The existence of three fold rotational, mirror and time reversal symmetries often give rise to the triply degenerate nodal point (TP) in the band structure of a material. Based on point group symmetry analysis and first principle electronic structure, we predict, in this article, a series of quaternary Heusler alloys host an ideal platform for the occurrence of TP. We simulated, the projection of these TPs onto the (111) and (100) surfaces lead to form topological Fermi arcs, which may further be detected by scanning tunneling spectroscopy and angle resolved photoemission spectroscopy. These Fermi arcs arise due to the symmetry protected band degeneracies, which are robust and can not be avoided due to the non-trivial band topology. Interestingly the TPs, in these class of Heusler alloys are far away from the Γ point along C3 axes, which allow to overcome the experimental difficulties over previously studied hexagonal and HgTe-type compounds.

In this article, we predict the emergence of non-trivial band topology in the family of XX′Bi compounds having (# 189) space group. Using first principles calculations within hybrid functional framework, we demonstrate that NaSrBi and NaCaBi are strong topological insulator under controlled band engineering. Here, we propose three different ways to engineer the band topology to get a non-trivial order: (i) hydrostatic pressure, (ii) biaxial strain (due to epitaxial mismatch), and (iii) doping. Non-triviality is confirmed by investigating bulk band inversion, topological Z2 invariant, surface dispersion and spin texture. Interestingly, some of these compounds also show a three dimensional topological nodal line semi-metal (NLS) state in the absence of spin orbit coupling (SOC). In these NLS phases, the closed loop of band degeneracy in the Brillouin zone lie close to the Fermi level. Moreover, a drumhead like flat surface state is observed on projecting the bulk state on the [001] surface. The inclusion of SOC opens up a small band gap making them behave like a topological insulator.

In this letter, we show the existence of Dirac like excitations in the elemental noble metal Ru, Re and Os based on symmetry analysis, first principle calculations and angle resolved photoemission (ARPES) experiment. This is the first report where unique Dirac surface states driven Fermi arcs are identified in Ru by ab-initio calculations, which are further confirmed by ARPES. We attribute these Dirac excitation mediated Fermi arc topology to be the possible reasons behind several existing transport anomalies, such as large non-saturating magneto resistance, anomalous Nernst electromotive force and its giant oscillations, magnetic breakdown etc. We further show that the Dirac like excitations in these elemental metal can further be tuned to three component Fermionic excitations, using symmetry allowed alloy mechanism for the binary alloys such as RuOs, ReOs and RuRe. arXiv:1910.00196v1 [cond-mat.mtrl-sci]

We report a van der Waals heterostructure based graphene–stanene–graphene (C–Sn–C) quantum wall (QW), which shows a topologically protected hybrid state in the graphene–stanene interface.