Electrically switchable Berry curvature dipole in the monolayer topological insulator WTe2

TL;DR

This paper reports the discovery of an electrically switchable Berry curvature dipole in monolayer WTe2, revealing new quantum geometrical effects enabled by its unique band structure and lattice, with potential applications in nonlinear Hall and chiral phenomena.

Contribution

It demonstrates the first observation of a controllable Berry curvature dipole in monolayer WTe2 using mid-infrared microscopy, linking wavefunction topology to optoelectronic responses.

Findings

Observation of in-plane circular photogalvanic current under normal incidence.

Electric field tuning of the Berry curvature dipole's magnitude and direction.

Identification of a novel Berry curvature dipole arising from the inverted band structure.

Abstract

Recent experimental evidence for the quantum spin Hall (QSH) state in monolayer WTe$_2$ has bridged two of the most active fields of condensed matter physics, 2D materials and topological physics. This 2D topological crystal also displays unconventional spin-torque and gate-tunable superconductivity. While the realization of QSH has demonstrated the nontrivial topology of the electron wavefunctions of monolayer WTe$_2$, the geometrical properties of the wavefunction, such as the Berry curvature, remain unstudied. On the other hand, it has been increasingly recognized that the Berry curvature plays an important role in multiple areas of condensed matter physics including nonreciprocal electron transport, enantioselective optical responses, chiral polaritons and even unconventional superconductivity. Here we utilize mid-infrared optoelectronic microscopy to investigate the Berry curvature in monolayer WTe$_2$. By optically exciting electrons across the inverted QSH gap, we observe an in-plane circular photogalvanic current even under normal incidence. The application of an out-of-plane displacement field further systematically controls the direction and magnitude of the photocurrent. Our observed photocurrent reveals a novel Berry curvature dipole that arises from the nontrivial wavefunctions near the inverted gap edge. These previously unrealized Berry curvature dipole and strong electric field effect are uniquely enabled by the inverted band structure and tilted crystal lattice of monolayer WTe$_2$. Such an electrically switchable Berry curvature dipole opens the door to the observation of a wide range of quantum geometrical phenomena, such as quantum nonlinear Hall, orbital-Edelstein and chiral polaritonic effects.