Recently, Yugui Yao and Junxi Duan’s group in School of Physics, Beijing Institute of Technology, reports the effective manipulation of a colossal second-order transverse response in twisted double bilayer graphene. The result was published in Nano Letters.

The research of the Hall effects is the pearl on the crown of condensed matter physics. Numerous extraordinary break-throughs of modern physics, such as the Berry phase and topological Chern number, are inspired during exploration of the Hall effects’ quantum nature. The study of exotic Hall effects has also induced revolutions among many quantum-technology fields, for instance the next generation nano device and the quantum information. However, within the linear response regime, a system requires the breaking of time-reversal symmetry to obtain a non-zero Hall signal, namely the long-range spin order or an external magnetic field is essential. This limitation greatly hampers further research and application. Recently, theories have been proposed that within the second-order nonlinear transport regime, these could exist transverse electronic transport even in time-reversal symmetric conditions. This nonlinear effect rapidly inspired a lot of researches and discussions. Not only can it be utilized to explore the electronic states and the higher-order topological properties of Bloch band geometry, but also has great application potential among the technique fields of signal rectification and energy harvest, which has been verified by many theoretical and experimental literatures. 

Within the second-order response regime, a non-zero Hall signal does not require the breaking of time-reversal symmetry. In contrast, a second-order Hall response could arise from the inversion symmetry broken. When the time-reversal symmetry is reserved, the Berry curvature contributes nothing to the systems’ topology. But in the condition of broken inversion-symmetry, the non-equilibrium distribution of Berry curvature, namely the Berry curvature dipole will induce a second-order transport. On the other hand, within the second-order response regime, the skew-scattering would also contribute to a large nonlinear transport with certain band geometry. When applying an AC external electric field, these two main contribution mechanisms can drive electrons to perform transverse movement with double frequency. Unfortunately, before this work, the reported second-order Hall conductivities in the literatures are relatively weak, hampering the further exploration and application. Meanwhile, the discovery and manipulation of the two contribution mechanisms in a single system have been lacking, limiting the deeper research.

The Yugui Yao and Junxi Duan group has been devoted for years aiming the electronic transport property of two-dimensional topological materials. Before this work, the Junxi Duan Lab has systematically analyzed the nonlinear Hall effects in twisted bilayer graphene (TBG) devices, and reported the interface-induced Berry curvature dipole in the surface of Fe5GeTe2/AlxO3 [Junxi Duan et.al., Phys. Rev. Lett. 129, 186801 (2022); Jinrui Zhong et.al., Phys. Rev. Applied 21, 024044 (2024)]. In this work, to study the effective manipulation of the second-order nonlinear transport and achieve a colossal nonlinear transverse transport signal, the team chose high-quality AB-BA stacked twisted double bilayer graphene (TDBG). The TDBG has the following advantages: (1) It has dual-gate highly tunable topological flat bands, in which electronic states and distributions of the Berry curvature can be delicately manipulated. (2) Compared with TBG, TDBG can be influenced by the displacement field D, where the Bloch band geometry and the Fermi energy can be separately changed. (3) In certain Fermi surface and band geometry, there exist van Hove singularities and extremely high carrier mobility, greatly enhancing the nonlinear transport signal.

P1. The record high second-order nonlinear Hall conductivity in TDBG.

The two highlights of this work: (1) In TDBG, by tuning the dual-gate voltage, the team successfully achieved a record high second-order nonlinear Hall conductivity. The largest second-order nonlinear Hall conductivity is two orders of magnitude higher than those reported in literatures before, as plotted in Figure 1b. (2) In the band edge of topological flat bands of TDBG, by changing the displacement filed D, the team is the first to effectively manipulate contributions of the two main mechanism of the second-order transport. Meanwhile, the team is the first to observe the crossover of two regimes dominated by these two mechanisms respectively, as in Figure 2. This experimental phenomenon indicates that TDBG is a promising platform for the studying of nonlinear transport, greatly expanding the research fields of nonlinear effects and Bloch band topological properties.

This work has attracted intense interest before its publication. It’s arXiv version has already been reported and discussed, using a whole paragraph, by a specially invited review article from a famous scientist Ning Wang of The Hong Kong University of Science and Technology (Ning Wang et.al., Acta Phys. Sin., 72(23): 237301 (2023)).

 P2. The manipulation of the two nonlinear transport mechanism contributions in TDBG.

The research was primary done in Beijing Institute of Technology. Junxi Duan and Yugui Yao in Beijing Institute of Technology, Jinhai Mao in University of Chinese Academy of Sciences, and Jianpeng Liu in ShanghaiTech University are the corresponding authors. Jinrui Zhong, a PhD candidate in Beijing Institute of Technology, and Shihao Zhang in Hunan university are the co-first authors.

The research was supported by the National Key R&D Program of China, the National Natural Science Foundation of China, the Beijing Natural Science Foundation, and the Strategic Priority Research Program of Chinese Academy of Sciences. The fabrication was supported by Micronano Center of Beijing Institute of Technology

Jinrui Zhong#, Shihao Zhang#, Junxi Duan*, Huimin Peng, Qi Feng, Yuqing Hu, Qinsheng Wang, Jinhai Mao*, Jianpeng Liu*, and Yugui Yao*，“Effective Manipulation of a Colossal Second-Order Transverse Response in an Electric-Field-Tunable Graphene Moiré System”，Nano Lett. 24, 5791−5798 (2024)   (# the authors contributed equally, * corresponding author)

URL：https://doi.org/10.1021/acs.nanolett.4c00933