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Two-dimensional semiconductors have a valley degree of freedom that could be used as a platform for future optoelectronic devices. The valley Hall effect, caused by electrons in different valleys having opposite Berry curvatures, is important for making such devices, but has only been reported with plasmonic structures or at cryogenic temperatures, limiting practical application. Here we report the observation of the valley Hall effect at room temperature in a molybdenum disulfide/tungsten diselenide van der Waals heterostructure. We show that the magnitude and polarity of the valley Hall effect in the heterostructure are gate tunable, which can be attributed to the contribution of the opposite valley Hall effect from electrons and holes in different layers. We use this gate tunability to create a bipolar valleytronic transistor. Ministry of Education (MOE) National Research Foundation (NRF) Submitted/Accepted version W.-b.G acknowledges financial support from the Singapore National Research Foundation through its Competitive Research Program (CRP award Nos. NRF-CRP21-2018-0007 and NRF-CRP22-2019-0004), QEP programme and Singapore Ministry of Education (MOE2016-T3-1-006 (S)). Q.X. gratefully acknowledges National Natural Science Foundation of China (no. 12020101003), support from the State Key Laboratory of Low-Dimensional Quantum Physics and Start-up Grant from Tsinghua University. C.J. acknowledges the National Natural Science Foundation of China (no. 61974075); the Key Laboratory of Photoelectronic Thin Film Devices and Technology of Tianjin; and the Engineering Research Center of Thin Film Optoelectronics Technology, Ministry of Education of China. H.M. gratefully acknowledges the National Natural Science Foundation of China (no. 61704121), China Scholarship Council (no. 201709345003) and the Tianjin Natural Science Foundation (no. 19JCQNJC00700)

While ferromagnets have been known and exploited for millennia, antiferromagnets (AFMs) were only discovered in the 1930s. The elusive nature indicates AFMs' unique properties: At large scale, due to the absence of global magnetization, AFMs may appear to behave like any non-magnetic material; However, such a seemingly mundane macroscopic magnetic property is highly nontrivial at microscopic level, where opposite spin alignment within the AFM unit cell forms a rich internal structure. In topological AFMs, such an internal structure leads to a new possibility, where topology and Berry phase can acquire distinct spatial textures. Here, we study this exciting possibility in an AFM Axion insulator, even-layered MnBi$_2$Te$_4$ flakes, where spatial degrees of freedom correspond to different layers. Remarkably, we report the observation of a new type of Hall effect, the layer Hall effect, where electrons from the top and bottom layers spontaneously deflect in opposite directions. Specifically, under no net electric field, even-layered MnBi$_2$Te$_4$ shows no anomalous Hall effect (AHE); However, applying an electric field isolates the response from one layer and leads to the surprising emergence of a large layer-polarized AHE (~50%$\frac{e^2}{h}$). Such a layer Hall effect uncovers a highly rare layer-locked Berry curvature, which serves as a unique character of the space-time $\mathcal{PT}$-symmetric AFM topological insulator state. Moreover, we found that the layer-locked Berry curvature can be manipulated by the Axion field, E$\cdot$B, which drives the system between the opposite AFM states. Our results achieve previously unavailable pathways to detect and manipulate the rich internal spatial structure of fully-compensated topological AFMs. The layer-locked Berry curvature represents a first step towards spatial engineering of Berry phase, such as through layer-specific moir\'e potential., Comment: A revised version of this article is published in Nature

The Berry curvature in the band structure of transition metal dichalcogenides (TMDs) introduces a valley-dependent effective magnetic field, which induces the valley Hall effect (VHE). Similar to the ordinary Hall effect, the VHE spatially separates carriers or excitons, depending on their valley index, and accumulates them at opposite sample edges. The VHE can play a key role in valleytronic devices, but previous observations of the VHE have been limited to cryogenic temperatures. Here, we report a demonstration of the VHE of interlayer excitons in a MoS2/WSe2 heterostructure at room temperature. We monitored the in-plane propagation of interlayer excitons through photoluminescence mapping and observed their spatial separation into two opposite transverse directions that depended on the valley index of the excitons. Our theoretical simulations reproduced the salient features of these observations. Our demonstration of the robust interlayer exciton VHE at room temperature, enabled by their intrinsically long lifetimes, will open up realistic possibilities for the development of opto-valleytronic devices based on TMD heterostructures. Ministry of Education (MOE) National Research Foundation (NRF) We acknowledge the support from the Singapore National Research Foundation (NRF-NRFF2015-03) and its Competitive Research Program (CRP Award NRF-CRP21-2018- 0007), Singapore Ministry of Education (MOE2016-T2-2- 077, MOE2016-T2-1-163, MOE2016-T3-1-006 (S)), and A*Star QTE programme. T.L. and K.D. were supported by the Singapore Ministry of Education (MOE2017-T2-1-001 and MOE2018-T3-1-002). The work done at Sun Yat-sen University was supported by the National Key R&D Program of China (2018YFA0306100), the National Natural Science Foundation of China (11874437), Guangzhou Science and Technology Project (201805010004) and the Natural Science Foundation of Guangdong (2018B030311027). We thank J. Kono for the helpful discussion.

In recent cryogenic measurements, narrow photoluminescence (PL) peaks due to diverse quantum emitters have been found at random locations of monolayer transition metal dichalcogenides (TMDs), which impedes precise optoelectronic applications. Thus, it is of great importance to truly regulate these localized exciton emissions by deterministic spatial and spectral control. Here, such desired emission is primarily demonstrated in monolayer WS2 nanodisks. The size-dependent PL studies indicate the clear evolution from the broad defect-band emission to a set of spectrally isolated narrow peaks (linewidth of ≈ sub nm) at 4.2 K, which is associated with the prevailing effect of edge defects with the shrinkage of the disk diameter, providing a narrow emission energy range for bound excitons. When the disk diameter is reduced to 300 nm, more than 80% of emitter peaks are located between 610 and 616 nm, verifying the effective control of emission wavelength of these photon emitters. Furthermore, the strategy is extended to prepare scalable WS2 nanodisk arrays based on flakes of hundreds of µm, and size-dependent narrow emissions of WSe2 nanodisks are testified. This work develops a defect-engineering strategy to generate localized exciton emitters toward the promising TMD-based optoelectronic applications. Ministry of Education (MOE) National Research Foundation (NRF) Submitted/Accepted version S.F. and C.Z. contributed equally to this work. S.F. is supported by a Marie Skłodowska-Curie Individual Fellowship H2020-MSCA-IF-2020 SingExTr (No. 101031596). This work was supported by the Singapore Ministry of Education (MOE) Tier 1 (No. RG199/17, RG93/19) and the Singapore National Research Foundation (NRF) under the Competitive Research Programs (NRF-CRP-212018-0007) and NRF QEP programme, National Natural Science Foundation of China (Nos. 61774040, 11774170, and 61904151), the National Young 1000 Talent Plan of China, the Shanghai Municipal Natural Science Foundation (No. 16ZR1402500), the Opening project of State Key Laboratory of Functional Materials for Informatics (Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences), Natural Science Foundation of Shaanxi under Grant No. 2020JM-108 and the Joint Research Funds of Department of Science & Technology of Shaanxi Province and Northwestern Polytechnical University (No. 2020GXLH-Z-020). J.Z. thanks the support from National Natural Science Foundation of China (NSFC) under the Grant No. 12174384.

Long valley polarization lifetime and tunable photoluminescence have been realized for interlayer excitons by layer engineering., Photoluminescence (PL) from excitons serves as a powerful tool to characterize the optoelectronic property and band structure of semiconductors, especially for atomically thin two-dimensional transition metal dichalcogenide (TMD) materials. However, PL quenches quickly when the thickness of TMD materials increases from monolayer to a few layers, due to the change from direct to indirect band transition. Here, we show that PL can be recovered by engineering multilayer heterostructures, with the band transition reserved to be a direct type. We report emission from layer-engineered interlayer excitons from these multilayer heterostructures. Moreover, as desired for valleytronics devices, the lifetime, valley polarization, and valley lifetime of the generated interlayer excitons can all be substantially improved as compared with that in the monolayer-monolayer heterostructure. Our results pave the way for controlling the properties of interlayer excitons by layer engineering.

Transition metal dichalcogenides have valley degree of freedom, which features optical selection rule and spin-valley locking, making them promising for valleytronics devices and quantum computation. For either application, a long valley polarization lifetime is crucial. Previous results showed that it is around picosecond in monolayer excitons, nanosecond for local excitons and tens of nanosecond for interlayer excitons. Here we show that the dark excitons in two-dimensional heterostructures provide a microsecond valley polarization memory thanks to the magnetic field induced suppression of valley mixing. The lifetime of the dark excitons shows magnetic field and temperature dependence. The long lifetime and valley polarization lifetime of the dark exciton in two-dimensional heterostructures make them promising for long-distance exciton transport and macroscopic quantum state generations., Owing to their valley degree of freedom, atomically thin transition metal dichalcogenides show promise for valleytronics. Here, the authors leverage the dark excitons of two-dimensional van der Waals heterostructures to demonstrate a microsecond valley polarization memory effect.