Giant nonlinear Hall effect in twisted bilayer WSe2

Abstract

The recently discovered nonlinear Hall effect (NHE) in a few non-interacting systems provides a novel mechanism for generating second harmonic electrical Hall signals under time-reversal-symmetric conditions. Here, we introduce a new approach to engineering NHE by using twisted moiré structures. We found that the twisted WSe2 bilayer exhibited a NHE when the Fermi level was tuned to the moiré flat bands. When the first moiré band was half-filled, the nonlinear Hall signal exhibited a sharp peak with a generation efficiency that was at least two orders of magnitude greater than those obtained in previous experiments. We discuss the possible origins of the diverging generation efficiency in twisted WSe2 based on resistivity measurements, such as moiré interface induced correlation effects and mass-diverging type continuous Mott transition. This study demonstrates not only how interaction effects can combine with Berry curvature dipoles to produce novel quantum phenomena, but also the potential of NHE measurements as a new tool for studying quantum criticality.

Ferroic Berry Curvature Dipole in a Topological Crystalline Insulator at Room Temperature

“…It is also noted that the magnitude is 2 orders of magnitude larger than the theoretically predicted value for SnTe . However, we emphasize that similar unprecedentedly large BCDs have been frequently reported in previous studies using twisted double bilayer graphene and twisted bilayer WSe 2 . , One possible origin of such discrepancies stems from extrinsic mechanisms, such as skew scattering and side-jumps. − However, it should be emphasized that the contribution of skew scattering to nonlinear transversal voltage generation reported in refs and can be neglected in our system because it is observed in a symmetry-protected system that enables suppression of the BCD and is considered to be independent of the switching of ferroelectric distortion. Regarding the other extrinsic contributions, although the scaling law has been proposed as a potential approach to distinguish the physical origins of the NLHE, there is a subtlety about whether the scaling law is obtained in the PST because the control of conductivity or disorders of PST is not easy without changing its ferroelectric nature and lattice inversion symmetry.…”

“…In particular, the Berry curvature dipole (BCD), which is the first-order moment of Berry curvature, has attracted a great deal of attention because of its production of nonlinear transport. A good example of such nonlinear transport is the nonlinear Hall effect (NLHE), the second-order Hall response, which appears in material systems with inversion symmetry breaking, such as T d phase transition metal dichalcogenides (TMDs), 1−4 strained TMD, 5 Moirésystems, 6,7 and monolayer ferroelectric materials. 8,9 The BCD along an arbitrary in-plane α direction, D α , in a twodimensional system is expressed as follows:…”

“…In particular, the Berry curvature dipole (BCD), which is the first-order moment of Berry curvature, has attracted a great deal of attention because of its production of nonlinear transport. A good example of such nonlinear transport is the nonlinear Hall effect (NLHE), the second-order Hall response, which appears in material systems with inversion symmetry breaking, such as T d phase transition metal dichalcogenides (TMDs), − strained TMD, Moiré systems, , and monolayer ferroelectric materials. , The BCD along an arbitrary in-plane α direction, D α , in a two-dimensional system is expressed as follows: D α = prefix∫ normald 2 bold-italick false( 2 π false) 2 .25em ∂ f 0 ∂ k α normalΩ ( k ) where k is a wave vector, k α is a wave vector along α, f 0 is the Fermi distribution function, and Ω( k ) is the Berry curvature. Under the time-reversal symmetric condition, D α is zero in centrosymmetric materials.…”

Giant Berry curvature dipole density in a ferroelectric Weyl semimetal

“…Moore and Orenstein 3 realized that the cancellation of Berry curvature contribution, which is enforced by time reversal symmetry, can be removed by driving the system into non-equilibrium state, i.e., external electric field (E). In accord with this view, a type of Hall effect with no external magnetic field, namely the nonlinear Hall effect (NLHE), was proposed 4 and experimentally verified 5,6 , and more recently observed in various materials without inversion symmetry, including artificially symmetry-broken or interface systems [7][8][9][10][11][12][13][14][15] . As opposed to the AHE described by the anomalous Hall conductivity, the NLHE is described by the second-order nonlinear susceptibility χ, which is in proportion to the Berry curvature dipole (D) defined as R k f 0 ∂Ω ∂k with the Berry curvature Ω and the Fermi distribution function f 0 4 .…”

Novel electrical properties of moiré graphene systems

“…我们相信在莫尔超晶格体系中的研究不会局 限于石墨烯体系, 而是会延伸到包含过渡金属二硫 化物 [207][208][209][210][211][212][213][214][215][216][217][218][219] 、二维磁性半导体 [220][221][222][223] 、石墨烯-绝缘 体异质结 [87,190,191] 等一系列的莫尔超晶格异质结体 系. 实验上已经成功制备了一系列过渡金属二硫化 物的莫尔体系, 并观测到了非常规莫尔激子 [207][208][209][210] 、 关联绝缘态 [211][212][213][214] 、轨道铁磁态 [211] 和非线性霍尔 效应 [215,224] 等一系列新奇现象. 特别地, Mak等 [214] 在MoTe 2 /WSe 2 的二维莫尔超晶格异质结体系中 发现了部分填充下的量子反常霍尔效应, 并通过光 学测量指出这是一个新奇的谷间相干的量子反常 霍尔态 [225] .…”