arXiv subjects

Shangkun Mo

Publications and source records attributed to Shangkun Mo.

7 recordsLinked to original sources

arXiv2026

One-Step Epitaxial Access to Rhombohedral Graphene Flat-Band States on Step-Bunched SiC

Rhombohedral graphene multilayers provide a moir\'e-free platform for correlated and topological flat-band physics, but direct, transfer-free epitaxial access to thickness-tunable multilayers remains limited. Here we report a one-step graphitization route on 4$^\circ$ off-axis 4H-SiC, in which high-temperature flash annealing simultaneously drives self-organized step bunching and multilayer graphene formation. Atomic-resolution cross-sectional scanning transmission electron microscopy identify local ABC registry and distinguish rhombohedral from Bernal stacking. The thickness is tuned from bilayer to more than twenty layers by varying single parameter, the annealing temperature. Angle-resolved photoemission spectroscopy directly tracks the thickness-dependent evolution from interface-dominated low-energy states toward pronounced near-Fermi-level flat-band spectral weight in thick multilayers. Low-temperature scanning tunneling microscopy and spectroscopy on a 17-layer film further reveal a 13.4 meV low-energy spectral reconstruction and a $\sqrt{3} \times \sqrt{3}$ Kekul\'e-like modulation, providing microscopic signatures consistent with an intervalley-mixed electronic texture. This one-step, transfer-free approach establishes step-bunched SiC as an epitaxial platform that links stacking engineering with moir\'e-free correlated flat-band electronic states.

cond-mat.mtrl-sci↗

arXiv2025

Quasi-steady electron-excitonic complexes coupling in a two-dimensional semiconductor

Excitons and their complexes govern optical-related behaviors in semiconductors. Here, using angle-resolved photoemission spectroscopy (ARPES), we have elucidated the light-matter interaction mediated by quasi-steady excitonic complexes within a monolayer of the prototypical two-dimensional (2D) semiconductor WSe2. Under continuous incident light, we have observed the generation of quasi-steady excitons and their complexes, encompassing ground and excited state excitons, trions, as well as their intricate interplay. We further show spectral evidence of electronic excitation states within the background of quasi-steady excitonic complexes, characterized by valence band (VB) effective mass renormalization, the enhanced spin-orbit coupling (SOC), the formation of an excitonic gap near the Fermi level (EF ) of the conduction band (CB), and intervalley excitonic band folding. Our findings not only unveil a quasi-steady excitonic complex background for the creation of diverse electronic excitations in 2D semiconductors but also offer new insights into the role of excitons in the charge density wave (CDW) formation mechanism and facilitate the advancement of correlated electronic state engineering based on the coupling between electrons and excitonic complexes in a quasi-equilibrium state.

cond-mat.str-el↗

arXiv2025

Surface band-selective moir\'e effect induces flat band in mixed-dimensional heterostructures

In this work, we reveal a curious type of moir\'e effect that selectively modifies the surface states of bulk crystal. We synthesize mixed-dimensional heterostructures consisting of a noble gas monolayer grow on the surface of bulk Bi(111), and determine the electronic structure of the heterostructures using angle-resolved photoemission spectroscopy. We directly observe moir\'e replicas of the Bi(111) surface states, while the bulk states remain barely changed. Meanwhile, we achieve control over the moir\'e period in the range of 25 {\AA} to 80 {\AA} by selecting monolayers of different noble gases and adjusting the annealing temperature. At large moir\'e periods, we observe hybridization between the surface band replicas, which leads to the formation of a correlated flat band. Our results serve as a bridge for understanding the moir\'e modulation effect from 2D to 3D systems, and provide a feasible approach for the realization of correlated phenomena through the engineering of surface states via moir\'e effects.

cond-mat.str-el↗

arXiv2025

Observation of quasi-steady dark excitons and gap phase in a doped semiconductor

Exciton plays an important role in optics and optics-related behaviors and leads to novel correlated phases like charge order, exciton insulator, and exciton-polariton condensation. Dark exciton shows distinct properties from bright one. However, it cannot be directly detected by conventional optic measurements. The electronic modulation effect of dark excitons in quasi-equilibrium distribution, critical for electronic devices in working status, is still elusive. Here, using angle-resolved photoemission spectroscopy, we report creating, detecting, and controlling dark excitons in the quasi-equilibrium distribution in a doped semiconductor SnSe2. Surprisingly, we observe an excitonic gap phase, with a conduction band opening an anisotropic gap. Our results broaden the scope of dark excitons, extending their studies from the picosecond timescale in the ultrafast photoemission process to conditions occurring under quasi-equilibrium. We reveal the light-matter interaction in the engineering of electronic structures and provide a new way to realize the excitonic gap phase in semiconductors with large band gaps.

cond-mat.mtrl-sci↗

arXiv2024

Quantum simulation of honeycomb lattice model by high-order moir\'e pattern

Moir\'e superlattices have become an emergent solid-state platform for simulating quantum lattice models. However, in single moir\'e device, Hamiltonians parameters like lattice constant, hopping and interaction terms can hardly be manipulated, limiting the controllability and accessibility of moire quantum simulator. Here, by combining angle-resolved photoemission spectroscopy and theoretical analysis, we demonstrate that high-order moir\'e patterns in graphene-monolayered xenon/krypton heterostructures can simulate honeycomb model in mesoscale, with in-situ tunable Hamiltonians parameters. The length scale of simulated lattice constant can be tuned by annealing processes, which in-situ adjusts intervalley interaction and hopping parameters in the simulated honeycomb lattice. The sign of the lattice constant can be switched by choosing xenon or krypton monolayer deposited on graphene, which controls sublattice degree of freedom and valley arrangment of Dirac fermions. Our work establishes a novel path for experimentally simulating the honeycomb model with tunable parameters by high-order moir\'e patterns.

cond-mat.mtrl-sci↗

arXiv2024

Surface region band enhancement in noble gas adsorption assisted ARPES on kagome superconductor RbV3Sb5

Electronic states near surface regions can be distinct from bulk states, which are paramount in understanding various physical phenomena occurring at surfaces and in applications in semiconductors, energy, and catalysis. Here, we report an abnormal surface region band enhancement effect in angle-resolved photoemission spectroscopy on kagome superconductor RbV3Sb5, by depositing noble gases with fine control. In contrast to conventional surface contamination, the intensity of surface region Sb band can be enhanced more than three times with noble gas adsorption. In the meantime, a hole-dope effect is observed for the enhanced surface region band, with other bands hardly changing. The doping effect is more pronounced with heavier noble gases. We propose that noble gas atoms selectively fill into alkali metal vacancy sites on the surface, which improves the surface condition, boosts surface region bands, and effectively dopes it with the Pauli repulsion mechanism. Our results provide a novel and reversible way to improve surface conditions and tune surface region bands by controlled surface noble gas deposition.

cond-mat.mtrl-sci↗

arXiv2022

Tailoring Dirac fermions by in-situ tunable high-order moire pattern in graphene-monolayer xenon heterostructure

A variety of novel quantum phases have been achieved in twist bilayer graphene (tBLG) and other moire superlattices recently, including correlated insulators, superconductivity, magnetism, and topological states. These phenomena are very sensitive to the moire superlattices, which can hardly be changed rapidly or intensely. Here, we report the experimental realization of a high-order moire pattern (a high-order interference pattern) in graphene-monolayer xenon heterostructure (G/mXe), with moire period in-situ tuned from few nanometers to infinity by changing the lattice constant of Xe through different annealing temperatures and pressures. We use angle-resolved photoemission spectroscopy to directly observe that replicas of graphene Dirac cone emerge and move close to each other in momentum-space as moire pattern continuously expands in real-space. When the moire period approaches infinity, the replicas finally overlap with each other and an energy gap is observed at the Dirac point induced by intervalley coupling, which is a manifestation of Kekule distortion. We construct a continuum moire Hamiltonian, which can explain the experimental results well. The form of moire Hamiltonian in G/mXe is similar to that in tBLG, and moire band with narrow bandwidth is predicted in G/mXe. However, the moire Hamiltonian couples Dirac fermions from different valleys in G/mXe, instead of ones from different layers in tBLG. Our work demonstrates a novel platform to study the continuous evolution of moire pattern and its modulation effect on electronic structure, and provides an unprecedented approach for tailoring Dirac fermions with tunable intervalley coupling.

cond-mat.mes-hall↗