Direct measurement of ferroelectric polarization in a tunable semimetal

Sergio C De La Barrera,Benjamin M Hunt,Yang Gao,Di Xiao,Jiaqiang Yan,Yuan Gao,Vineetha S Bheemarasetty,Wenguang Zhu,David G Mandrus,Qingrui Cao

doi:10.1038/s41467-021-25587-3

Abstract

Ferroelectricity, the electrostatic counterpart to ferromagnetism, has long been thought to be incompatible with metallicity due to screening of electric dipoles and external electric fields by itinerant charges. Recent measurements, however, demonstrated signatures of ferroelectric switching in the electrical conductance of bilayers and trilayers of WTe2, a semimetallic transition metal dichalcogenide with broken inversion symmetry. An especially promising aspect of this system is that the density of electrons and holes can be continuously tuned by an external gate voltage. This degree of freedom enables measurement of the spontaneous polarization as free carriers are added to the system. Here we employ capacitive sensing in dual-gated mesoscopic devices of bilayer WTe2 to directly measure the spontaneous polarization in the metallic state and quantify the effect of free carriers on the polarization in the conduction and valence bands, separately. We compare our results to a low-energy model for the electronic bands and identify the layer-polarized states that contribute to transport and polarization simultaneously. Bilayer WTe2 is thus shown to be a fully tunable ferroelectric metal and an ideal platform for exploring polar ordering, ferroelectric transitions, and applications in the presence of free carriers.

Highlights

Ferroelectricity, the electrostatic counterpart to ferromagnetism, has long been thought to be incompatible with metallicity due to screening of electric dipoles and external electric fields by itinerant charges
Our devices each consist of a bilayer WTe2 crystal encapsulated by two hexagonal boron nitride dielectric layers, with metallic top and bottom gates, and contacts integrated into the top hBN layer[23] (Fig. 1a)
Broken inversion and mirror-z symmetries together with the ultrathin, layered structure conspire to allow layer-polarized states at the Fermi level. These broken symmetries and large spin–orbit coupling further contribute to the pronounced e–h asymmetry

Summary

Introduction

Ferroelectricity, the electrostatic counterpart to ferromagnetism, has long been thought to be incompatible with metallicity due to screening of electric dipoles and external electric fields by itinerant charges. We employ capacitive sensing in dual-gated mesoscopic devices of bilayer WTe2 to directly measure the spontaneous polarization in the metallic state and quantify the effect of free carriers on the polarization in the conduction and valence bands, separately. Combined with low carrier density and a thickness less than the out-of-plane screening length[11,20], these factors conspire to stabilize a ferroelectric metal state in bilayer WTe2 These findings are exciting given the semimetallic and tunable nature of bilayer WTe2, which enables reaching both electron and hole bands by electrostatic gating in the ferroelectric state. Plane polarization[10] but prevented independent control of the charge density and electric field This limitation precluded observing the effect of free carriers on the polarization and its dependence on carrier type and density, a fundamental open question for ferroelectric metals. The parallel-plate geometry enables this charge sensing with simultaneous and independent control of the vertical electric field and the carrier density in the bilayer by electrostatic gating

Methods

Results

Conclusion