Valley-Dependent Interlayer Excitons in Magnetic WSe2/CrI3

Novel synthesis approach for highly crystalline CrCl3/MoS2 van der Waals heterostructures unaffected by strain

Controlling the layer-by-layer chemistry and structure of nanomaterials remains a crucial focus in nanoscience and nanoengineering. Specifically, the integration of atomically thin semiconductors with antiferromagnetic two-dimensional materials holds great promise for advancing research. In this work, we successfully demonstrate a new synthesis approach for high-crystallinity CrCl3/MoS2 van der Waals heterostructures via a thermodynamically optimized chemical vapor transport (CVT) process on c-sapphire (0001) substrates. The 2H-MoS2 layers can be grown as monolayers or with varying twist angles whereas the deposition of CrCl3 layers in a second step forms the well-defined heterostructure. Of particular significance are the sharp and clean edges and faces of the crystals, indicating high-quality interfaces in the heterostructures. Raman spectroscopy, AFM and HRTEM confirm the monocrystalline character and precise structure of these layered nanomaterials, in which their intrinsic properties are preserved and unaffected by strain. This can pave the way for next-generation applications, particularly in valleytronics, opto-spintronics, and quantum information processing.

Robust second-order topological insulator in 2D van der Waals magnet CrI3

CrI3 offers an intriguing platform for exploring fundamental physics and the innovative design of spintronics devices in two-dimensional (2D) magnets, and moreover has been instrumental in the study of topological physics. However, the 2D CrI3 monolayer and bilayers have long been thought to be topologically trivial. Here we uncover a hidden facet of the band topology of 2D CrI3 by showing that both the CrI3 monolayer and bilayers are second-order topological insulators (SOTIs) with a nonzero second Stiefel-Whitney number w2 = 1. Furthermore, the topologically nontrivial nature can be explicitly confirmed via the emergence of floating edge states and in-gap corner states. Remarkably, in contrast to most known magnetic topological states, we put forward that the SOTIs in 2D CrI3 monolayer and bilayers are highly robust against magnetic transitions, which remain intact under both ferromagnetic and antiferromagnetic configurations. These interesting predictions not only provide a comprehensive understanding of the band topology of 2D CrI3 but also offer a favorable platform to realize magnetic SOTIs for spintronics applications.

Spin-Enhanced Self-Powered Light Helicity Detecting Based on Vertical WSe2-CrI3 p-n Heterojunction

Two-dimensional (2D) magnetic semiconductors offer an intriguing platform for investigating magneto-optoelectronic properties and hold immense potential in developing prospective devices when they are combined with valley electronic materials like 2D transition-metal dichalcogenides. Herein, we report various magneto-optoelectronic response features of the vertical hBN-FLG-CrI3-WSe2-FLG-hBN van der Waals heterostructure. Through a sensible layout and exquisite manipulation, an hBN-FLG-CrI3-FLG-hBN heterostructure was also fabricated on identical CrI3 and FLGs for better comparison. Our results show that the WSe2-CrI3 heterostructure, acting as a p-n heterojunction, has advantageous capability in light detection, especially in self-powered light helicity detecting. In the WSe2-CrI3 heterojunction, the absolute value of photocurrent IPH exhibits obvious asymmetry with respect to the bias V, with the IPH of reversely biased WSe2-CrI3 p-n heterojunction being larger. When the CrI3 is fully spin-polarized under a 3 T magnetic field, the reversely biased WSe2-CrI3 heterojunction exhibits advantageous capability in light helicity detecting. Both the short-circuit currents ISC and IPH show one-cycle fluctuation behaviors when the quarter-wave plate rotates 180°, and the corresponding photoresponsivity helicities can be as high as 18.0% and 20.1%, respectively. We attribute the spin-enhanced photovoltaic effect in the WSe2-CrI3 heterojunction and its contribution to circularly polarized light detection to the coordination function of the spin-filter CrI3, the valley electronic monolayer WSe2, and the spin-dependent charge transfer between them. Our work helps us understand the interplay between the magnetic and optoelectronic properties of WSe2-CrI3 heterojunctions and promotes the developing progress of prospective 2D spin optoelectronic devices.

Charge Transfer and Asymmetric Coupling of MoSe2 Valleys to the Magnetic Order of CrSBr

van der Waals heterostructures composed of two-dimensional (2D) transition metal dichalcogenides and vdW magnetic materials offer an intriguing platform to functionalize valley and excitonic properties in nonmagnetic TMDs. Here, we report magneto photoluminescence (PL) investigations of monolayer (ML) MoSe2 on the layered A-type antiferromagnetic (AFM) semiconductor CrSBr under different magnetic field orientations. Our results reveal a clear influence of the CrSBr magnetic order on the optical properties of MoSe2, such as an anomalous linear-polarization dependence, changes of the exciton/trion energies, a magnetic-field dependence of the PL intensities, and a valley g-factor with signatures of an asymmetric magnetic proximity interaction. Furthermore, first-principles calculations suggest that MoSe2/CrSBr forms a broken-gap (type-III) band alignment, facilitating charge transfer processes. The work establishes that antiferromagnetic-nonmagnetic interfaces can be used to control the valley and excitonic properties of TMDs, relevant for the development of opto-spintronics devices.

Tunable valley splitting in two-dimensional CrBr3/VSe2van der Waals heterostructure under strains and electric fields

Valleytronics opens up fascinating opportunities for using the valley degree of freedom in information storage and quantum computation. Here, based on the first-principles calculations, we investigate the effects of biaxial strains and electric fields on the magnetic, electronic, and valleytronic properties of two-dimensional CrBr3/VSe2van der Waals (vdW) heterostructure consisting of two ferromagnetic monolayers. An interlayer magnetic phase transition from parallel to antiparallel is found when a compressive strain exceeds-2%or a tensile strain exceeds 4% is applied, while the interlayer magnetic configuration remains parallel under perpendicular electric fields. The valley splitting in the conduction bands is significantly enhanced by a compressive strain or an electric field pointing from the VSe2to the CrBr3layer. Specifically, a large valley splitting about 30.8 meV is obtained in the system with antiparallel interlayer magnetic configurations under a compressive strain of-4%, which is more than three times that of pristine CrBr3/VSe2heterostructure. Our findings provide new insights into the future valleytronic applications for two-dimensional magnetic vdW heterostructures.

The Bulk van der Waals Layered Magnet CrSBr is a Quasi-1D Material

Correlated quantum phenomena in one-dimensional (1D) systems that exhibit competing electronic and magnetic order are of strong interest for the study of fundamental interactions and excitations, such as Tomonaga-Luttinger liquids and topological orders and defects with properties completely different from the quasiparticles expected in their higher-dimensional counterparts. However, clean 1D electronic systems are difficult to realize experimentally, particularly for magnetically ordered systems. Here, we show that the van der Waals layered magnetic semiconductor CrSBr behaves like a quasi-1D material embedded in a magnetically ordered environment. The strong 1D electronic character originates from the Cr-S chains and the combination of weak interlayer hybridization and anisotropy in effective mass and dielectric screening, with an effective electron mass ratio of m_X^e/m_Y^e ∼ 50. This extreme anisotropy experimentally manifests in strong electron-phonon and exciton-phonon interactions, a Peierls-like structural instability, and a Fano resonance from a van Hove singularity of similar strength to that of metallic carbon nanotubes. Moreover, because of the reduced dimensionality and interlayer coupling, CrSBr hosts spectrally narrow (1 meV) excitons of high binding energy and oscillator strength that inherit the 1D character. Overall, CrSBr is best understood as a stack of weakly hybridized monolayers and appears to be an experimentally attractive candidate for the study of exotic exciton and 1D-correlated many-body physics in the presence of magnetic order.

Signatures of Electric Field and Layer Separation Effects on the Spin-Valley Physics of MoSe2/WSe2 Heterobilayers: From Energy Bands to Dipolar Excitons

Multilayered van der Waals heterostructures based on transition metal dichalcogenides are suitable platforms on which to study interlayer (dipolar) excitons, in which electrons and holes are localized in different layers. Interestingly, these excitonic complexes exhibit pronounced valley Zeeman signatures, but how their spin-valley physics can be further altered due to external parameters-such as electric field and interlayer separation-remains largely unexplored. Here, we perform a systematic analysis of the spin-valley physics in MoSe2/WSe2 heterobilayers under the influence of an external electric field and changes of the interlayer separation. In particular, we analyze the spin (Sz) and orbital (Lz) degrees of freedom, and the symmetry properties of the relevant band edges (at K, Q, and Γ points) of high-symmetry stackings at 0° (R-type) and 60° (H-type) angles-the important building blocks present in moiré or atomically reconstructed structures. We reveal distinct hybridization signatures on the spin and the orbital degrees of freedom of low-energy bands, due to the wave function mixing between the layers, which are stacking-dependent, and can be further modified by electric field and interlayer distance variation. We find that H-type stackings favor large changes in the g-factors as a function of the electric field, e.g., from -5 to 3 in the valence bands of the Hhh stacking, because of the opposite orientation of Sz and Lz of the individual monolayers. For the low-energy dipolar excitons (direct and indirect in k-space), we quantify the electric dipole moments and polarizabilities, reflecting the layer delocalization of the constituent bands. Furthermore, our results show that direct dipolar excitons carry a robust valley Zeeman effect nearly independent of the electric field, but tunable by the interlayer distance, which can be rendered experimentally accessible via applied external pressure. For the momentum-indirect dipolar excitons, our symmetry analysis indicates that phonon-mediated optical processes can easily take place. In particular, for the indirect excitons with conduction bands at the Q point for H-type stackings, we find marked variations of the valley Zeeman (∼4) as a function of the electric field, which notably stands out from the other dipolar exciton species. Our analysis suggests that stronger signatures of the coupled spin-valley physics are favored in H-type stackings, which can be experimentally investigated in samples with twist angle close to 60°. In summary, our study provides fundamental microscopic insights into the spin-valley physics of van der Waals heterostructures, which are relevant to understanding the valley Zeeman splitting of dipolar excitonic complexes, and also intralayer excitons.

Spontaneous Valley Polarization in a Ferromagnetic Fe(OH)2 Monolayer

At present, creating sizable spontaneous valley polarization is at the center of the study of valleytronics, which, however, is still a huge challenge. In this work, we determined that the ferromagnetic Fe(OH)₂ monolayer of the hexagonal lattice is a highly appealing candidate for valleytronics by using first-principles calculations in conjunction with tight-binding model analysis. In light of the simultaneous inversion symmetry breaking and time-reversal symmetry breaking, we illustrated that the strong spin-orbit coupling and robust ferromagnetic exchange interaction cause a spontaneous valley polarization as large as 67 meV for Fe(OH)₂, indicative of room-temperature application. In addition, the physics of valley-selective circular dichroism, spin/valley Hall effects, and topological phase transition were also discussed.

All-optical control of spin in a 2D van der Waals magnet

Two-dimensional (2D) van der Waals magnets provide new opportunities for control of magnetism at the nanometre scale via mechanisms such as strain, voltage and the photovoltaic effect. Ultrafast laser pulses promise the fastest and most energy efficient means of manipulating electron spin and can be utilized for information storage. However, little is known about how laser pulses influence the spins in 2D magnets. Here we demonstrate laser-induced magnetic domain formation and all-optical switching in the recently discovered 2D van der Waals ferromagnet CrI₃. While the magnetism of bare CrI₃ layers can be manipulated with single laser pulses through thermal demagnetization processes, all-optical switching is achieved in nanostructures that combine ultrathin CrI₃ with a monolayer of WSe₂. The out-of-plane magnetization is switched with multiple femtosecond pulses of either circular or linear polarization, while single pulses result in less reproducible and partial switching. Our results imply that spin-dependent interfacial charge transfer between the WSe₂ and CrI₃ is the underpinning mechanism for the switching, paving the way towards ultrafast optical control of 2D van der Waals magnets for future photomagnetic recording and device technology.

The use of light in driving the magnetization of materials has great technological potential, as well as allowing for insights into the fast dynamics of magnetic systems. Here, the authors combine CrI₃, a van der Waals magnet, with WSe₂, and demonstrate all optical switching of the resulting heterostructure.

Chirality of Valley Excitons in Monolayer Transition-Metal Dichalcogenides

By enabling control of valley degrees of freedom in transition-metal dichalcogenides, valley-selective circular dichroism has become a key concept in valleytronics. Herein, we show that valley excitons, bound electron-hole pairs formed at the K or K̅ valleys upon absorption of circularly polarized light, are chiral quasiparticles characterized by a finite orbital angular momentum (OAM). We further formulate an ab initio many-body theory of valley-selective circular dichroism and valley excitons based on the Bethe-Salpeter equation. Besides governing the interaction with circularly polarized light, the OAM confers upon excitons a finite magnetization that manifests itself through an excitonic Zeeman splitting upon interaction with external magnetic fields. The good agreement between our ab initio calculations and recent experimental measurements of the exciton Zeeman shifts corroborates this picture, indicating that valley excitons can carry angular momentum even in their singlet state.

Spin-Selective Hole-Exciton Coupling in a V-Doped WSe2 Ferromagnetic Semiconductor at Room Temperature

While valley polarization with strong Zeeman splitting is the most prominent characteristic of two-dimensional (2D) transition metal dichalcogenide (TMD) semiconductors under magnetic fields, enhancement of the Zeeman splitting has been demonstrated by incorporating magnetic dopants into the host materials. Unlike Fe, Mn, and Co, V is a distinctive dopant for ferromagnetic semiconducting properties at room temperature with large Zeeman shifting of band edges. Nevertheless, little known is the excitons interacting with spin-polarized carriers in V-doped TMDs. Here, we report anomalous circularly polarized photoluminescence (CPL) in a V-doped WSe₂ monolayer at room temperature. Excitons couple to V-induced spin-polarized holes to generate spin-selective positive trions, leading to differences in the populations of neutral excitons and trions between left and right CPL. Using transient absorption spectroscopy, we elucidate the origin of excitons and trions that are inherently distinct for defect-mediated and impurity-mediated trions. Ferromagnetic characteristics are further confirmed by the significant Zeeman splitting of nanodiamonds deposited on the V-doped WSe₂ monolayer.