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Register a new userNonlinear Hall Effects in Strained Twisted Bilayer WSe$_2$
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Recently, it has been pointed out that the twisting of bilayer WSe$_2$ would generate topologically non-trivial flat bands near the Fermi energy. In this work, we show that twisted bilayer WSe$_2$ (tWSe$_2$) with uniaxial strain exhibits a large nonlinear Hall (NLH) response due to the non-trivial Berry curvatures of the flat bands. Moreover, the NLH effect is greatly enhanced near the topological phase transition point which can be tuned by a vertical displacement field. Importantly, the nonlinear Hall signal changes sign across the topological phase transition point and provides a way to identify the topological phase transition and probe the topological properties of the flat bands. The strong enhancement and high tunability of the NLH effect near the topological phase transition point renders tWSe$_2$ and related moire materials new platforms for rectification and second harmonic generations.
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A finite Berry curvature dipole can induce a nonlinear Hall effect in which a charge current induces a second harmonic transverse electric voltage under time-reversal-symmetric condition. Here, we report the transport measurement of giant nonlinear Hall effect in twisted WSe$_2$ homobilayers as evidenced by the dominated second harmonic Hall voltage that scales quadratically with the injection current. Benefited from strain-induced symmetry breaking, the nonlinear Hall effects are measurable globally along all in-plane directions. At the half-filling of the hole moire superlattice band in twisted WSe$_2$ where interaction effects are strong, we observe a record high nonlinear Hall responsivity of 10$^{10}$ V W$^{-1}$. Our work demonstrates a new and highly tunable correlated system to achieve nonlinear Hall effect and provides potential device applications using artificially constructed van der Waals superlattices.
Recent studies have shown that moir{e} flat bands in a twisted bilayer graphene(TBG) can acquire nontrivial Berry curvatures when aligned with hexagonal boron nitride substrate [1, 2], which can be manifested as a correlated Chern insulator near the 3/4 filling [3, 4]. In this work, we show that the large Berry curvatures in the moir{e} bands lead to strong nonlinear Hall(NLH) effect in a strained TBG with general filling factors. Under a weak uniaxial strain $sim 0.1%$, the Berry curvature dipole which characterizes the nonlinear Hall response can be as large as $sim$ 200{AA}, exceeding the values of all previously known nonlinear Hall materials [5-14] by two orders of magnitude. The dependence of the giant NLH effect as a function of electric gating, strain and twist angle is further investigated systematically. Importantly, we point out that the giant NLH effect appears generically for twist angle near the magic angle due to the strong susceptibility of nearly flat moir{e} bands to symmetry breaking induced by strains. Our results establish TBG as a practical platform for tunable NLH effect and novel transport phenomena driven by nontrivial Berry phases.
Twisted bilayer graphene provides a new two-dimensional platform for studying electron interaction phenomena and flat band properties such as correlated insulator transition, superconductivity and ferromagnetism at certain magic angles. Here, we present strong evidence of correlated insulator states and superconductivity signatures in p-type twisted double-bilayer WSe$_2$. Enhanced interlayer interactions are observed when the twist angle decreases to a few degrees as reflected by the high-order satellites in the electron diffraction patterns taken from the 2H/3R-stacked domains reconstructed from a conventional Moire superlattice. In contrast to twisted bilayer graphene, there is no specific magic angle for twisted WSe$_2$. The flat band properties are observed at twist angles ranging from 1 to 4 degrees. The highest superconducting transition temperature observed by transport measurement is 6 K. Our work has facilitated future study in the area of flat band related properties in twisted transition metal dichalcogenide layered structures.
The quantum spin Hall (QSH) effect, characterized by topologically protected spin-polarized edge states, was recently demonstrated in monolayers of the transition metal dichalcogenide (TMD) 1T-WTe$_2$. However, the robustness of this topological protection remains largely unexplored in van der Waals heterostructures containing one or more layers of a QSH insulator. In this work, we use scanning tunneling microscopy and spectroscopy (STM/STS), to study twisted bilayer (tBL) WTe$_2$ with three different orientations and compare it to a topologically trivial as-grown bilayer. We observe the characteristic spectroscopic signature of the QSH edge state in the twisted bilayers, including along a coinciding edge where two sets of QSH edge states sit on top of the other. By comparing our experimental observations to first principles calculations, we conclude that the twisted bilayers are weakly coupled, preserving the QSH states and preventing back scattering.
We investigate the steady-state out-of-plane spin diffusion in p-type bilayer WSe2 in the presence of the Rashba spin-orbit coupling and Hartree-Fock effective magnetic field. The out-of-plane components of the Rashba spin-orbit coupling serve as the opposite Zeeman-like fields in the two valleys. Together with the identical Hartree-Fock effective magnetic fields, different total effective magnetic field strengths in the two valleys are obtained. It is further revealed that due to the valley-dependent total effective magnetic field strength, similar (different) spin-diffusion lengths in the two valleys are observed at small (large) spin injection. Nevertheless, it is shown that the intervalley hole-phonon scattering can suppress the difference in the spin-diffusion lengths at large spin injection due to the spin-conserving intervalley charge transfers with the opposite transfer directions between spin-up and -down holes. Moreover, with a fixed large pure spin injection, we predict the build-up of a steady-state valley polarization during the spin diffusion with the maximum along the diffusion direction being capable of exceeding 1 %. It is revealed that the valley polarization arises from the induced quasi hot-hole Fermi distributions with different effective hot-hole temperatures between spin-up and -down holes during the spin diffusion, leading to the different intervalley charge transfer rates in the opposite transfer directions. Additionally, it is also shown that by increasing the injected spin polarization, the hole density or the impurity density, the larger valley polarization can be obtained.
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