Weihua Wang

Berry curvature in transition metal dichalcogenides (TMDs) critically governs spin‐valley degrees of freedom, nonlinear/quantum electrical transport, and magnetoelectric coupling, underpinning emergent functionalities in valleytronic, magnetoelectric, and optoelectronic devices. Self‐intercalated Cr _{1+ δ} Te ₂ , as a representative magnetic semimetal among TMDs, enables controlled the Berry curvature due to its rigid band shift under external stimuli. In this work, we study the thickness‐tuned Berry curvature, manifested by intrinsic anomalous Hall effect (AHE), in van der Waals (vdW) epitaxial Cr ₃ Te ₄ ( δ = 0.5) ultrathin films. At low temperatures, the intrinsic AHE of Cr ₃ Te ₄ ultrathin film has a negative‐polarity below 13 monolayers and positive‐polarity for 22 monolayers. This sign reversal, indicating the Berry curvature modulation, could be attributed to the Fermi level shift caused by Te(CrTe ₂ )‐termination at the surface and interface. These findings highlight the crucial role of controlling the electronic structure of vdW materials at atomic scale and pave the way for the design of high‐density, miniaturized topological spintronic devices.

Crystal structure engineering is pivotal for tailoring material properties, particularly in atomically thin two-dimensional (2D) transition-metal chalcogenides that exhibit rich, crystal-phase-dependent electronic, optical, and magnetic properties. However, their crystal-phase-controlled synthesis remains a critical challenge that limits their practical applications. Here, we report the phase-controlled synthesis of atomically thin chromium telluride films, including CrTe₃, CrTe₂, Cr_1+δTe₂, and CrTe, on graphene/SiC substrates via precise tuning of molecular beam epitaxy parameters. The growth mechanism is further revealed. We have systematically characterized the different structures and electronic, chemical, and magnetic properties of these films using scanning tunneling microscopy/spectroscopy, scanning transmission electron microscopy, density functional theory calculations, X-ray photoelectron/absorption spectroscopy, and circular-polarized X-ray photoemission electron microscopy. Heterostructures of different chromium telluride phases are further fabricated, demonstrating band engineering enabled by phase control of chromium tellurides.

As the conventional binary CMOS technology approaches its fundamental limits in device scaling and energy efficiency, multivalued logic (MVL) has emerged as a promising strategy to enhance information density and computational capability. Two-dimensional tellurium (Te), owing to its outstanding hole transport properties, provides an attractive material platform for advanced logic devices. In this work, surface selenium-doped tellurium thin films (Te_xSe_1–x) are employed as a key functional layer, exhibiting gate-tunable, p-type-dominated ambipolar transport behavior. By integrating Te_xSe_1–x with n-type two-dimensional semiconductors, heterojunction devices with pronounced antiambipolar transport characteristics are constructed, establishing a solid foundation for novel logic functionalities. Based on these heterostructures, two representative logic device demonstrations are realized: (i) a binary inverter based on a Te_xSe_1–x/MoS₂ heterojunction, exhibiting a characteristic Λ-shaped voltage transfer curve; and (ii) a stable ternary inverter implemented in a Te_xSe_1–x/MoSe₂ device, showing a distinct W-shaped transfer characteristic with a logic-state switching ratio approaching 10⁴. These results highlight the significant potential of tellurium-based heterostructures for scalable binary and multivalued logic devices, offering a viable platform for next-generation MVL circuits.

The development of two-dimensional (2D) semiconductors is limited by the lack of doping methods. We propose surface isovalent substitution as an efficient doping mechanism for 2D semiconductors by revealing the evolution of the structure and electronic properties of 2D Se/Te. Because of the different electronegativity of Se and Te, Se substitution for Te at the specific lattice sites introduces electric dipoles and leads to charge redistribution, which lowers the work function and tunes the Te films from p-type to n-type semiconductors. This differs from the gap enlargement of alloy films with random Se-Te substitutions. This doping method minimizes the change of lattice structure and surface roughness, which benefits structure stacking. Further increasing the Se content leads to the formation of two types of 2D semiconducting Se polymorphs.

2D magnetic crystals with atomic thickness exhibit intriguing physical properties, which have attracted considerable research interest in the related materials’ family, both in fundamental research and in developing spintronic devices. The recent discovery of some non‐van der Waals 2D magnetic crystals expands the systems. Nevertheless, the relationship between the dimensionality of microscopic magnetic exchange interactions and macroscopic magnetic properties at the 2D limit remains to be fully elucidated. Here, we have fabricated mono‐phased continuous ultrathin CrTe₂ and Cr₃Te₄ films by molecular beam epitaxy and elucidated the diverse magnetism tuned by the dimensionality of exchange interactions by a joint study of spin‐polarized scanning tunneling microscopy, magnetization, magneto‐transport measurements, and density functional theory calculations. The transition from a zigzag‐antiferromagnetic order in the monolayer CrTe₂ to a ferromagnetic (FM) order in the second‐layer CrTe₂ is confirmed, which is driven by their varied in‐plane lattice constants induced change of 2D exchange interactions. A robust FM state with large perpendicular magnetic anisotropy in Cr₃Te₄ is observed, originating from its strong 3D exchange interactions. The observed evolution of magnetism demonstrates that the dimensionality of magnetic exchange interactions strongly influences magnetism even at the 2D limit.

The remarkable increase in superconducting transition temperature (T_c) observed at the interface of one-unit-cell FeSe films on SrTiO₃ substrates (1 uc FeSe/STO)¹ has attracted considerable research into the interface effects^2,3,4,5,6. Although this high T_c is thought to be associated with electron–phonon coupling (EPC)², the microscopic coupling mechanism and its role in the superconductivity remain elusive. Here we use momentum-selective high-resolution electron energy loss spectroscopy to atomically resolve the phonons at the FeSe/STO interface. We uncover new optical phonon modes, coupling strongly with electrons, in the energy range of 75–99 meV. These modes are characterized by out-of-plane vibrations of oxygen atoms in the interfacial double-TiO_x layer and the apical oxygens in STO. Our results also demonstrate that the EPC strength and superconducting gap of 1 uc FeSe/STO are closely related to the interlayer spacing between FeSe and the TiO_x terminated STO. These findings shed light on the microscopic origin of the interfacial EPC and provide insights into achieving large and consistent T_c enhancement in FeSe/STO and potentially other superconducting systems.

One-dimensional (1D) π-d conjugated metal-organic frameworks (c-MOFs) have garnered widespread research interest in chemical energy storage and conversion. In this work, we introduce a universal principle to engineer the topological bands of 1D c-MOFs. Connected by d orbitals of transition metals, two equivalent hidden molecular π orbitals in 1D c-MOFs can generate a staggered hopping within and between the organic ligands, forming Su-Schrieffer-Heeger-shaped 1D topological bands. Guided by this discovery, we investigate the electronic structures of the typical 1D c-MOF assembled from Ni atoms and 2HQDI (QDI = 2,5-diamino-1,4-benzoquinonediimine) precursors (NiQDI) by first-principles calculations, revealing 1D topological bands around the Fermi level. Due to local bonding variations at the QDI terminations, these two hidden molecular π orbitals become atomically bonded but electronically separated at the edge QDI, creating spatially localized in-gap topological edge states at the end of the NiQDI chain. This definitive signature for 1D topological bands is identified through differential conductance spectra in scanning tunneling microscopy measurements. Our results provide conclusive experimental evidence for topological bands in 1D c-MOFs, paving the way for exploring the topological physics in organic materials through frontier molecular orbitals.

The electronic coupling was examined in 1D CMOF by investigating the electronic states in the FeQDI polymer at the atomic scale.

Phonon splitting of the longitudinal and transverse optical modes (LO-TO splitting), a ubiquitous phenomenon in three-dimensional polar materials, will break down in two-dimensional (2D) polar systems. Theoretical predictions propose that the LO phonon in 2D polar monolayers becomes degenerate with the TO phonon, displaying a distinctive “V-shaped” nonanalytic behavior near the center of the Brillouin zone. However, the full experimental verification of these nonanalytic behaviors has been lacking. Here, using monolayer hexagonal boron nitride (h-BN) as a prototypical example, we report the comprehensive and direct experimental verification of the nonanalytic behavior of LO phonons by inelastic electron scattering spectroscopy. Interestingly, the slope of the LO phonon in our measurements is lower than the theoretically predicted value for a freestanding monolayer due to the screening of the Cu foil substrate. This enables the phonon polaritons in monolayer h-BN/Cu foil to exhibit ultra-slow group velocity (~5 × 10⁻⁶c, c is the speed of light) and ultra-high confinement (~ 4000 times smaller wavelength than that of light). These exotic behaviors of the optical phonons in h-BN presents promising prospects for future optoelectronic applications.

Due to the diversity of molecular building blocks, the two-dimensional (2D) metal-organic frameworks (MOFs) are ideal platforms to realize exotic lattice models in condensed matter theory. In this work, we demonstrate the universal existence of topological corner states in 2D MOFs with a star lattice configuration, and confirm the intriguing higher-order nontrivial topology in the energy window between two Kagome-bands, or between Dirac-band and four-band. Furthermore, combining first-principles calculations and scanning tunneling microscopy measurements, the unique topological corner state is directly identified in monolayer Ni₃(HITP)₂ (HITP = 2,3,6,7,10,11-hexaiminotriphenylene) grown on the Au(111) substrate. Our results not only illustrate the first organic topological state in the experiments, but also offer an exciting opportunity to study higher-order topology in 2D MOFs with the large insulating band gap.