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Abstract A synaptic memristor using 2D ferroelectric junctions is a promising candidate for future neuromorphic computing with ultra‐low power consumption, parallel computing, and adaptive scalable computing technologies. However, its utilization is restricted due to the limited operational voltage memory window and low on/off current (I_ON/OFF) ratio of the memristor devices. Here, it is demonstrated that synaptic operations of 2D In₂Se₃ferroelectric junctions in a planar memristor architecture can reach a voltage memory window as high as 16 V (±8 V) and I_ON/OFFratio of 10⁸, significantly higher than the current literature values. The power consumption is 10⁻⁵ W at the on state, demonstrating low power usage while maintaining a large I_ON/OFFratio of 10⁸compared to other ferroelectric devices. Moreover, the developed ferroelectric junction mimicked synaptic plasticity through pulses in the pre‐synapse. The nonlinearity factors are obtained 1.25 for LTP, −0.25 for LTD, respectively. The single‐layer perceptron (SLP) and convolutional neural network (CNN) on‐chip training results in an accuracy of up to 90%, compared to the 91% in an ideal synapse device. Furthermore, the incorporation of a 3 nm thick SiO₂interface between the α‐In₂Se₃and the Au electrode resulted in ultrahigh performance among other 2D ferroelectric junction devices to date. 

We report the nonvolatile modulation of microwave conductivity in ferroelectric PbZr0.2Ti0.8O3-gated ultrathin LaNiO3/La0.67Sr0.33MnO3 correlated oxide channel visualized by microwave impedance microscopy. Polarization switching is obtained by applying a tip bias above the coercive voltage of the ferroelectric layer. The microwave conductivity of the correlated channel underneath the up- and down-polarized domains has been quantified by finite-element analysis of the tip-sample admittance. At room temperature, a resistance on/off ratio above 100 between the two polarization states is sustained at frequencies up to 1 GHz, which starts to drop at higher frequencies. The frequence-dependence suggests that the conductance modulation originates from ferroelectric field-effect control of carrier density. The modulation is nonvolatile, remaining stable after 6 months of domain writing. Our work is significant for potential applications of oxide-based ferroelectric field-effect transistors in high-frequency nanoelectronics and spintronics. 

We report the implementation of a dilution refrigerator-based scanning microwave impedance microscope with a base temperature of ∼100 mK. The vibration noise of our apparatus with tuning-fork feedback control is as low as 1 nm. Using this setup, we have demonstrated the imaging of quantum anomalous Hall states in magnetically (Cr and V) doped (Bi, Sb)2Te3 thin films grown on mica substrates. Both the conductive edge modes and topological phase transitions near the coercive fields of Cr- and V-doped layers are visualized in the field-dependent results. Our study establishes the experimental platform for investigating nanoscale quantum phenomena at ultralow temperatures. 

Abstract Implementing microelectromechanical system (MEMS) resonators calls for detailed microscopic understanding of the devices, such as energy dissipation channels, spurious modes, and imperfections from microfabrication. Here, we report the nanoscale imaging of a freestanding super-high-frequency (3 – 30 GHz) lateral overtone bulk acoustic resonator with unprecedented spatial resolution and displacement sensitivity. Using transmission-mode microwave impedance microscopy, we have visualized mode profiles of individual overtones and analyzed higher-order transverse spurious modes and anchor loss. The integrated TMIM signals are in good agreement with the stored mechanical energy in the resonator. Quantitative analysis with finite-element modeling shows that the noise floor is equivalent to an in-plane displacement of 10 fm/√Hz at room temperatures, which can be further improved under cryogenic environments. Our work contributes to the design and characterization of MEMS resonators with better performance for telecommunication, sensing, and quantum information science applications. 

The research on two-dimensional (2D) van der Waals ferroelectrics has grown substantially in the last decade. These layered materials differ from conventional thin-film oxide ferroelectrics in that the surface and interface are free from dangling bonds. Some may also possess uncommon properties, such as bandgap tunability, mechanical flexibility, and high carrier mobility, which are desirable for applications in nanoelectronics and optoelectronics. This Tutorial starts by reviewing the theoretical tools in 2D ferroelectric studies, followed by discussing the material synthesis and sample characterization. Several prototypical electronic devices with innovative functionalities will be highlighted. Readers can use this article to obtain a basic understanding of the current status, challenges, and future prospects of 2D ferroelectric materials. 

Moiré superlattices host a rich variety of correlated electronic phases. However, the moiré potential is fixed by interlayer coupling, and it is dependent on the nature of carriers and valleys. In contrast, it has been predicted that twisted hexagonal boron nitride (hBN) layers can impose a periodic electrostatic potential capable of engineering the properties of adjacent functional layers. Here, we show that this potential is described by a theory of electric polarization originating from the interfacial charge redistribution, validated by its dependence on supercell sizes and distance from the twisted interfaces. This enables controllability of the potential depth and profile by controlling the twist angles between the two interfaces. Employing this approach, we further demonstrate how the electrostatic potential from a twisted hBN substrate impedes exciton diffusion in semiconductor monolayers, suggesting opportunities for engineering the properties of adjacent functional layers using the surface potential of a twisted hBN substrate.