Electrically switchable Berry curvature dipole in the monolayer topological insulator WTe2

Recent experimental evidence for the quantum spin Hall (QSH) state in monolayer WTe2 has linked the fields of two-dimensional materials and topological physics(1-7). This two-dimensional topological crystal also displays unconventional spin-torque(8) and gate-tunable superconductivity(7). Whereas the realization of the QSH has demonstrated the nontrivial topology of the electron wavefunctions of monolayer WTe2, the geometrical properties of the wavefunction, such as the Berry curvature(9), remain unstudied. Here we utilize mid-infrared optoelectronic microscopy to investigate the Berry curvature in monolayer WTe2. 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 allows further control of the direction and magnitude of the photocurrent. The observed photocurrent reveals a Berry curvature dipole that arises from the nontrivial wavefunctions near the inverted gap edge. The Berry curvature dipole and strong electric field effect are enabled by the inverted band structure and tilted crystal lattice of monolayer WTe2. Such an electrically switchable Berry curvature dipole may facilitate the observation of a wide range of quantum geometrical phenomena such as the quantum nonlinear Hall(10,11), orbital-Edelstein(12) and chiral polaritonic effects(13,14).