Topological phase transitions of (BixSb1-x)2Se3 alloys by density functional theory

Topological Phases in Graphene Nanoribbons Tuned by Electric Fields

Graphene nanoribbons (GNRs) possess distinct symmetry-protected topological phases. We show, through first-principles calculations, that by applying an experimentally accessible transverse electric field, certain boron and nitrogen periodically codoped GNRs have tunable topological phases. The tunability arises from a field-induced band inversion due to an opposite response of the conduction- and valence-band states to the electric field. With a spatially varying applied field, segments of GNRs of distinct topological phases are created, resulting in a field-programmable array of topological junction states, each may be occupied with charge or spin. Our findings not only show that electric field may be used as an easy tuning knob for topological phases in quasi-one-dimensional systems, but also provide new design principles for future GNR-based quantum electronic devices through their topological characters.

Interplay between magnetic, metal/insulator and topological phases in Hg1-x Mn x Te alloys: prediction of a ferromagnetic Weyl semimetal at x = 0.25

We report a theoretical investigation of magnetic, electronic, and topological properties of Hg_1-x Mn _x Te alloys. We consider periodic structures with Mn concentrations as x  =  0, 0.25, 0.5, 0.75, and 1. Our hybrid DFT/Hartree-Fock calculations for the bandgaps of antiferromagnetic (ground-state) phases are in good agreement with experiments. The calculations also show that the modification of the magnetic ordering from anti- to ferromagnetic leads to a significant bandgap reduction, resulting in a metal/insulator transition at higher Mn concentrations. We show that a ferromagnetic Weyl semimetal phase is achieved for x  =  0.25, where a single pair of Weyl nodes mirrored by the [Formula: see text] point in the momentum space is observed. The non-trivial topological property of the ferromagnetic Hg_0.75Mn_0.25Te is confirmed by the calculation of the chirality of each Weyl node, which are connected by a surface Fermi arc of a semi infinite Hg_0.75Mn_0.25Te.

Controlling Topological States in Topological/Normal Insulator Heterostructures

We have performed a systematic investigation of the nature of the nontrivial interface states in topological/normal insulator (TI/NI) heterostructures. On the basis of first principles and a recently developed scheme to construct ab initio effective Hamiltonian matrices from density functional theory calculations, we studied systems of realistic sizes with high accuracy and control over the relevant parameters such as TI and NI band alignment, NI gap, and spin-orbit coupling strength. Our results for IV-VI compounds show the interface gap tunability by appropriately controlling the NI thickness, which can be explored for device design. Also, we verified the preservation of an in-plane spin texture in the interface-gaped topological states.