Li Yang

Ferroelectric HfO2 is a promising candidate for next-generation memory devices due to its CMOS compatibility and ability to retain polarization at nanometer scales. However, the polar orthorhombic phase responsible for ferroelectricity is metastable and requires extrinsic stabilization, complicating integration with silicon. Here, we predict that twist-induced symmetry breaking in bilayer 1T-HfO2 enables robust, switchable out-of-plane (OOP) polarization. First-principles density functional theory shows that monolayer 1T-HfO2, cleaved from the (111) surface of cubic HfO2, is dynamically stable. Twisting two aligned monolayers forms a moiré superlattice that breaks interlayer symmetry and yields bistable OOP polarization. At a twist angle of 7.34°, the bilayer exhibits ∼16 μC/cm2 polarization, primarily from polar displacements within AB stacking domains. The polarization can be switched via interlayer sliding with a low energy barrier (∼8 meV per formula unit), enabling electric-field tunability in atomically thin ferroelectric devices.

Metallic environments are generally expected to suppress excitons through strong dielectric screening, yet their influence can differ in composite systems where metallic and insulating regions coexist. Here, we investigate excitonic states in Pd _x Cu _1-x CrO ₂ thin films across a percolation-driven metal-insulator transition. Optical spectroscopy and many-body GW calculations show that CuCrO ₂ hosts strongly bound excitons with a binding energy of about 489 meV. With increasing Pd substitution, the system approaches an insulator-to-metal transition near x = 0.5, consistent with the site-percolation threshold of a triangular lattice. In the pre-percolation regime, the excitonic resonance redshifts by 241 meV while the Tanguy continuum onset remains nearly unchanged, consistent with a substantial increase in exciton binding energy before metallization. An image-charge-based excitonic hydrogen model shows that isolated metallic regions can enhance electron-hole binding through image-charge interactions, whereas conventional screening is recovered once a continuous metallic network forms. Although this model provides a possible interpretation of the observed excitonic evolution, an alternative scenario in which metallic and excitonic responses originate from electronically distinct states and evolve independently cannot be excluded. These results reveal unusual metallic-excitonic coexistence near a percolation-driven metal-insulator transition and suggest nanoscale metallic proximity as a possible route for modifying excitonic interactions.

Moiré superlattices provide a powerful platform for flat bands and correlated topological phases, yet most established examples rely on valley-contrasting Berry curvature in hexagonal lattices. Here, we propose a different route to achieve topological moiré minibands based on noncollinear spin–orbit coupling (SOC) in centrosymmetric type-II SOC bilayers. Interlayer hybridization opens local pseudogaps and produces sharply localized Berry curvature, which twisting reconstructs into isolated topological minibands without requiring valley degrees of freedom or hexagonal symmetry. We demonstrate this mechanism in tetragonal Dresselhaus-SOC HgI2, where a Lieb-like moiré potential yields topological flat bands. To improve miniband isolation, we develop a physics-informed machine-learning surrogate that identifies stronger SOC as a key design principle and guides the replacement of Hg by Pb. The resulting PbI2 minibands are narrower and better isolated, supporting correlation-driven magnetism and tunable quantum spin Hall and Chern insulating phases, thereby providing an optimized material realization for experimental exploration.

We predict a so-called axial Hall effect, a Berry-curvature-driven anomalous Hall response, in Lieb-lattice altermagnets. Using a tight-binding model, we reveal a hidden topological degree of freedom: the axial pseudospin. Breaking the double degeneracy of axial symmetry exposes this pseudospin, generating a substantial Berry curvature and pronounced intrinsic anomalous Hall conductivity. Subsequent first-principles calculations confirm the emergence of this effect in the family of strained ternary transition-metal dichalcogenides. Focusing on Mn₂WS₄, we demonstrate that the axial Hall effect essentially arises from the interplay between Dresselhaus spin-orbit coupling and the piezomagnetic response, resulting in highly localized and enhanced Berry curvature. Remarkably, the magnitude of this effect is significant and remains constant under varying strain, underscoring the topological nature of the axial degree of freedom. Beyond monolayers, this effect manifests as a distinctive thickness-dependent modulation of both anomalous and spin Hall responses in multilayer structures. These findings emphasize the critical role of spin-orbit coupling and noncollinear spin textures in altermagnets, an area that has received limited attention, and open new pathways for exploring and tuning intrinsic Hall phenomena and spintronic applications in topological altermagnets.

Altermagnets have recently been recognized as a distinct class of magnetic materials characterized by alternative spin-split electronic structures without net magnetization. Despite intensive studies on their single-particle spintronic and valleytronic properties, many-electron interactions and optical responses of altermagnets remain less explored. In this work, we employ many-body perturbation theory to investigate excited states and their strain tunability. Using monolayer Mn2WS4 as a representative candidate, we uncover a novel spin valley-dependent excitonic selection rule in two-dimensional altermagnetic Lieb lattices. In addition to strongly bound excitons, we find that linearly polarized light selectively excites valley spin-polarized excitons. Moreover, due to the interplay between altermagnetic spin symmetry and electronic orbital character, we predict that applying uniaxial strain can lift valley degeneracy and enable the selective excitation of spin-polarized excitons, an effect not achievable in previously studied transition-metal dichalcogenides. These spin-valley-locked excitonic states and their strain tunability offer a robust mechanism for four-fold symmetric altermagnets to encode, store, and read valley/spin information.

Two-dimensional (2D) materials offer strong light-matter interaction and design flexibility beyond bulk semiconductors, but an intrinsic limit is the low absorption imposed by the atomic thickness. A long-sought-after goal is to achieve complementary absorption enhancement through energy transfer (ET) to break this limit. However, it is found challenging due to the competing charge transfer (CT) process and lack of resonance in exciton states. Here, we report highly efficient ET in a 2D hybrid organic-inorganic heterostructure (HOIST) of Me-PTCDI/WS ₂ . Resonant ET is observed leading to enhanced WS ₂ photoluminescence (PL) by 124 times. We identify Dexter exchange between the Frenkel state in donor and an excited 2 s state in acceptor as the main ET mechanism, as supported by density functional theory calculations. We further demonstrate ET-enhanced phototransistor devices with enhanced responsivity by nearly 1000 times without sacrificing the response time. Our results expand the understanding of interlayer relaxation processes in 2D materials and open opportunities in optoelectronic devices.

Altermagnetism, featuring alternating spin structures in reciprocal space, has sparked growing interest. Here, we predict novel real-space alternative piezomagnetic and piezoelectric responses in an emerging altermagnetic family of Lieb lattices, specifically transition-metal chalcogenides M₂WS₄ (M = Mn, Fe, Co). The unique S₄T crystal-spin symmetry leads to distinct magnetic and electric responses depending on the direction of applied stress. When subjected to axial stress, they exhibit a giant piezomagnetic response, which is about 1-2 orders of magnitude larger than that of most piezomagnetic materials, while the residual C₂ symmetry suppresses the piezoelectric effect. In contrast, diagonal stress induces an imbalance of oppositely aligned electric-dipole moments and a significant piezoelectric response, while in-plane mirror symmetry inhibits the piezomagnetic effect. This alternative piezoresponse offers an unprecedented opportunity to precisely control electric and magnetic properties independently, opening new avenues for altermagnetic materials in high-fidelity multifunctional memory and sensor applications.

α-phase Ruthenium (III) chloride (α-RuCl3) has attracted significant attention because of its potential for realizing Kitaev quantum spin liquid. In this work, we employ first-principles many-body perturbation theory to study its many-electron interactions and excited-state properties. We find enhanced many-electron interactions that dominate quasiparticle energies and optical responses in α-RuCl3. Our calculated quasiparticle band gap of bulk structure is about 1.75 eV that agrees well with recent scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy measurements. Furthermore, our calculated first and second primary bright excitons are located at 1.23 eV and 1.98 eV, respectively. These excitons show good consistency with the main features observed in the optical absorption spectrum. We extend our investigation to monolayer α-RuCl3, examining the zigzag antiferromagnetic (AFM) and ferromagnetic (FM) phases. In addition to significant excitonic effects, the optical spectrum of the zigzag AFM phase exhibits anisotropic behavior, while the FM phase demonstrates isotropic characteristics. The different optical response behaviors provide an efficient approach to identify the energy nearly degenerate magnetic states, which can both potentially exist in fabricated samples.