Guangyao Miao

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.

Single-layer transition metal dihalides grown on conducting substrates were shown to host stable polarons. Here, we investigate polarons in insulating single-layer MnBr$_2$ grown by molecular beam epitaxy on three different substrates, namely graphene on Ir(110), graphene on Ir(111), and Au(111). The number densities and species of polarons observed vary strongly as a function of the substrate. For MnBr$_2$ grown on Ir(110) the largest number of polaron species is observed, namely four, of which three show clear similarities with the species observed for CoCl$_2$ on graphite. Polarons in single-layer MnBr$_2$ are observed up to 300\,K. They can be created, converted, and moved by the STM tip when a tunneling current flows at a proper bias voltage. For graphene on Ir(110) as a substrate, mobile polarons in MnBr$_2$ are guided through the periodic potential imposed from the super-moiré resulting from the interaction of MnBr$_2$ with graphene and Ir(110). Our findings indicate that modeling of polarons in such single-layer insulators in contact with a conducting substrate requires to take the substrate explicitly into account.

Nonadiabatic effects profoundly influence lattice dynamics, resulting in phonon renormalizations not only at the center of Brillouin zone (BZ), but also across the entire dispersion at finite momenta. These nonadiabatic phenomena exhibit clear dimensional dependencies and remain largely unexplored experimentally in low-dimensional systems. Here, we utilize high-resolution electron energy loss spectroscopy to investigate nonadiabatic phonon dispersion renormalization in monolayer graphene (MLG) and Bernal bilayer graphene (BLG). We present comprehensive phonon spectra measurements for both MLG and BLG across the full BZ. The high-resolution data reveal an intriguing "W" -shaped dispersion for the longitudinal optical phonon in MLG and a "V" -shaped dispersion in BLG near the BZ center, in contrast to the conventional "U" -shaped parabolic dispersions. Combining theoretical analysis, these anomalous phonon renormalizations are demonstrated to originate from nonadiabatic electron-phonon couplings. The comparative study of MLG and BLG gives a generic understanding of the impact of nonadiabatic effects on phonon dispersions in doped two-dimensional systems.

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.

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.

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.

In the pursuit of manipulating the properties of single atoms, the surface-supported metal–organic frameworks (MOFs) provide us opportunities to individually address the electronic and magnetic properties of coordinated metal atoms by scanning tunneling microscopy. Recently, we have synthesized Ni-TPyP (TPyP = 5,10,15,20-tetra-(4-pyridyl) porphyrin) networks with dinuclear Ni centers on a Au(111) surface, in which the top-Ni atoms are sitting above the molecular plane. Here, we investigate the top-Ni atoms and their hydrogenated derivatives by low-temperature scanning tunneling microscopy and spectroscopy, and show that the electronic and magnetic states of top-Ni atoms can be manipulated by hydrogen adsorption. Specifically, by fitting the spin-flip spectra in vertical magnetic field, we find the spin state of top-Ni atoms is tuned from S = 1/2 to S = 1 by attaching one H atom and S = 3/2 by attaching two H atoms. Our work demonstrates atomic-scale control over the electronic and magnetic properties of coordinated metal atoms in a surface-supported MOF.

Trigonal tellurium with helical structure exhibits intriguing properties due to its inherent chirality, which has attracted considerable research interest recently. Using cryogenic scanning tunneling microscopy/spectroscopy and density functional theory calculations, we investigated quasiperiodic defects at particular edges of Te nanoribbons grown on graphene/SiC(0001) substrate. The detailed topography of edge defects reveals the chirality of Te nanoribbon, and the defects induce a quasiperiodic modulation of the electronic states along the edge. The average period of the edge defects varies with the twist angle between Te nanoribbon and substrate, implying the electronic modulation can be tuned by the interfacial interaction. Based on the morphology analyses and modeled calculations, we further propose the growth mechanism of the Te nanoribbons.