Topological band dispersions other than the standard Dirac or Weyl fermions have garnered the increasing interest in condensed matter physics. Among them, the cubic Dirac fermions were recently proposed in the family of quasi-one-dimensional conductors A 2 Mo 6 X 6 (A= Na, K, In, Tl; X= S, Se, Te), where the band crossing is characterized by a linear dispersion in one k-space direction but the cubic dispersion in the plane perpendicular to it. It is not yet clear, however, how the external perturbations can alter these nontrivial carriers and ultimately induce a new distinct quantum phase. Here we study the evolution of Dirac fermions, in particular the cubic Dirac crossing, under external pressure in the representative quasi-one-dimensional Tl 2 Mo 6 Se 6 via the first-principles calculations. Specifically, it is found that the topological properties, including the bulk Dirac crossings and the topological surface states, change progressively under pressure up to 50 GPa where it undergoes a structural transition from the hexagonal phase to body-centered tetragonal phase. Above 50 GPa, the system is more likely to be topologically trivial. Further, we also investigate its phonon spectra, which reveals a gradual depletion of the negative phonon modes with pressure, consistent with the more three-dimensional Fermi surface in the high-pressure phase.Our work may provide a useful guideline for further experimental search and the band engineering of the topologically nontrivial fermions in this intriguing state of matter.

Insulating ferromagnets with high T C are required for many new magnetic devices.More complexity arises when strongly correlated 3d ions coexist with strongly spin-orbit coupled 5d ones in a double perovskite. Here, we perform the structural, magnetic, and density functional theory (DFT) study of such double perovskite La 2 CuIrO 6 . A new P2 1 /n polymorph is found according to the comprehensive analysis of x-ray, Raman scattering and phonon spectrum. The magnetization reveals a weak ferromagnetic (FM) transition at T C = 62 K and short range FM order in higher temperature range. A huge coercivity is found as high as H C~1 1.96 kOe at 10K, which, in combination with the negative trapped field, results in the magnetization reversal in the zero field cooling measurement. The first principle calculations confirm the observed FM state and suggest La 2 CuIrO 6 of this polymorph is a Mott insulating ferromagnet assisted by the spin-orbit coupling.