Electrically switchable Berry curvature dipole in the monolayer topological insulator WTe2

Abstract

Recent experimental evidence for the quantum spin Hall (QSH) state in monolayer WTe 2 has linked the fields of twodimensional materials and topological physics [1][2][3][4][5][6][7] . This twodimensional 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 WTe 2 , 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 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 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 WTe 2 . 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 . One of the early landmarks of condensed matter physics was the classification of metals, insulators and semiconductors by studying the energy-momentum dispersion-or band structure-of the electrons in crystalline solids. Despite this remarkable success, the quantum nature of the electron states means that they can only be fully described by their quantum wavefunctions, whereas the band structure concerns only the energy and momentum eigenvalues of the wavefunctions. Therefore, a central question in modern condensed matter physics is whether there exist new phenomena that arise from other properties of quantum wavefunctions beyond the band structure 9 . For instance, the study of the global-or topological-properties of electronic wavefunctions continues to give rise to novel topological phases, including the QSH states, three-dimensional topological insulators and Weyl semimetals. These topological materials feature robust surface states and often exhibit protected transport and optical responses. Another direction is to study the local-or geometrical-properties of wavefunctions. One important property is the local curvature of the wavefunction, defined as the Berry curvature. Originally employed to explain the anomalous Hall conductivities of ferromagnets 9 , the importance of the Berry curvature is increasingly recognized in a wide range of areas in condensed matter physics, including nonlocal transport and chiral optical responses in noncentrosymmetric metals and semiconductors [10][11][12][15][16][17][18][19][20] , unconventional pairing in superconductors 21 , and topological plasmonic and excitonic ...

Prediction of ferroelectricity-driven Berry curvature enabling charge- and spin-controllable photocurrent in tin telluride monolayers

“…[16][17][18][19] Under out-of-equilibrium electron distributions, the BC dipole can enable nonlinear optoelectronic transport, which has been realized in photogalvanic experiments. 20,21 Since level crossing can generate a singular BC distribution, topological materials that possess small inverted band gaps or band crossings have been mostly studied as efficient platforms for hosting a large BC dipole. [20][21][22][23][24][25][26] For instance, a small-gap quantum spin Hall WTe2 monolayer shows a large inter-band BC and its dipole is manipulated by an external electric field, resulting in the circular photogalvanic effect.…”

Spontaneous gyrotropic electronic order in a transition-metal dichalcogenide

“…The in-plane CPGE under oblique or normal incidence has been studied in a wide range of materials [31][32][33][52][53][54][55][56][57][58][59]. Any intrinsic CPGE requires the material to be inversion breaking [31][32][33][49][50][51][52][53][54][55][56][57][58][59]. [50] Hosur, P. Circular photogalvanic effect on topological insulator surfaces: Berry-curvature- There are three inequivalent L points (L 1 , L 2 and L 3 ).…”

Berry Curvature Dipole in Strained Graphene: A Fermi Surface Warping Effect

“…Single-layer transition metal dichalcogenides MX 2 (M=Mo,W and X=S, Se, Te) [10] have been theoretically predicted [11,12] and experimentally verified [13][14][15] as material platforms supporting large Berry curvature dipoles. These materials possess large spin-orbit coupling and lack of inversion symmetry in their T d structure.…”