Metallic Quantized Anomalous Hall Effect without Chiral Edge States

TL;DR

This paper reports the discovery of a metallic quantum anomalous Hall effect in a topological insulator film, characterized by quantized Hall conductance without chiral edge states, due to massless Dirac cones and quantum anomaly.

Contribution

It introduces a metallic QAHE state in a magnetic heterostructure, challenging the traditional association of QAHE with chiral edge states and Chern numbers.

Findings

Hall conductance quantized to e^2/h with finite longitudinal conductance

Presence of massless Dirac cones contributing to quantum anomaly

Absence of chiral edge states in the metallic QAHE state

Abstract

The quantum anomalous Hall effect (QAHE) is a topological state of matter with a quantized Hall resistance. It has been observed in some two-dimensional insulating materials such as magnetic topological insulator films and twisted bilayer graphene. These materials are insulating in the bulk, but possess chiral edge states carrying the edge current around the systems. Here we discover a metallic QAHE in a topological insulator film with magnetic sandwich heterostructure, in which the Hall conductance is quantized to $e^{2}/h$, but the longitudinal conductance remains finite. This effect is attributed to the existence of a pair of massless Dirac cones of surface fermions, with each contributing half of the Hall conductance due to quantum anomaly. It is not characterized by a Chern number and not associated to any chiral edge states. Our study offers novel insights into topological transport phenomena and topological metallic states of matter.