Achieving electrical injection of exciton-polaritons, half-light, half-matter quasiparticles arising from the strong coupling between photonic and excitonic resonances, is a crucial milestone to scale up polaritonic devices such as optical computers, quantum simulators and inversionless lasers. Here we present a new approach to achieve strong coupling between electrically injected excitons and photonic bound states in the continuum of a dielectric metasurface monolithically patterned in the channel of a light-emitting transistor. Exciton-polaritons are generated by coupling electrically injected excitons in the gate-induced transport channel with a Bloch mode of the metasurface, and decay into photons emitted from the top surface of the transistor. Thanks to the high-finesse of the metasurface cavity, we achieve a large Rabi splitting of ~200 meV and more than 50-fold enhancement of the polaritonic emission over the intrinsic excitonic emission of the perovskite film. Moreover, we show that the directionality of polaritonic electroluminescence can be dynamically tuned by varying the source-drain bias which controls the radiative recombination zone of the excitons. We argue that this approach provides a new platform to study strong light-matter interaction in dispersion engineered photonic cavities under electrical injection, and paves the way to solution-processed electrically pumped polariton lasers.

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High-crystalline halide perovskite nanostructures [such as nanowires and nanoplates (NPs)] provide good potential in realizing nanoscale solid light sources for on-chip optical communication, high-density storage, and life science applications. However, it remains a great challenge to fabricate nanoscale perovskite light-emitting devices using traditional fabrication methods because the perovskite nanomaterials will be dissolved in polar solvents. Developing new device configurations to enhance radiative recombination efficiency as well as device stability is one of the most important research topics in nanoscale perovskite light-emitting devices. Here, we demonstrate nanoscale perovskite electroluminescence (EL) using a single-crystalline CsPbBr3 NP as the active layer. The device is based on a hybrid capacitance structure, where an underlying few-layer graphene (FLG) electrode, a single-crystalline CsPbBr3 NP, a thin hexagonal boron nitride (hBN) flake, and another FLG top electrode are stacking in sequence, forming a van der Waals heterostructure. A strong EL emission peak with a narrow linewidth (∼1.09 nm) is observed at 2 K. Alternating current voltage/frequency-dependent EL spectra are studied in detail. We attribute the superior EL behavior of the as-fabricated nanoscale perovskite light-emitting devices to (1) the high-quality single-crystalline CsPbBr3 NPs synthesized, (2) the hBN encapsulation, which enhances the device stability by providing a large heat dissipation pathway for CsPbBr3 NP and protecting it from the polar solvents, (3) the capacitance structure, which facilitates the injection of both electrons and holes. Our work demonstrates a method to construct nanoscale perovskite (with well-defined geometry) light sources, providing an opportunity for realizing a nanoscale electrically driven perovskite laser.

Natural superlattice structures MnBi2Te4(Bi2Te3)n (n = 1, 2, ...), in which magnetic MnBi2Te4 layers are separated by nonmagnetic Bi2Te3 layers, hold band topology, magnetism and reduced interlayer coupling, providing a promising platform for the realization of exotic topological quantum states. However, their magnetism in the two-dimensional limit, which is crucial for further exploration of quantum phenomena, remains elusive. Here, complex ferromagnetic-antiferromagnetic coexisting ground states that persist down to the 2-septuple layers limit are observed and comprehensively investigated in MnBi4Te7 (n = 1) and MnBi6Te10 (n = 2). The ubiquitous Mn-Bi site mixing modifies or even changes the sign of the subtle interlayer magnetic interactions, yielding a spatially inhomogeneous interlayer coupling. Further, a tunable exchange bias effect, arising from the coupling between the ferromagnetic and antiferromagnetic components in the ground state, is observed in MnBi2Te4(Bi2Te3)n (n = 1, 2), which provides design principles and material platforms for future spintronic devices. Our work highlights a new approach toward the fine-tuning of magnetism and paves the way for further study of quantum phenomena in MnBi2Te4(Bi2Te3)n (n = 1, 2) as well as their magnetic applications.

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The exchange bias (EB) effect is a fundamental phenomenon in conventional systems containing interfaces of ferromagnetic (FM) and antiferromagnetic (AFM) materials and plays a crucial role in magnetic memory technologies. Due to the rapid development of van der Waals (vdW) magnets, the EB effect can be constructed by assembling vdW FM and AFM materials together without the constraints of lattice matching, greatly broadening the understanding of two-dimensional (2D) magnetism. However, the EB effect in singular 2D magnets down to a monolayer has not been realized, where material design plays an important role. Here, we report a distinct EB effect in singular FM MnSb2Te4 flakes, which can be achieved by applying asymmetric field-sweeping ranges. We conjecture that the EB effect is related to the Mn–Sb site-mixing behavior in MnSb2Te4, giving rise to the presence of the pinning sites. The EB field can reach more than 30 mT at 2 K, and its direction and magnitude can be modulated by changing the field-sweeping range, providing a degree of freedom for controlling the EB effect. This work reveals intriguing EB effects in singular vdW magnetic materials and paves the way for low-dimensional spintronic devices.

Two-dimensional (2D) magnetic materials provide an ideal platform for investigating novel magnetism and spin behavior in low-dimensional systems while being restricted by the deficiency of accurate bottom-up synthesis. To overcome this difficulty, a facile and universal flux-assisted growth (FAG) method is proposed to synthesize the multicomponent Fe x GeTe2 (x = 3–5) with different Fe contents and even alloyed with hetero metal atoms. This one-to-one method ensures the stoichiometry consistency from the Fe x GeTe2 and M y Fe5–y GeTe2 (M = Co, Ni) bulk crystal precursors to the 2D nanosheets, with controllable composition. Tuning the growth temperatures can provide thickness-tunable products. Changeable magnetic properties of Fe x GeTe2 and alloyed Co y Fe5–y GeTe2 are substantiated by the superconducting quantum interference device and reflective magnetic circular dichroism. This method generates thickness-tunable high-crystallinity Fe x GeTe2 samples without phase separation and exhibits a high tolerance to different substrates and a large temperature window, providing a new avenue to synthesize and explore such multicomponent 2D magnets and even the alloyed ones.

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