Nonlinear optical diode effect in a magnetic Weyl semimetal.

Diode effects are of great interest for both fundamental physics and modern technologies. Electrical diode effects (nonreciprocal transport) have been observed in Weyl systems. Optical diode effects arising from the Weyl fermions have been theoretically considered but not probed experimentally. Here, we report the observation of a nonlinear optical diode effect (NODE) in the magnetic Weyl semimetal CeAlSi, where the magnetization introduces a pronounced directionality in the nonlinear optical second-harmonic generation (SHG). We demonstrate a six-fold change of the measured SHG intensity between opposite propagation directions over a bandwidth exceeding 250 meV. Supported by density-functional theory, we establish the linearly dispersive bands emerging from Weyl nodes as the origin of this broadband effect. We further demonstrate current-induced magnetization switching and thus electrical control of the NODE. Our results advance ongoing research to identify novel nonlinear optical/transport phenomena in magnetic topological materials and further opens new pathways for the unidirectional manipulation of light.

Dual quantum spin Hall insulator by density-tuned correlations in TaIrTe4.

The convergence of topology and correlations represents a highly coveted realm in the pursuit of new quantum states of matter1. Introducing electron correlations to a quantum spin Hall (QSH) insulator can lead to the emergence of a fractional topological insulator and other exotic time-reversal-symmetric topological order2-8, not possible in quantum Hall and Chern insulator systems. Here we report a new dual QSH insulator within the intrinsic monolayer crystal of TaIrTe4, arising from the interplay of its single-particle topology and density-tuned electron correlations. At charge neutrality, monolayer TaIrTe4 demonstrates the QSH insulator, manifesting enhanced nonlocal transport and quantized helical edge conductance. After introducing electrons from charge neutrality, TaIrTe4 shows metallic behaviour in only a small range of charge densities but quickly goes into a new insulating state, entirely unexpected on the basis of the single-particle band structure of TaIrTe4. This insulating state could arise from a strong electronic instability near the van Hove singularities, probably leading to a charge density wave (CDW). Remarkably, within this correlated insulating gap, we observe a resurgence of the QSH state. The observation of helical edge conduction in a CDW gap could bridge spin physics and charge orders. The discovery of a dual QSH insulator introduces a new method for creating topological flat minibands through CDW superlattices, which offer a promising platform for exploring time-reversal-symmetric fractional phases and electromagnetism2-4,9,10.

Quantum metric nonlinear Hall effect in a topological antiferromagnetic heterostructure.

Quantum geometry in condensed-matter physics has two components: the real part quantum metric and the imaginary part Berry curvature. Whereas the effects of Berry curvature have been observed through phenomena such as the quantum Hall effect in two-dimensional electron gases and the anomalous Hall effect (AHE) in ferromagnets, the quantum metric has rarely been explored. Here, we report a nonlinear Hall effect induced by the quantum metric dipole by interfacing even-layered MnBi2Te4 with black phosphorus. The quantum metric nonlinear Hall effect switches direction upon reversing the antiferromagnetic (AFM) spins and exhibits distinct scaling that is independent of the scattering time. Our results open the door to discovering quantum metric responses predicted theoretically and pave the way for applications that bridge nonlinear electronics with AFM spintronics.

Axion optical induction of antiferromagnetic order.

Using circularly polarized light to control quantum matter is a highly intriguing topic in physics, chemistry and biology. Previous studies have demonstrated helicity-dependent optical control of chirality and magnetization, with important implications in asymmetric synthesis in chemistry; homochirality in biomolecules; and ferromagnetic spintronics. We report the surprising observation of helicity-dependent optical control of fully compensated antiferromagnetic order in two-dimensional even-layered MnBi2Te4, a topological axion insulator with neither chirality nor magnetization. To understand this control, we study an antiferromagnetic circular dichroism, which appears only in reflection but is absent in transmission. We show that the optical control and circular dichroism both arise from the optical axion electrodynamics. Our axion induction provides the possibility to optically control a family of [Formula: see text]-symmetric antiferromagnets ([Formula: see text], inversion; [Formula: see text], time-reversal) such as Cr2O3, even-layered CrI3 and possibly the pseudo-gap state in cuprates. In MnBi2Te4, this further opens the door for optical writing of a dissipationless circuit formed by topological edge states.

Approaching the Intrinsic Threshold Breakdown Voltage and Ultrahigh Gain in a Graphite/InSe Schottky Photodetector.

Realizing both ultralow breakdown voltage and ultrahigh gain is one of the major challenges in the development of high-performance avalanche photodetector. Here, it is reported that an ultrahigh avalanche gain of 3 × 105 can be realized in the graphite/InSe Schottky photodetector at a breakdown voltage down to 5.5 V. Remarkably, the threshold breakdown voltage can be further reduced down to 1.8 V by raising the operating temperature, approaching the theoretical limit of 1.5 E g \[{{\cal E}_{\bf g}}\] /e, with E g ${{\cal E}_{\bf g}}$ the bandgap of semiconductor. A 2D impact ionization model is developed and it is uncovered that observation of high gain at low breakdown voltage arises from reduced dimensionality of electron-phonon scattering in the layered InSe flake. These findings open up a promising avenue for developing novel weak-light detectors with low energy consumption and high sensitivity.

Networking retinomorphic sensor with memristive crossbar for brain-inspired visual perception.

Compared to human vision, conventional machine vision composed of an image sensor and processor suffers from high latency and large power consumption due to physically separated image sensing and processing. A neuromorphic vision system with brain-inspired visual perception provides a promising solution to the problem. Here we propose and demonstrate a prototype neuromorphic vision system by networking a retinomorphic sensor with a memristive crossbar. We fabricate the retinomorphic sensor by using WSe2/h-BN/Al2O3 van der Waals heterostructures with gate-tunable photoresponses, to closely mimic the human retinal capabilities in simultaneously sensing and processing images. We then network the sensor with a large-scale Pt/Ta/HfO2/Ta one-transistor-one-resistor (1T1R) memristive crossbar, which plays a similar role to the visual cortex in the human brain. The realized neuromorphic vision system allows for fast letter recognition and object tracking, indicating the capabilities of image sensing, processing and recognition in the full analog regime. Our work suggests that such a neuromorphic vision system may open up unprecedented opportunities in future visual perception applications.

Layer Hall effect in a 2D topological axion antiferromagnet.

Whereas ferromagnets have been known and used for millennia, antiferromagnets were only discovered in the 1930s1. At large scale, because of the absence of global magnetization, antiferromagnets may seem to behave like any non-magnetic material. At the microscopic level, however, the opposite alignment of spins forms a rich internal structure. In topological antiferromagnets, this internal structure leads to the possibility that the property known as the Berry phase can acquire distinct spatial textures2,3. Here we study this possibility in an antiferromagnetic axion insulator-even-layered, two-dimensional MnBi2Te4-in which spatial degrees of freedom correspond to different layers. We observe a type of Hall effect-the layer Hall effect-in which electrons from the top and bottom layers spontaneously deflect in opposite directions. Specifically, under zero electric field, even-layered MnBi2Te4 shows no anomalous Hall effect. However, applying an electric field leads to the emergence of a large, layer-polarized anomalous Hall effect of about 0.5e2/h (where e is the electron charge and h is Planck's constant). This layer Hall effect uncovers an unusual layer-locked Berry curvature, which serves to characterize the axion insulator state. Moreover, we find that the layer-locked Berry curvature can be manipulated by the axion field formed from the dot product of the electric and magnetic field vectors. Our results offer new pathways to detect and manipulate the internal spatial structure of fully compensated topological antiferromagnets4-9. The layer-locked Berry curvature represents a first step towards spatial engineering of the Berry phase through effects such as layer-specific moiré potential.

Low Voltage Operating 2D MoS2 Ferroelectric Memory Transistor with Hf1-xZrxO2 Gate Structure.

Ferroelectric field effect transistor (FeFET) emerges as an intriguing non-volatile memory technology due to its promising operating speed and endurance. However, flipping the polarization requires a high voltage compared with that of reading, impinging the power consumption of writing a cell. Here, we report a CMOS compatible FeFET cell with low operating voltage. We engineer the ferroelectric Hf1-xZrxO2 (HZO) thin film to form negative capacitance (NC) gate dielectrics, which generates a counterclock hysteresis loop of polarization domain in the few-layered molybdenum disulfide (MoS2) FeFET. The unstabilized negative capacitor inherently supports subthermionic swing rate and thus enables switching the ferroelectric polarization with the hysteresis window much less than half of the operating voltage. The FeFET shows a high on/off current ratio of more than 107 and a counterclockwise memory window (MW) of 0.1 V at a miminum program (P)/erase (E) voltage of 3 V. Robust endurance (103 cycles) and retention (104 s) properties are also demonstrated. Our results demonstrate that the HZO/MoS2 ferroelectric memory transistor can achieve new opportunities in size- and voltage-scalable non-volatile memory applications.

Gate-tunable van der Waals heterostructure for reconfigurable neural network vision sensor.

Early processing of visual information takes place in the human retina. Mimicking neurobiological structures and functionalities of the retina provides a promising pathway to achieving vision sensor with highly efficient image processing. Here, we demonstrate a prototype vision sensor that operates via the gate-tunable positive and negative photoresponses of the van der Waals (vdW) vertical heterostructures. The sensor emulates not only the neurobiological functionalities of bipolar cells and photoreceptors but also the unique connectivity between bipolar cells and photoreceptors. By tuning gate voltage for each pixel, we achieve reconfigurable vision sensor for simultaneous image sensing and processing. Furthermore, our prototype vision sensor itself can be trained to classify the input images by updating the gate voltages applied individually to each pixel in the sensor. Our work indicates that vdW vertical heterostructures offer a promising platform for the development of neural network vision sensor.

Room-temperature valleytronic transistor.

Valleytronics, based on the valley degree of freedom rather than charge, is a promising candidate for next-generation information devices beyond complementary metal-oxide-semiconductor (CMOS) technology1-4. Although many intriguing valleytronic properties have been explored based on excitonic injection or the non-local response of transverse current schemes at low temperature4-7, demonstrations of valleytronic building blocks similar to transistors in electronics, especially at room temperature, remain elusive. Here, we report a solid-state device that enables a full sequence of generating, propagating, detecting and manipulating valley information at room temperature. Chiral nanocrescent plasmonic antennae8 are used to selectively generate valley-polarized carriers in MoS2 through hot-electron injection under linearly polarized infrared excitation. These long-lived valley-polarized free carriers can be detected in a valley Hall configuration9-11 even without charge current, and can propagate over 18 µm by means of drift. In addition, electrostatic gating allows us to modulate the magnitude of the valley Hall voltage. The electrical valley Hall output could drive the valley manipulation of a cascaded stage, rendering the device able to serve as a transistor free of charge current with pure valleytronic input/output. Our results demonstrate the possibility of encoding and processing information by valley degree of freedom, and provide a universal strategy to study the Berry curvature dipole in quantum materials.

Edge-Epitaxial Growth of InSe Nanowires toward High-Performance Photodetectors.

Semiconducting nanowires offer many opportunities for electronic and optoelectronic device applications due to their unique geometries and physical properties. However, it is challenging to synthesize semiconducting nanowires directly on a SiO2 /Si substrate due to lattice mismatch. Here, a catalysis-free approach is developed to achieve direct synthesis of long and straight InSe nanowires on SiO2 /Si substrates through edge-homoepitaxial growth. Parallel InSe nanowires are achieved further on SiO2 /Si substrates through controlling growth conditions. The underlying growth mechanism is attributed to a selenium self-driven vapor-liquid-solid process, which is distinct from the conventional metal-catalytic vapor-liquid-solid method widely used for growing Si and III-V nanowires. Furthermore, it is demonstrated that the as-grown InSe nanowire-based visible light photodetector simultaneously possesses an extraordinary photoresponsivity of 271 A W-1 , ultrahigh detectivity of 1.57 × 1014 Jones, and a fast response speed of microsecond scale. The excellent performance of the photodetector indicates that as-grown InSe nanowires are promising in future optoelectronic applications. More importantly, the proposed edge-homoepitaxial approach may open up a novel avenue for direct synthesis of semiconducting nanowire arrays on SiO2 /Si substrates.

Robust Impact-Ionization Field-Effect Transistor Based on Nanoscale Vertical Graphene/Black Phosphorus/Indium Selenide Heterostructures.

Maintaining the rapid development of information technology by scaling down a metal-oxide semiconductor field-effect transistor faces two serious challenges. First, the gate field loses control of the channel as it continuously decreases. Second, the fundamental thermionic limit restricts the reduction in supply voltage. Thus, further scaling down necessitates alternative device structures and different switching mechanisms. Here, we report impact-ionization transistors (IITs) based on nanoscale (∼30 nm) vertical graphene/black phosphorus (BP)/indium selenide (InSe) heterostructures. By facilitating the carrier multiplication of the ballistic impact-ionization process as the internal gain mechanism in sub-mean-free-path (sub-MFP) channels, the IITs exhibit a low average subthreshold swing (SS < 1 mV/dec) over five current levels. High stability (>10 000 cycles) and small hysteresis (<1%) switching properties are also obtained. The experimental demonstration of such transistor combining steep SS, high ON-state current density, reliable robustness, miniature footprint, and low bias voltage approaches fulfillments of targets for next-generation devices in the International Technology Roadmap for Semiconductors.

Plasmon Excited Ultrahot Carriers and Negative Differential Photoresponse in a Vertical Graphene van der Waals Heterostructure.

Photogenerated nonequilibrium hot carriers play a key role in graphene's intriguing optoelectronic properties. Compared to conventional photoexcitation, plasmon excitation can be engineered to enhance and control the generation and dynamics of hot carriers. Here, we report an unusual negative differential photoresponse of plasmon-induced "ultrahot" electrons in a graphene-boron nitride-graphene tunneling junction. We demonstrate nanocrescent gold plasmonic nanostructures that substantially enhance the absorption of long-wavelength photons whose energy is greatly below the tunneling barrier and significantly boost the electron thermalization in graphene. We further analyze the generation and transfer of ultrahot electrons under different bias and power conditions. We find that the competition among thermionic emission, the carrier-cooling effect, and the field effect results in a hitherto unusual negative differential photoresponse in the photocurrent-bias plot. Our results not only exemplify a promising platform for detecting low-energy photons, enhancing the photoresponse, and reducing the dark current but also reveal the critically coupled pathways for harvesting ultrahot carriers.

Observation of ballistic avalanche phenomena in nanoscale vertical InSe/BP heterostructures.

Impact ionization, which supports carrier multiplication, is promising for applications in single photon detection1 and sharp threshold swing field effect devices2. However, initiating the impact ionization of avalanche breakdown requires a high applied electric field in a long active region, which hampers carrier multiplication with a high gain, low bias and superior noise performance3,4. Here we report the observation of ballistic avalanche phenomena in sub-mean free path (MFP) scaled vertical InSe/black phosphorus (BP)5-9 heterostructures10. We use these heterojunctions to fabricate avalanche photodetectors (APDs) with a sensitive mid-infrared light detection (4 µm wavelength) and impact ionization transistors with a steep subthreshold swing (<0.25 mV dec-1). The devices show a low avalanche threshold (<1 V), low noise figure and distinctive density spectral shape. Our transport measurements suggest that the breakdown originates from a ballistic avalanche phenomenon, where the sub-MFP BP channel support the lattice impact ionization by electrons and holes and the abrupt current amplification without scattering from the obstacles in a deterministic nature. Our results provide new strategies for the development of advanced photodetectors1,11,12 via efficient carrier manipulation at the nanoscale.

Electrically tunable optical properties of few-layer black arsenic phosphorus.

Black arsenic phosphorus (b-AsP) is a promising two-dimensional material for various optoelectronic applications, bridging the wavelength gap between two-dimensional molybdenum disulfide and graphene. In particular, it has intriguing potential in photodetectors and great advantages in the mid-infrared field. However, its optoelectronic modulation has yet to be elucidated, which requires a fundamental understanding of its field-effect optical modulation. Here, we report the measurements of the lower-energy infrared anisotropic optical response of thin b-AsP under different electrical gating. We reveal that in addition to band edge absorption, amplitude modulation of sub-band absorption up to ten percent is also obtained in reflection extinction. These in-gap absorptions are attributed to spin-orbital coupling and free carrier absorption. Our results suggest the important potential for use of b-AsP in mid-infrared optoelectronic modulator applications.

Negative Photoconductance in van der Waals Heterostructure-Based Floating Gate Phototransistor.

van der Waals (vdW) heterostructures made of two-dimensional materials have been demonstrated to be versatile architectures for optoelectronic applications due to strong light--matter interactions. However, most light-controlled phenomena and applications in the vdW heterostructures rely on positive photoconductance (PPC). Negative photoconductance (NPC) has not yet been reported in vdW heterostructures. Here we report the observation of the NPC in the ReS2/h-BN/MoS2 vdW heterostructure-based floating gate phototransistor. The fabricated devices exhibit excellent performance of nonvolatile memory without light illumination. More interestingly, we observe a gate-tunable transition between the PPC and the NPC under the light illumination. The observed NPC phenomenon can be attributed to charge transfer between the floating gate and the conduction channel. Furthermore, we show that control of NPC through light intensity is promising in realization of light-tunable multibit memory devices. Our results may enable potential applications in multifunctional memories and optoelectronic devices.

Damage-free and rapid transfer of CVD-grown two-dimensional transition metal dichalcogenides by dissolving sacrificial water-soluble layers.

As one of the most important family members of two-dimensional (2D) materials, the growth and damage-free transfer of transition metal dichalcogenides (TMDs) play crucial roles in their future applications. Here, we report a damage-free and highly efficient approach to transfer single and few-layer 2D TMDs to arbitrary substrates by dissolving a sacrificial water-soluble layer, which is formed underneath 2D TMD flakes simultaneously during the growth process. It is demonstrated, for monolayer MoS2, that no quality degradation is found after the transfer by performing transmission electron microscopy, Raman spectroscopy, photoluminescence and electrical transport studies. The field effect mobility of the post-transfer MoS2 flakes was found to be improved by 2-3 orders compared with that of the as-grown ones. This approach was also demonstrated to be applicable to other TMDs, other halide salts as precursors, or other growth substrates, indicating its universality for other 2D materials. Our work may pave the way for material synthesis of future integrated electronic and optoelectronic devices based on 2D TMD materials.

Room temperature high-detectivity mid-infrared photodetectors based on black arsenic phosphorus.

The mid-infrared (MIR) spectral range, pertaining to important applications, such as molecular "fingerprint" imaging, remote sensing, free space telecommunication, and optical radar, is of particular scientific interest and technological importance. However, state-of-the-art materials for MIR detection are limited by intrinsic noise and inconvenient fabrication processes, resulting in high-cost photodetectors requiring cryogenic operation. We report black arsenic phosphorus-based long-wavelength IR photodetectors, with room temperature operation up to 8.2 µm, entering the second MIR atmospheric transmission window. Combined with a van der Waals heterojunction, room temperature-specific detectivity higher than 4.9 × 109 Jones was obtained in the 3- to 5-µm range. The photodetector works in a zero-bias photovoltaic mode, enabling fast photoresponse and low dark noise. Our van der Waals heterojunction photodetectors not only exemplify black arsenic phosphorus as a promising candidate for MIR optoelectronic applications but also pave the way for a general strategy to suppress 1/f noise in photonic devices.

Broadband Photovoltaic Detectors Based on an Atomically Thin Heterostructure.

van der Waals junctions of two-dimensional materials with an atomically sharp interface open up unprecedented opportunities to design and study functional heterostructures. Semiconducting transition metal dichalcogenides have shown tremendous potential for future applications due to their unique electronic properties and strong light-matter interaction. However, many important optoelectronic applications, such as broadband photodetection, are severely hindered by their limited spectral range and reduced light absorption. Here, we present a p-g-n heterostructure formed by sandwiching graphene with a gapless band structure and wide absorption spectrum in an atomically thin p-n junction to overcome these major limitations. We have successfully demonstrated a MoS2-graphene-WSe2 heterostructure for broadband photodetection in the visible to short-wavelength infrared range at room temperature that exhibits competitive device performance, including a specific detectivity of up to 10(11) Jones in the near-infrared region. Our results pave the way toward the implementation of atomically thin heterostructures for broadband and sensitive optoelectronic applications.