Pan He

Low-symmetry materials unlock rich Berry curvature physics and anomalous transport phenomena that are forbidden in high-symmetry quantum systems through rigorous crystalline symmetry constraints. Current approaches rely on external fields or complex heterogeneous stacking to break crystalline symmetries, which hinders scalable device integration. In this work, we demonstrate surface premelting engineering to create wafer-scale low-symmetry graphene on germanium-110 [Ge(110)]. Controlled premelting forms striped germanium surface patterns that reduce graphene symmetry from D _6h to C _1v , producing a room-temperature nonlinear Hall conductivity of ~11 micrometers per volt per ohm. First-principles calculations attribute this to originating from overtilted massive Dirac cones of hybridized germanium bands and the graphene Dirac cone. Integrated nonlinear Hall rectifiers generate >20 millivolts of output from radio frequency input and drive commercial voltage boosters and light-emitting diodes, establishing a complementary metal-oxide semiconductor–compatible route toward wafer-scale nonlinear quantum devices.

The nonlinear Hall effect (NHE) can enable rectification and energy harvesting, and its control by external fields, including gate, strain and magnetic field, has been pursued intensively. However, existing tuning pathways rely predominantly on fully quantum mechanical effects and are typically inefficient, resulting in weak NHE signals that limit further progress. In this work, we report the discovery of a distinct type of NHE in a graphene-hBN moire superlattice, which arises from a classical-quantum cooperative effect called Lorentz skew scattering (LSK), induced by a perpendicular magnetic field. This field-driven NHE exhibits a linear dependence on magnetic field and a pronounced unidirectional angular dependence. Remarkably, its magnitude reaches up to 32% of the linear Hall signal. We show that this giant, field-tunable NHE originating from LSK follows a unique quartic scaling law and produces a record-high nonlinear Hall conductivity (36000 μmV-1Ω-1) near van Hove singularities of moire minibands, which is over an order of magnitude larger than all previously reported NHEs. Our findings establish an efficient, magnetic-field-driven route to giant Hall rectification in high-mobility materials, offering a broadly applicable paradigm for modulating the NHE beyond electrostatic gating.

The nonlinear Hall effect (NLHE) in two-dimensional moiré superlattice materials offers promising opportunities for radio frequency (RF) energy harvesting, but challenges persist in achieving both room-temperature controllable NLHE and scalable fabrication. Here, we report the manipulation of NLHE in the 4-inch epitaxial graphene-germanium (Gr-Ge) heterostructures for practical energy harvesting. The stacking of stripe-phase Ge(110) surface with monolayer graphene generates periodic hexagonal moiré superlattices exhibiting reduced C1v symmetry configuration. The intrinsic symmetry breaking at the Gr-Ge hetero-interface enables remarkable room-temperature NLHE strength of ~103 μmV-1, which exceeds all state-of-the-art moiré or bulk materials systems. Moreover, by adopting the series-connected nonlinear Hall rectifiers (NRs) array configuration and a dual-functional layout design with back-end-of-line (BEOL) processes, we develop self-contained rectenna dies incorporating a 4×4 NRs array with the remarkable rectified voltage exceeding 20 mV at -16 dBm input power, which is sufficient for driving commercial booster converter operation. Besides, we demonstrate the use of NRs array to realize ambient RF energy harvesting by powering an LED at room temperature. Our results demonstrate the successful transition of NLHE-based rectification from theoretical concept to practical applications. Beyond RF energy harvesting, the low-symmetry Gr-Ge moiré superlattices open new avenues for next-generation application in nonlinear electronics, photonics, and optoelectronic technologies.

The Nernst effect, that is, the generation of a transverse voltage in response to a temperature gradient, enables thermoelectric energy conversion. In the absence of an external magnetic field, the linear Nernst effect is forbidden in non-magnetic materials because of time-reversal symmetry constraints, but the recently predicted nonlinear Nernst effect (NNE) is allowed. Here we report the experimental observation of the NNE in non-magnetic ABA trilayer graphene, even in the absence of an external magnetic field. This effect is detected via electric harmonic measurements under an alternating temperature gradient at temperatures below 12 K. The NNE exhibits a quadratic dependence on the temperature gradient. It is notably enhanced near the charge neutrality point and reaches a giant effective Nernst coefficient of up to 300 µV K^-1 at 2 K, surpassing the linear coefficients of magnetic materials. Moreover, we establish a scaling law between the NNE and the linear Seebeck effect, confirming the dominance of a skew scattering mechanism in driving the NNE. Our findings demonstrate an alternative approach for thermoelectric energy harvesting and cooling applications via nonlinear thermoelectric responses, which may, in the long run, offer alternative approaches towards the development of advanced thermoelectric devices.

Spin-triplet pairing of superconductivity, drawing huge attention by enabling future technologies in quantum computing, spintronics, and other fields, can host unique transport phenomena. Here, we report gigantically enhanced nonreciprocal transport in the superconducting fluctuation regime of KTaO_{3}-based interface superconductors. Notably, the observed nonreciprocity undergoes a sign reversal compared to the normal state. Combining experimental observations with theoretical modeling, we identify its dominant mechanism as the vortex motion and parity mixing of spin-singlet and spin-triplet pairings. We also provide evidence for an attractive spin-triplet pairing interaction. Furthermore, the nonreciprocal response is highly tunable by orders of magnitude using electrostatic gating. These findings shed light on the pairing mechanisms in Rashba superconductors and offer new pathways for developing nonlinear superconducting devices.

The valley Hall effect (VHE) holds great promise for valleytronic applications by leveraging the valley degree of freedom. To date, research on VHE has focused on its linear response to an applied current, leaving nonlinear valley responses undetected and nonlinear valleytronic devices undeveloped. Here, we report the experimental observation of a nonlinear VHE in a graphene-hBN moire superlattice, evidenced by the generation of second-harmonic nonlocal voltages under AC currents. Remarkably, the nonlinear VHE has magnitude surpassing the linear VHE and is highly tunable via a gate voltage, which exhibits a pair of opposite peaks on the two sides of a Dirac gap. The nonlinear signal shows quadratic scaling with driving current and quartic scaling with local resistance, setting it apart from the linear counterpart. These experimental features are consistent with the theoretical picture of nonlocal transport mediated by nonlinear VHE and linear inverse VHE. We further reveal a nonlinear inverse VHE by observing the third- and fourth-harmonic nonlocal voltages. The nonlinear VHE provides a novel mechanism for valley manipulation and enables a novel valleytronic device, the valley rectifier, that converts AC charge current into DC valley current.

自旋电子学利用电子的自旋属性来实现信息存储和处理，有望克服传统电子学面临的能耗、微型化以及抗辐照等瓶颈问题。近年来，非线性自旋电子学作为自旋电子学领域的新兴分支，受到一定的关注。非线性自旋电子学结合了非线性动力学和自旋电子学，开辟了超越传统自旋相关效应的新途径。本文综述了非线性自旋电子学的最新进展，重点介绍几种典型的非线性自旋输运现象、物理机制、实验表征方法以及在自旋器件中的应用，旨在展示其重要性以激发新的相关研究。

Two-dimensional electron gas (2DEG) at oxide interfaces exhibits various exotic properties stemming from interfacial inversion and symmetry breaking. In this work, we report large nonlinear transverse conductivities in the LaAlO₃/KTaO₃ interface 2DEG under zero magnetic field. Skew scattering was identified as the dominant origin based on the cubic scaling of nonlinear transverse conductivity with linear longitudinal conductivity and 3-fold symmetry. Moreover, gate-tunable nonlinear transport with pronounced peak and dip was observed and reproduced by our theoretical calculation. These results indicate the presence of Berry curvature hotspots and thus a large Berry curvature triplet at the oxide interface. Our theoretical calculations confirm the existence of large Berry curvatures from the avoided crossing of multiple 5d-orbit bands, orders of magnitude larger than that in transition-metal dichalcogenides. Nonlinear transport offers a new pathway to probe the Berry curvature at oxide interfaces and facilitates new applications in oxide nonlinear electronics.