Electronic structure of icosahedral quasicrystals and ferromagnetic shape memory alloy

Jayita Nayak, UGC-DAE Consortium for Scientific Research, Khandwa Road, Indore 452001, India

ABSTRACT
We have used hard x-ray photoelectron spectroscopy (HAXPES) to investigate the bulk electronic structure of quasicrystals to resolve the controversy about the existence of pseudogap in these materials. The near E_F spectra of Al-Pd-Mn as well as Al-Cu-Fe are strongly suppressed in intensity at E_F, which provides strong evidence for the existence of the pseudogap in contrast to surface sensitive low energy photoemission spectra that shows a metallic Fermi edge. From fitting of the spectra by inverted Lorenzian function the evidence of fully formed pseudogap in Al-Cu-Fe demonstrate that the DOS at E_F is zero indicating that it is close to a metal-insulator phase boundary[1]. The existence of deeper pseudogap in bulk has been supported by the theoretical calculations. On the other hand, the HAXPES near E_F spectrum of both Zn-Mg-Dy and Zn-Mg-Y exhibits shallower pseudogap compared to the Al based quasicrystals (Al-Pd-Mn and Al-Cu-Fe) that may be related to their larger carrier concentration compared to Al-Pd-Mn. Although, semiconducting quasicrystals demonstrate a deeper pseudogap but no evidence of opening up of a real gap has been observed for both the quasicrystals.

Electronic structure and morphology of ferromagnetetic shape memory alloy Ni₂MnGa(100) has been studied by photoemission and scanning probe techniques. Our UPS demonstrated the existence of Charge density Wave (CDW) through the appearance of a pseudogap at the Fermi level (E_F) in the premartensite phase. In the martensite phase, the LEED spot profiles show presence of an incommensurate modulation for the stoichiometric surface. STM measurement in the premartensite phase at 230 K exhibit the evidence of twinned morphology and reveals the presence of parallel bands that are perpendicular to the twin boundary on one of the twin bands. In the premartensite phase, another type of nanostructure is observed in the flat regions of the surface where parallel bands are observed on both sides of a boundary and the parallel bands are non-periodic. On the other hand, in the martensite phase, equispaced parallel bands with separation of 1.5 nm are observed. The parallel bands in the premartensite as well as in the martensite phase possibly indicate presence of a CDW state[2].

1 J. Nayak et al, Phys. Rev. Lett. 109, 216403 (2012).
2 J. Nayak et al, (manuscript under preparation).