Title: Electronic structure of topological materials and quasicrystals

Speaker: Dr. Jayita Nayak, UC Davis

Abstract: The discovery of topological materials has created tremendous attention in the scientific community and beyond because these materials can exhibit a novel state of matter in which topologically protected gapless states on their edges or surfaces are present. Such states are protected from scattering by impurities and thus have important technological applications. The rare-earth monopnictide LaBi, LaSb exhibits very large, unusual magnetoresistance which stimulates the interest in directly observing any topological surface states. Although band inversions have been postulated to induce a topological phase in LaBi but there was no experimental evidence for topological surface states in this compound. By using angle-resolved photoemission spectroscopy (ARPES) and ab-initio calculations, we have revealed the existence of topological surface states of LaBi through the observation of three Dirac cones: two coexist at the corners and one appears at the centre of the Brillouin zone. The odd number of surface Dirac cones is a direct proof of the topological nature of LaBi compound. Moreover, our ARPES measurement in conjugation with band structure calculation established the Type II Dirac semi metallic phase in PtSe2. In my recent work, six fold degeneracy has been observed at the crossing point in PdSb2 compound in contrast to Dirac and Weyl semimetals.

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 Fermi edge (EF) spectra of Al-Pd-Mn as well as Al-Cu-Fe are strongly suppressed in intensity at EF, 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 EFis zero indicating that it is close to a metal-insulator phase boundary. The existence of deeper pseudogap in bulk has been supported by the theoretical calculations. On the other hand, the HAXPES near EF 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. In this context,I will discuss about my future research.