Ab initio: MnBi2Te4 with non-magnetic GeBi2Te4 overlayer hosts half-integer quantum anomalous Hall phase
In a preprint calculation, a non-magnetic overlayer nearly closes the exchange gap of one Dirac cone in MnBi2Te4; the authors say this can raise the observation temperature.
An arXiv · preprint, not yet peer reviewed, reports an ab initio study of a film of the antiferromagnetic topological insulator MnBi2Te4 in contact with a non-magnetic GeBi2Te4 or Bi2Te3 overlayer. According to the authors, the junction removes the exchange splitting of one of the Dirac cones of the MnBi2Te4 (0001) surface and thereby produces a half-integer quantum anomalous Hall phase arxiv.org
arXiv · препринтarxiv.orgOpen source ↗. The work is theoretical; no measurements are reported.
Heterostructure and surface states
The magnetic part consists of 6 or 7 septuple layers of MnBi2Te4, enough to exclude interaction between its two surface states. The overlayer is a septuple-layer GeBi2Te4 or a quintuple-layer Bi2Te3 film, chosen for in-plane lattice matching and suitable work function alignment arxiv.org
arXiv · препринтarxiv.orgOpen source ↗. The structure is asymmetric, so the two MnBi2Te4 surfaces differ: one pristine, one at the interface.
For one septuple layer of GeBi2Te4 on MnBi2Te4, the calculation gives two spin-split Dirac states on opposite sides of the heterostructure. On the pristine surface the exchange splitting is 60 meV. On the interface side it drops to 2 meV, because the surface state redistributes into the non-magnetic block arxiv.org
arXiv · препринтarxiv.orgOpen source ↗. The authors attribute the remaining gap to the tail of the state in the interface MnBi2Te4 layer and to magnetization induced in the surface GeBi2Te4 block. The spectrum has no additional interface states at the Fermi level, which the authors link to the alignment of work functions of the two parts.
Hall response
Within the narrow window of the small gap on the interface side, the Hall conductivity behaves as in an axion insulator of free-standing MnBi2Te4: a zero plateau for an even number of magnetic septuple layers and a quantum anomalous Hall plateau for an odd number arxiv.org
arXiv · препринтarxiv.orgOpen source ↗. The deviation from quantization is tiny and, per the authors, is a residual finite-size effect. Outside this window, within the exchange gap of the pristine surface, the Hall conductivity approaches the half-quantized value.
For context, the half-integer effect was earlier demonstrated in "semi-magnetic" topological insulators: Cr-doped (Bi/Sb)2Se3 at 2 K and, with a magnetic overlayer in (Bi,Sb)2Se3/Cr2Ge2Te6, at up to 13 K. There, a single Dirac cone is gapped on the overlayer side. The authors state that their approach allows a higher temperature of observation and suits other layered antiferromagnetic topological insulators; they also say other overlayers and magnetic topological insulator films can be used.
