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Register a new userGiant nonlinear Hall effect in twisted WSe$_2$
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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.
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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.
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.
While the anomalous Hall effect can manifest even without an external magnetic field, time reversal symmetry is nonetheless still broken by the internal magnetization of the sample. Recently, it has been shown that certain materials without an inversion center allow for a nonlinear type of anomalous Hall effect whilst retaining time reversal symmetry. The effect may arise from either Berry curvature or through various asymmetric scattering mechanisms. Here, we report the observation of an extremely large $c$-axis nonlinear anomalous Hall effect in the non-centrosymmetric T$_d$ phase of MoTe$_2$ and WTe$_2$ without intrinsic magnetic order. We find that the effect is dominated by skew-scattering at higher temperatures combined with another scattering process active at low temperatures. Application of higher bias yields an extremely large Hall ratio of $E_perp /E_parallel$=2.47 and corresponding anomalous Hall conductivity of order 8x10$^7$S/m.
Lattice reconstruction in twisted transition-metal dichalcogenide (TMD) bilayers gives rise to piezo- and ferroelectric moire potentials for electrons and holes, as well as a modulation of the hybridisation across the bilayer. Here, we develop hybrid $mathbf{k}cdot mathbf{p}$ tight-binding models to describe electrons and holes in the relevant valleys of twisted TMD homobilayers with parallel (P) and anti-parallel (AP) orientations of the monolayer unit cells. We apply these models to describe moire superlattice effects in twisted WSe${}_2$ bilayers, in conjunction with microscopic emph{ab initio} calculations, and considering the influence of encapsulation, pressure and an electric displacement field. Our analysis takes into account mesoscale lattice relaxation, interlayer hybridisation, piezopotentials, and a weak ferroelectric charge transfer between the layers, and describes a multitude of possibilities offered by this system, depending on the choices of P or AP orientation, twist angle magnitude, and electron/hole valley.
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.
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