Nan Xu

Moiré superlattices provide a versatile means of modifying electronic bands through periodic potentials generated by lattice mismatch or rotational misalignment. Over the past decade, this concept has reshaped the study of low-dimensional quantum materials, enabling the observation of flat bands, correlated insulating behavior, unconventional superconductivity, excitonic states, and topological responses. More recently, moiré physics has begun to move beyond the conventional limit of atomically thin two-dimensional (2D) heterostructures. Experimental studies have shown that a moiré potential formed at a 2D interface can propagate into adjacent three-dimensional materials and reconstruct their bulk electronic states, giving rise to mixed-dimensional moiré systems. This review summarizes recent progress in moiré effects from 2D platforms to mixed-dimensional architectures. We first present the fundamental physical definition of moiré patterns and trace their historical development. We then review graphene-based moiré systems with different layer numbers, where twist-angle control modifies band dispersion, enhances electronic correlations, and leads to superconductivity near magic angles. The discussion is extended to transition metal dichalcogenide moiré heterostructures, with emphasis on excitonic physics, interlayer hybridization, correlation effects, and nontrivial band topology. We then focus on 2D–3D mixed-dimensional heterostructures, where interfacial moiré modulation penetrates into bulk materials and produces electronic reconstruction beyond the interface. Finally, we discuss future directions involving broader material choices, improved control over stacking and twist geometry, advanced probes of buried moiré potentials, and the search for unexplored correlated and topological phases.

Excitons underpin optical phenomena and can intertwine with correlated electronic states. Unlike bright excitons, dark excitons evade conventional optical detection, and their impact on quasiequilibrium electronic structures remains largely unexplored. Here, using angle-resolved photoemission spectroscopy, we report creating, detecting, and controlling dark excitons in the quasiequilibrium distribution of a doped semiconductor SnSe_{2}. By tuning doping and temperature, we control the excitonic spectral weight and observe a concomitant anisotropic gapped state. Our results broaden the scope of dark excitons beyond ultrafast photoemission studies and highlight a correlation between dark excitons and correlated electrons under quasiequilibrium conditions.

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.

4Hb-TaS_2, a van der Waals superlattice comprising alternate stacked Ising superconducting 1H-TaS_2 and cluster Mott insulating 1T-TaS_2, exhibits emergent properties beyond those of its constituent layers. Notable phenomena include time-reversal-symmetry-breaking superconductivity and spontaneous vortex phases, which are driven by nontrivial interlayer interactions that remain debated. Using area-selective angle-resolved photoemission spectroscopy, we provide direct spectroscopic evidence of such interaction by systematically probing the electronic structures of 1T- and 1H-terminted surfaces of 4Hb-TaS_2. The metallic states of subsurface 1H-layers are folded to the Brillouin zone center by the sqrt(13) by sqrt(13) modulation of the surface 1T-layer, forming chiral "windmill" Fermi surfaces via Umklapp scattering. These conducting states further hybridize with the incipient flat band of the surface 1T-layer, producing a Kondo-like peak at the Fermi level. Interlayer charge transfer induces distinct 3 by 3 and 2 by 2 charge orders on the surface and subsurface 1H-layers, respectively, which result in characteristic segmented Fermi surfaces and dichotomously shift the van Hove singularities. These findings reconcile the competing Kondo and Mott-Hubbard models in this material and emphasize the interplay of flat bands, van hove singularities, charge orders, and unconventional superconductivity in correlated superlattices.

Bilayer rhombohedral-stacked transition-metal dichalcogenides (3R-TMDCs) combining high carrier mobility, good electrostatic control, and exotic switchable polarization are emerging as promising semiconducting channels for beyond-silicon electronics. However, despite great efforts, the growth of wafer-scale bilayer 3R-TMDCs single crystals remains difficult due to challenges in the synergistic control of phase structure and grain orientation. Here we design a hole-doping-assisted strategy to synthesize a series of two-inch bilayer 3R-TMDCs single crystals on c-plane sapphire. The introduction of hole dopants (e.g. Hf, V, Nb, Ta) not only increases the interlayer coupling to break the formation energy degeneracy of bilayer 3R-stacked and hexagonal-stacked TMDCs, but also promotes the parallel steps formation on sapphire surfaces to induce the unidirectionally aligned bilayer grain nucleation. The fabricated ferroelectric semiconductor field-effect transistors based on bilayer Hf-MoS₂ demonstrate high endurance (more than 10⁵ cycles) and long retention time (exceeding one year) due to the restriction of interlayer charge defect migration/aggregation caused by sliding ferroelectricity. This work proposes a promising strategy for synthesizing wafer-scale ferroelectric semiconductor single crystals, which could promote the further exploration of logic-in-memory chips.

In this work, we reveal a curious type of moiré 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 moiré replicas of the Bi(111) surface states, while the bulk states remain barely changed. Meanwhile, we achieve control over the moiré period in the range of 25 Å to 80 Å by selecting monolayers of different noble gases and adjusting the annealing temperature. At large moiré 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 moiré 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 moiré effects.

Manipulating magnetism in two-dimensional (2D) van der Waals (vdW) materials arouses considerable and ongoing interest in fundamental physics and potential applications in next-generation spintronics. Here, we have investigated the underlying electronic structures of bulk vdW magnets CrTe₂ and NaCrTe₂, by carrying out high-resolution angle-resolved photoemission spectroscopy studies and first-principles calculations. In CrTe₂, strong out-of-plane band dispersions and metallic Fermi surface are observed, accompanied by temperature-dependent ferromagnetic (FM) energy gain behavior which directly confirms its itinerant origin. In sharp contrast, NaCrTe₂ turns into an A-type antiferromagnetic (AFM) semiconductor in calculations and transport measurements. Remarkably, robust ferromagnetism inherited from CrTe₂ is distinguished on the polar surface of AFM NaCrTe₂. Our results not only reveal the itinerant FM mechanism of CrTe₂, but also demonstrate that chemical doping is a powerful tuning knob to manipulate the properties of 2D magnetic material. Moreover, we establish NaCrTe₂ as an excellent AFM semiconductor platform with robust surface ferromagnetism, which is promising in practical applications.

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.

Benefit to layer-dependent bandgap, van der Waals materials with surface photovoltaic effect (SPV) enable photodetection over a tunable wavelength range with low power consumption. However, sensitive SPV in the infrared region, especially in a quasi-steady illumination condition, is still elusive in layered semiconductors. Here, using angle-resolved photoemission spectroscopy, we report a sensitive SPV in quasi-equilibrium in NbSi0.5Te2, with photoresponsivity up to 2.4*10^6 V/(W*cm^(-2)) at low intensity low temperature condition (LILT). The sensitive SPV is further confirmed by observing the Dember effect, where the photogenerated carrier density is high enough and diffusion currents suppress SPV. Temperature-dependent measurements indicate that intrinsic carriers freezing at low temperature leads to the ultrahigh photoresponse, while a small amount of photon-generated carriers in quasi-equilibrium dominate the system. Our work not only provides a promising layered semiconductor for Infrared optoelectronic devices with strong infrared SPV at LILT, which has application potential in fields such as quantum information and deep-space exploration, but also paves a novel way to enhance light-matter interaction effect by freezing bulk carriers.