic structure of a Ni-Fe(Co)-Ga/MgO(100), Heusler-type, ferromagnetic shape-memory alloy thin film has been followed by a bulk-sensitive hard x-ray photoelectron spectroscopy, element-selective soft x-ray magnetic circular dichroism, and first-principles calculation. The reversible changes of the electronic states near the Fermi energy show a hysteresis associated with the martensitic phase transition (MPT), where the pseudogap opens on cooling and closes again on warming. In addition, the Ni $3d$ spin magnetic moment increases by approximately two times across the MPT, whereas the change of Fe $3d$ moment is moderate. By comparing the experimental results with the calculated spin-resolved density of states, we conclude that the band Jahn-Teller effect of Ni $3d$ and Fe $3d$ orbitals is responsible for MPT.

Trabajo presentado a la 16th International Conference on Solid Films and Surfaces celebrada en Genova (Italia) del 1 al 6 de julio de 2012.

The C_2v symmetry of the W(110) surface influences strongly the spin-polarized Dirac-cone-like surface state within a spin-orbit-induced symmetry gap. We present a detailed angle-resolved photoemission study with s- and p-polarized light along three different symmetry lines. The Dirac-cone-like feature appears along Γ̅H̅ and Γ̅S̅, while it is strongly deformed along Γ̅N̅. A two-fold Σ_3 symmetry of the d-type surface state is identified from photoemission experiments using linear polarized light. Our results are well described by model calculations based on an effective Hamiltonian with C_2v symmetry including Rashba parameters up to third order. The flattened Dirac cone of the surface state is caused by hybridization with bulk continuum states of Σ_1 and Σ_2 symmetry. The spin texture of this state obtained from the model calculations shows a quasi-one dimensional behavior. This finding opens a new avenue in the study of d-electron-based persistent spin helix systems and/or weak topological insulators.