Single-Spin Dirac Fermion and Chern Insulator Based on Simple Oxides

Abstract

It is highly desirable to combine recent advances in the topological quantum phases with technologically relevant materials. Chromium dioxide (CrO2) is a half-metallic material, widely used in high-end data storage applications. Using first principles calculations, we show via interfacial orbital design that a novel class of half semi-metallic Dirac electronic phase emerges at the interface CrO2 with TiO2 in both thin film and superlattice configurations, with four spin-polarized Dirac points in momentum-space (k-space) band structure. When the spin and orbital degrees of freedom are allowed to couple, the CrO2/TiO2 superlattice becomes a Chern insulator without external fields or additional doping. With topological gaps equivalent to 43 Kelvin and a Chern number 2, the ensuing quantization of Hall conductance to 2e 2 /h will enable potential development of these highly industrialized oxides for applications in topologically high fidelity data storage and energy-efficient electronic and spintronic devices.The defining feature of massless Dirac Fermions [1,2] is the linear energy dispersion, implying that a quasiparticle moves at a constant speed independent of its energy, as governed by relativistic quantum mechanics. The exploration of novel Dirac materials with bulk Dirac or Dirac-Weyl states remains a great challenge in condensed matter and materials physics. [3][4][5][6][7][8][9][10] In particular, there has yet been neither theoretical proposal nor observation of materials systems with Dirac points a single spin species, which may be called a half-semimetal. Half metals are a class of ferromagnetic materials in which the conduction is dominated by one spin component while the other spin is gapped, which stands to enable reduced device size and non-volatile memory. At a rudimentary level, a half semi-metal will harbour the fascinating Dirac physics [1][2][3][4][5][6][7][8][9][10] all with a single spin. And as a not-so-trivial consequence, it may lead to Chern insulators [11][12][13][14] or valleytronics materials [15][16][17][18] when the low-energy excitations turn massive. In particular, a Chern insulator exhibits quantum anomalous Hall (QAH) effect in the absence of external magnetic field. This important fundamental phenomenon was established experimentally recently in magnetically doped topological insulator. [14] A spontaneous half semimetal will lead to a Chern insulator with appropriate spin-orbit coupling (SOC) and symmetry, which is advantageous for it requires no addition of impurity to the system. Precisely controlled growth of heterojunctions of oxides offers a unique avenue for exploring novel interfacial quantum phases, in which the interplay of orbital, charge, spin and lattice degrees of freedom gives rise to remarkably rich chemical, structural, electronic and magnetic varieties. Bulk CrO 2 (structure shown in Fig. 1) is a robust half metal even at elevated temperatures, [19][20][21] and widely used in enterprisegrade data storage. Here, we consider the possibility to fashion the CrO 2 int...

Colloquium: Topological band theory

“…(Tokura and Hwang 2008;Heber 2009) First-principles calculations indicate that the electronic structures of bulk YBiO3, BaBiO3 (Yan, Jansen, and Felser 2013), (111) bilayer of LuAlO3 , and a superlattice of CrO2/TiO2 (Cai et al 2015) are potential TI and QAH candidates. [Iridates are related materials discussed in Sec.…”

Geometrical lattice engineering of complex oxide heterostructures: a designer approach to emergent quantum states

“…Conventionally, for rather thick bulk-like films, the effect of those variations is often negligible (apart from anisotropy), whereas for the ultra-thin samples it becomes much more dominant in determining the electronic and magnetic properties. Following this idea, in the pursuit of exotic quantum states many interesting material systems have been proposed theoretically [10][11][12][13][14][15][16][17][18][19][20][21][22] , while the experimental work on GLE has been primarily focused on growth of cubic or pseudocubic (pc) (111)-oriented artificial lattices [23][24][25][26][27][28] . The rest of the paper is organized as follows.…”

First-Principles Prediction of Spin-Polarized Multiple Dirac Rings in Manganese Fluoride

“…They can also break the protection of time-reversal symmetry (TRS) but maintain the linear relation of energy-momentum dispersion at the Fermi level, resulting in outstanding transport properties. Such a type of material was theoretically proposed in a triangular ferrimagnet in recent years [24], but this novel class of structures is very rare and has only been predicted in a limited number of configurations such as CrO 2 =TiO 2 heterostructure [26], Mn-intercalated graphene [13], and the NiCl 3 monolayer [27]. Yet no Dirac half-metal has been experimentally synthesized [28].…”