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Atomically thin magnets are the key element to build up spintronics based on twodimensional materials. The surface nature of two-dimensional ferromagnet opens up opportunities to improve the device performance efficiently. Here, we report the intrinsic ferromagnetism in atomically thin monolayer CrBr3, directly probed by polarization resolved magnetophotoluminescence. The spontaneous magnetization persists in monolayer CrBr3 with a Curie temperature of 34 K. The development of magnons by the thermal excitation is in line with the spin-wave theory. We attribute the layer-number dependent hysteresis loops in thick layers to the 2 magnetic domain structures. As a stable monolayer material in air, CrBr3 provides a convenient platform for fundamental physics and pushes the potential applications of the two-dimensional ferromagnetism.Ferromagnetism in atomically thin magnet has been studied in a variety of van der Waals materials 1, 2 , including metallic Fe3GeTe2 3, 4 , semiconducting Cr2Ge2Te6 5 and insulating CrI3 6 .Even though the long-range magnetic order is highly suppressed by the thermal excitation of magnons in a two-dimensional (2D) magnet at finite temperature 7 , the magnetic anisotropy opens an energy gap in the magnon spectra and therefore, protects the ferromagnetism in two dimensions.The magnon-magnon interaction in such van der Waals ferromagnets also provides a platform to study the fundamental topological spin excitation, for example, Dirac magnon 8 and topological magnon surface state 9 . Moreover, in contrast to the three-dimensional ferromagnet, magnetic 2D materials show tunable magnetic properties due to their surface nature 1-3, 10-13 . Particularly the layer-number dependent 4, 6, 14 and gate-tunable magnetism 3, 10-13 opens a new way to build spintronic devices with high accuracy and efficiency 15-20 .Among various van der Waals ferromagnets, CrBr3 is an interesting platform to study the magnetism in low dimensions and light matter interactions in magnetic materials. The neutron scattering has revealed the Dirac points in bulk CrBr3 21, 22 , formed by acoustic and optical spinwave modes, where both intralayer and interlayer exchange interactions play an important role.On the other hand, optical absorption spectra in CrBr3 have shown the out-of-plane magnetic field dependence 23 , suggesting potential applications in optoelectronics. However, magnetism in atomically thin CrBr3, especially in monolayer limit, is still unknown.

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.

We report the study of the helicity-driven photocurrents in graphene excited by midinfrared light of a CO 2 laser. Illuminating an unbiased monolayer sheet of graphene with circularly polarized radiation generatesunder oblique incidence-an electric current perpendicular to the plane of incidence, whose sign is reversed by switching the radiation helicity. We show that the current is caused by the interplay of the circular ac Hall effect and the circular photogalvanic effect. By studying the frequency dependence of the current in graphene layers grown on the SiC substrate, we observe that the current exhibits a resonance at frequencies matching the longitudinal optical phonon in SiC.

Internal magnetic moments induced by magnetic dopants in MoS2 monolayers are shown to serve as a new means to engineer valley Zeeman splitting (VZS). Specifically, successful synthesis of monolayer MoS2 doped with the magnetic element Co is reported, and the magnitude of the valley splitting is engineered by manipulating the dopant concentration. Valley splittings of 3.9, 5.2, and 6.15 meV at 7 T in Co‐doped MoS2 with Co concentrations of 0.8%, 1.7%, and 2.5%, respectively, are achieved as revealed by polarization‐resolved photoluminescence (PL) spectroscopy. Atomic‐resolution electron microscopy studies clearly identify the magnetic sites of Co substitution in the MoS2 lattice, forming two distinct types of configurations, namely isolated single dopants and tridopant clusters. Density functional theory (DFT) and model calculations reveal that the observed enhanced VZS arises from an internal magnetic field induced by the tridopant clusters, which couples to the spin, atomic orbital, and valley magnetic moment of carriers from the conduction and valence bands. The present study demonstrates a new method to control the valley pseudospin via magnetic dopants in layered semiconducting materials, paving the way toward magneto‐optical and spintronic devices.

Introducing the chiral spacers to two-dimensional (2D) lead halide perovskites (LHPs) enables them to exhibit circularly polarized photoluminescence (CPPL), which could have applications in chiral-optics and spintronics. Despite that a great deal of effort has been made in this field, the reported polarization degree of CPPL at ambient conditions is still very limited, and the integration of multiple functionalities also remains to be explored. Here we report the structures, CPPL, and piezoelectric energy harvesting properties of chiral 2D LHPs, [R-1-(4-bromophenyl)­ethylaminium]2PbI4 (R-[BPEA]2PbI4) and [S-1-(4-bromophenyl)­ethylaminium]2PbI4 (S-[BPEA]2PbI4). Our results show that these chiral perovskites are direct bandgap semiconductors and exhibit CPPL centered at ∼513 nm with a maximum degree of polarization of up to 11.0% at room temperature. In addition, the unique configurational arrangement of the chiral spacers is found to be able to reduce the interlayer π–π interactions and consequently result in strong electron–phonon coupling. Furthermore, the intrinsic chirality of both R-[BPEA]2PbI4 and S-[BPEA]2PbI4 enables them to be piezoelectric active, and their composite films can be applied to generate voltages and currents up to ∼0.6 V and ∼1.5 μA under periodic impacting with a strength of 2 N, respectively. This work not only reports a high degree of CPPL but also demonstrates piezoelectric energy harvesting behavior for realizing multifunctionalities in chiral 2D LHPs.

Atomically thin monolayer transition metal dichalcogenides possess coupling of spin and valley degrees of freedom. The chirality is locked to identical valleys as a consequence of spin–orbit coupling and inversion symmetry breaking, leading to a valley analog of the Zeeman effect in presence of an out-of-plane magnetic field. Owing to the inversion symmetry in bilayers, the photoluminescence helicity should no longer be locked to the valleys. Here we show that the Zeeman splitting, however, persists in 2H-MoTe2 bilayers, as a result of an additional degree of freedom, namely the layer pseudospin, and spin–valley-layer locking. Unlike monolayers, the Zeeman splitting in bilayers occurs without lifting valley degeneracy. The degree of circularly polarized photoluminescence is tuned with magnetic field from −37% to 37%. Our results demonstrate the control of degree of freedom in bilayer with magnetic field, which makes bilayer a promising platform for spin-valley quantum gates based on magnetoelectric effects.

Abstract2D hybrid organic–inorganic perovskites (HOIPs) are highly responsive to external stimuli and therefore have application potential as sensing materials. Though their optical properties upon singular thermal or pressure stimulation have been recently investigated, their dual‐stimuli‐responsive behaviors have not yet been explored. Here, the dual‐stimuli‐responsive luminescence of a pair of new enantiomeric 2D Dion–Jacobson HOIPs, R+[(4‐aminophenyl)ethylamine]PbI4 and S‐[(4‐aminophenyl)ethylamine]PbI4, is reported. The photoluminescence results show that their 485 nm emissions can be red‐shifted by ≈6 nm upon heating, and further increased to 529 nm under pressure. Such dual‐stimuli‐responsive emissions expand their Commission Internationale de L'Eclairage coordinates successively from (0.140, 0.272) to (0.283, 0.473). Detailed structural analysis and first principles calculations reveal that the temperature‐ and pressure‐responsive behaviors arise from the predominant electron–phonon interactions over thermal expansion effect and pressure‐induced in‐plane PbI bond contraction, respectively. The findings open up a new pathway to successively tune the optical emission of 2D HOIPs via a dual‐stimuli‐responsive approach.