Interlayer sliding as a multifunctional knob for simultaneous valley polarization in altermagnetic Cr2MoX4 bilayer

Interlayer excitons (IXs) in van der Waals (vdW) heterostructures offer prolonged lifetimes and electrically tunable dipoles, enabling advanced excitonic devices and coherent light sources. However, achieving stable and efficient room temperature IX emission for applications requires vdW systems with both momentum-matched band alignment and feasible scalable fabrication capability, which is still challenging. Here, we propose to address this issue by demonstrating vdW epitaxy of a uniformly distributed bilayered 2H-WSe2/PbI2 heterojunction, which exhibits uniform and stable IX emission at 1.36 eV at room temperature. First-principle calculations and experiments confirm that the momentum-direct IX emission at the Γ point is possible. Thanks to the inorganic nature of PbI2, the occurrence of IX emission is air stable. The IX emission intensity retains 84.2% of its initial intensity after 4 months in ambient condition. The high IX binding energy (85.3 meV), long lifetime (3.77 ns), and large blueshift (45 meV) during power-dependent emission spectra demonstrate the strong Coulomb interactions and robust nature of the IXs. More delightfully, the valley information in IXs is preserved, showing a stable valley polarization degree of 19.58% at 83 K. These results indicate that the bilayered 2H-WSe2/PbI2 heterojunction is a promising platform for promoting the development of IX-related fundamental science research and applications.

Valley-dependent topological physics offers a promising avenue for designing nanoscale devices based on gapless single-layer graphene. To demonstrate this potential, we investigate an electrical bias-controlled topological discontinuity in valley polarization within a two-segment armchair nanoribbon of gapless single-layer graphene. This discontinuity is created at the interface by applying opposite in-plane, transverse electrical biases to the two segments. An efficient tight-binding theoretical formulation is developed to calculate electron states in the structure. In a reference configuration, we obtain energy eigenvalues and probability distributions that feature interface-confined electron eigenstates induced by the topological discontinuity. Moreover, to elucidate the implications of interface confinement for electron transport, a modified configuration is introduced to transform the eigenstates into transport-active, quasi-localized ones. We show that such states result in Fano “anti-resonances” in transmission spectra. The resilience of these quasi-localized states and their associated Fano fingerprints is examined with respect to fluctuations. Finally, a proof-of-concept band-stop electron energy filter is presented, highlighting the potential of this confinement mechanism and, more broadly, valley-dependent topological physics in designing nanoscale devices in gapless single-layer graphene.

Altermagnets, a novel class of collinear magnetic materials, exhibit unique spin-split band structures, yet topological insulating states in intrinsic altermagnetic systems are rare. Here, we identify monolayer Fe2X2O (X = Cl, Br, I) as a new family of 2D altermagnetic real Chern insulators. These materials display robust d-wave altermagnetic ordering, semiconducting band gaps, and nontrivial real Chern numbers per spin channel, yielding spin-polarized topological corner modes. They also feature spin-polarized valleys with strong altermagnetism-valley-spin-lattice coupling, enabling valley-selective excitation via linear dichroism and strain-induced valley polarization. In multiferroic Fe2Cl2O, magnetism coexists with ferroelasticity, and an applied strain can switch the Néel vector. These findings position 2D iron oxyhalides as a promising platform for exploring the altermagnetism and magnetic topological states for spintronics and valleytronics.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged as a unique class of materials that host robust valley degrees of freedom. This opens exciting possibilities for valleytronic applications where information is encoded in the electronic valleys of momentum space. The inherent inversion symmetry breaking and strong spin-orbit coupling in monolayer TMDs give rise to valley-selective optical selection rules and spin-valley locking, making them ideal candidates for manipulating valley pseudospins in next-generation quantum devices. This Perspective highlights the fundamental valley physics focusing on valley dynamics in 2D TMDs, including valley polarization and coherence, and reviews recent experimental advancements in valley control using optical, electrical, and magnetic means. We further discuss emerging directions such as valley-based quantum computing, coherent valley manipulation at ultrafast time scales, and hybrid valleytronic architectures that integrate TMDs with other quantum materials. The Perspective outlines key challenges, including valley depolarization mechanisms, material quality, and scalability, and proposes strategic approaches to overcome these hurdles. As valleytronics research transitions from proof-of-concept studies toward real-world technologies, 2D TMDs stand at the forefront of a promising frontier in quantum information science.

Two-dimensional (2D) ferrovalley materials with spontaneous valley polarization have emerged as pivotal platforms for valleytronic applications. Generally, 1T-phase 2D materials are overlooked as ferrovalley candidates due to their intrinsic inversion...

Using first-principles calculations, we explore the electronic and topological properties of Janus VAZ3H single layers (A = Si, Ge; Z = N, P) that are dynamically and thermally stable. In the strain-free state, VSiN3H, VSiP3H, and VGeN3H demonstrate direct bandgap ferrovalley (FV) semiconducting properties, while VGeP3H displays an indirect bandgap. The easy magnetization axis varies among these materials, with VSiN3H and VGeN3H preferring in-plane magnetization, whereas VSiP3H and VGeP3H favor out-of-plane magnetization. Furthermore, the electronic structure analysis reveals valley polarization at the K and K' points. When subjected to strain, these systems experience phase transitions, such as direct-to-indirect bandgap shift, the evolution from FV semiconducting to half-valley metal (HVM), and the emergence of a quantum anomalous Hall (QAH) phase within certain strain intervals. The QAH phase is identified by chiral edge states and quantized anomalous Hall conductivity (AHC), supported by an integer AHC plateau of 1e2/h and a Chern number of 1. These results highlight the tunability of VAZ3H SLs through strain engineering, providing a potential platform for valleytronic and topological applications.

Manipulation of valley polarization and anomalous valley Hall effect in monolayer ferrovalley AgMoP2S6

Efficient control of the valley index is of great importance for both fundamental research and device applications, yet it remains a challenging problem. Here, through symmetry analysis and an effective k·p model, we propose a novel mechanism for coupling valley index with ferroelectricity in a two-dimensional (2D) multiferroic lattice. The physics behind this is that inequivalent potential arising from ferroelectricity can reverse and annihilate nonsymmetric trigons. Owing to the intimate connection between nonsymmetric trigons and valley physics, the valley index is locked to ferroelectric polarization. This enables the efficient electrical reversal of valley index for carriers and the electrical creation/annihilation of valley polarization. Moreover, based on first-principles calculations, we validate this mechanism in the 2D multiferroic semiconductor TiCr2O4, which favors the paraelectric state as a metastable state. Our work establishes a new paradigm for the design and optimization of valleytronic devices.

Realize a strain-driven valley-polarization effect in the altermagnetic material Nb 2 Cl 2 .

Optical control of topology, particularly in the presence of electron correlations, is an interesting topic with broad scientific and technological impact1-4. Twisted MoTe2 bilayer (tMoTe2) is a zero-field fractional Chern insulator (FCI)5-10, exhibiting the fractionally quantized anomalous Hall effect11-14. As the chirality of the edge states and sign of the Chern number are determined by the underlying ferromagnetic polarization15,16, manipulation of ferromagnetism would realize control of the Chern insulator (CI)/FCI states. Here we demonstrate control of ferromagnetic polarization, and thus the CI and FCI states, by circularly polarized optical pumping in tMoTe2. At low excitation power, we achieve on-demand preparation of ferromagnetic polarization by optical training, that is, electrically tuning the system from non-ferromagnetic to desirable ferromagnetic states under helicity-selective optical pumping. With increased excitation power, we further realize direct optical switching of ferromagnetic polarization at a temperature far below the Curie temperature17,18. Both optical training and direct switching are most effective near CI and FCI states, which we attribute to a gap-enhanced valley polarization of optically pumped holes. The magnetization can be dynamically switched by modulating the helicity of optical excitation. Spatially resolved measurements further demonstrate optical writing of ferromagnetic, and thus CI (or FCI) domains. Our work realizes precise optical control of a topological quantum many-body system with potential applications in topological spintronics, quantum memories and creation of exotic edge states by programmable patterning of integer and fractionally quantized anomalous Hall domains4,19.

The integration of non-volatile spin and valley control in 2D quantum systems remains a pivotal challenge for spintronic and valleytronic functionalities. Here, we demonstrate persistent spin and valley polarizations in a van der Waals heterostructure comprising bulk antiferromagnetic CrPS4 and monolayer MoSe2, achieved via interfacial magnetic proximity effects. The 1L-MoSe2/bulk-CrPS4 heterostructure exhibits non-volatile hysteresis in chiral photoluminescence (PL), directly linked to the antiferromagnetic ordering of bulk-CrPS4. This helicity of PL persists at zero field, enabled by the spin-polarized charge transfer from the K valley in the MoSe2 monolayer to the conduction band of CrPS4, breaking the valley degeneracy without external stimuli. Remarkably, the PL helicity switches surprisingly at a magnetic field of ∼ 0.5 T, a 17-fold smaller than the spin-flip field of ∼8.5 T in bulk CrPS4. Our work establishes bulk antiferromagnet-based heterostructure as a robust platform for low-energy, magnetically tunable quantum devices, bridging the gap between the transient valleytronic phenomena and practical non-volatile applications.

Two-dimensional transition metal dichalcogenides (TMDs) heterostructures formed moiré superlattices have emerged as a new platform for exploring correlated excitonic states and valleytronic phenomena. Despite the significant progress in moiré-trapped single excitons, the valley polarization and fine structure of moiré-trapped biexcitons remain poorly understood, with the existing studies reporting only limited or zero polarization and lacking insight into the underlying mechanisms. Here, we study the moiré-trapped interlayer biexcitons in WS2/WSe2 heterostructures through power- and temperature-dependent photoluminescence (PL) spectroscopy. We find that the valley polarization of these biexcitons can be effectively tuned, reaching ~ at 120 K. This behavior is attributed to the different occupation of intravalley and intervalley biexcitons within the fine structure, with the intravalley biexcitons playing a dominant role. The power-dependent energy splitting and temperature-dependent polarization trends further confirm the existence of biexciton fine structure and its influence on valley polarization. Furthermore, the experiment revealed a fine structure splitting of 2.77 meV, consistent with theoretical calculations. Our study provides new insight into the rational control of excitonic states in moiré superlattices and establishes a basis for developing advanced valleytronic devices, such as polarization-sensitive photodetectors and quantum light sources.

Mixed effects of ferromagnetic metal stripes, strains and Schottky metal stripe on valley polarization of electrons in a graphene

Many topological phases host gapless boundary modes that can be markedly modified by electronic interactions. Even for the long-studied edge modes of quantum Hall phases1,2, forming at the boundaries of two-dimensional electron systems, the nature of such interaction-induced changes has been elusive. Despite advances made using local probes3-13, key experimental challenges persist: the lack of direct information about the internal structure of edge states on microscopic scales, and complications from edge disorder. Here we use scanning tunnelling microscopy to image pristine electrostatically defined quantum Hall edge states in graphene with high spatial resolution and demonstrate how correlations dictate the structures of edge channels on both magnetic and atomic length scales. For integer quantum Hall states in the zeroth Landau level, we show that interactions renormalize the edge velocity, dictate the spatial profile for co-propagating modes and induce unexpected edge valley polarization, which differs from the bulk. Although some of our findings can be understood by mean-field theory, others show breakdown of this picture, highlighting the roles of edge fluctuations and inter-channel couplings. We also extend our measurements to spatially resolve the edge state of fractional quantum Hall phases and detect spectroscopic signatures of interactions in this chiral Luttinger liquid. Our study establishes scanning tunnelling microscopy as a promising tool for exploring the edge physics of the rapidly expanding group of two-dimensional topological phases, including recently realized fractional Chern insulators.

The ability to control spin and valley degrees of freedom in two-dimensional materials offers promising prospects for next-generation electronic and spintronic devices. However, achieving tunable valley polarization and intrinsic anomalous Hall effect (AHE) within a single material system without an external magnetic or optical field remains challenging. Here, we present two complementary mechanisms to realize robust valley polarization control in TiInSe3. In the monolayer, electron doping modifies the magnetic easy axis, inducing spontaneous valley polarization. More importantly, in bilayers with specific stacking configurations, intrinsic interlayer charge transfer breaks inversion symmetry, enabling spontaneous valley polarization and a switchable layer-resolved AHE even in the absence of doping. These findings establish fundamental strategies to manipulate spin and valley degrees of freedom through doping and stacking engineering.

Quantum anomalous Hall (QAH) and valley-polarized QAH (VP-QAH) effects offer dissipationless edge transport, which is essential for low-power electronics and valleytronics devices. However, finding both phenomena in a single material remains significantly challenging. Herein, we develop a first-principles screening approach to identify QAH and VP-QAH effects in functionalized Ti-Cr-C MXenes due to their intrinsic magnetism and strong spin-orbit coupling. Out of 100 noncentrosymmetric MXenes, 28 are found to be dynamically stable, among which 14 (13) exhibit in-plane (out-of-plane) ferromagnetism and one shows out-of-plane stripy antiferromagnetism in the respective ground states. Ferromagnetic Ti-Cr-C-H-OCN MXene shows the QAH effect, while Ti-Cr-C-CN-Cl and Ti-Cr-C-SCN-O exhibit the strain-induced VP-QAH effect. To further enrich the search space, we explore heterostructuring and, notably, find that compressed Ti-Cr-C-H-OCN MXene/2H-CrS2 van der Waals heterostructure hosts the VP-QAH effect. Importantly, electric-field-induced topological phase transitions in the Ti-Cr-C-H-OCN MXene/2H-CrS2 heterostructure and pronounced valley polarization (∼33.7-35.3 meV) in Ti-Cr-C-SCN-O MXene are observed under an applied field, demonstrating strong potential for valleytronics applications.

Abstract The quantum anomalous valley Hall effect (QAVHE) represents an important research direction in topological electronics. In this work, based on first-principles calculations combined with a tight-binding (TB) model, we predict that the two-dimensional (2D) ferromagnetic material Pt 4 Hg 2 Cl 3 X 3 (X = F, Br) can realize the QAVHE. The system exhibits a Curie temperature of 219 K, and its magnetic easy axis can be tuned from the in-plane to the out-of-plane direction under biaxial strain, leading to a remarkable change in valley polarization and enabling effective control of the valley degree of freedom. A strain-driven topological phase transition from a trivial to a nontrivial state is revealed, resulting in the coexistence of the anomalous valley Hall effect (AVHE) and the quantum anomalous Hall effect (QAHE), characterized by a Chern number ( C = 1) and chiral spin-valley-locked states. The coexistence of robust valley polarization and a high Curie temperature makes this system an ideal platform for realizing dissipationless electronic transport and topological quantum computation, offering valuable prospects for the development of valleytronics, spintronics, and topological quantum materials.

Two-dimensional (2D) ferrovalley bilayers with coupled spin, valley, and layer degrees of freedom offer a promising platform for valleytronic applications. However, a unified understanding of how these properties can be controlled by external stimuli remains lacking. Here, we demonstrate a universal mechanism for electrically tuning valley splitting and the spin-valley-layer polarized anomalous Hall effect (AHE) in Janus YFI bilayers. We systematically examine how interlayer sliding and external electric fields modulate the band structure and valley polarization in 2H-YFI bilayers and construct a two-band k·p model to reveal the underlying physics. Interestingly, interlayer antiferromagnetic coupling and layer-asymmetric charge redistribution in YFI bilayers give rise to a spin-valley-layer polarized AHE. Both interlayer sliding and external electric fields effectively control the valley polarization and Hall response, showing functional equivalence in modulating these properties. Our work establishes a unified electrical control strategy for bilayer ferrovalley systems, facilitating the design of nonvolatile valleytronic devices.

Abstract The presence of two degenerate unconventional quantum states, known as valleys, in 2D materials is often regarded as analogous to binary operational units – representing 0 and 1. These valley states can also exist in superposition, effectively functioning as qubits. The unique properties of valleys make 2D materials a viable possibility for integration into devices designed for coherent encoding, processing, and reading of both classical and quantum information. To realize such applications, precise control over valley states is essential, enabling the implementation of valley‐based devices for practical purposes. A novel approach was introduced to exercise necessary control in achieving valley polarization using a nonresonant single color linearly polarized laser pulse. By exploiting the asymmetric waveform of the linear pulse with skewed polarization, valley polarization was successfully induced. Remarkably, the method enables the complete reversal of valley polarization between valleys in 2D semiconductors. The asymmetric waveform also generates a valley current, which serves as a quantifiable marker for induced valley polarization. This approach provides a means to induce valley polarization, achieve its complete reversal, and quantify it through the associated valley current. The present findings hold significant potential for advancing valleytronics at ambient conditions by showcasing an all‐optical approach to effectively harness and manipulate valley states in 2D semiconductors.