Ferroelectric nonlinear anomalous Hall effect in few-layer WTe2

Xiaofeng Qian,Hua Wang

doi:10.1038/s41524-019-0257-1

Abstract

Under broken time reversal symmetry such as in the presence of external magnetic field or internal magnetization, a transverse voltage can be established in materials perpendicular to both longitudinal current and applied magnetic field, known as classical Hall effect. However, this symmetry constraint can be relaxed in the nonlinear regime, thereby enabling nonlinear anomalous Hall current in time-reversal invariant materials – an underexplored realm with exciting new opportunities beyond classical linear Hall effect. Here, using group theory and first-principles theory, we demonstrate a remarkable ferroelectric nonlinear anomalous Hall effect in time-reversal invariant few-layer WTe2 where nonlinear anomalous Hall current switches in odd-layer WTe2 except 1T′ monolayer while remaining invariant in even-layer WTe2 upon ferroelectric transition. This even-odd oscillation of ferroelectric nonlinear anomalous Hall effect was found to originate from the absence and presence of Berry curvature dipole reversal and shift dipole reversal due to distinct ferroelectric transformation in even and odd-layer WTe2. Our work not only treats Berry curvature dipole and shift dipole on an equal footing to account for intraband and interband contributions to nonlinear anomalous Hall effect, but also establishes Berry curvature dipole and shift dipole as new order parameters for noncentrosymmetric materials. The present findings suggest that ferroelectric metals and Weyl semimetals may offer unprecedented opportunities for the development of nonlinear quantum electronics.

Highlights

In classical linear Hall effect, a transverse voltage can be developed in materials with broken time-reversal symmetry only due to Onsager’s relation
Monolayer 1T′ WTe2 is centrosymmetric with vanishing evenorder nonlinear current response, vertical electric field can break its two-fold screw rotation symmetry, generate Berry curvature dipole (BCD), and induce second-order nonlinear anomalous Hall current.[5,6,7,8]
In contrast to monolayer WTe2, bilayer WTe2 is naturally noncentrosymmetric due to the loss of two-fold screw rotation symmetry, resulting in intrinsic nontrivial BCD in bilayer WTe2.9–11 Surprisingly, ferroelectric switching was recently discovered in semimetallic bilayer and few-layer WTe2,21 quite unusual as ferroelectricity and semimetallicity normally do not coexist in the same material.[22]

Summary

Introduction

In classical linear Hall effect, a transverse voltage can be developed in materials with broken time-reversal symmetry only (e.g. in the presence of external magnetic field or internal magnetization) due to Onsager’s relation. PE state of trilayer WTe2 has C2h point group, its out-of-plane polarization vanishes due to inversion symmetry.

Results

Conclusion