Yugui Yao

Directional control of band structures at ultrafast time scales is a key requirement for nanoscale optoelectronics. Here, we realize cooperative band engineering in the quasi-one-dimensional (quasi-1D) van der Waals topological insulator α-Bi4Br4 through joint carrier-phonon band renormalization. Femtosecond excitation drives quasi-1D carrier accumulation, generating a strongly nonlinear many-body band renormalization that redshifts an optical transition by up to ∼25 meV, with >90% polarization anisotropy. Superimposed Ag coherent phonons further modulate the same transition by ∼1 meV on intrinsic lattice time scales. By implementing phase-controlled dual-pump excitation, we achieve deterministic and reversible positioning of the renormalized band edge with ∼100 μeV precision within a ∼1.5 meV tuning window. First-principles calculations with charge-neutral photoexcitation constraints qualitatively reproduce the carrier-induced narrowing of the relevant interband transition, while phonon-mode analysis reveals that coherent Ag phonons modulate the same transition through interchain hopping and orbital polarization. These results establish cooperative carrier-phonon band renormalization as a scalable, all-optical mechanism for ultrafast, polarization-sensitive band engineering in quasi-1D van der Waals nanomaterials, enabling dynamically reconfigurable optoelectronic functionality.

Understanding how hydrogen-bond interactions govern nonequilibrium photochemical reactivity is essential for controlling the stability and functionality of molecular crystals. Using crystalline 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) as a representative molecular crystal, we investigate the microscopic origin of its anomalous photosensitivity. We find that decomposition is initiated by coupled proton–electron dynamics, which drive directional charge redistribution along amino-nitro hydrogen-bond motifs. The resulting enhancement of donor–acceptor polarization destabilizes the aromatic π-framework, induces anisotropic structural deformation, and ultimately promotes bond-selective molecular dissociation. This electronically directed pathway is fundamentally distinct from thermally activated decomposition and provides a microscopic explanation for the longstanding stability–sensitivity paradox of TATB. Our findings establish a structure-dynamics-reactivity relationship linking hydrogen-bond-assisted proton–electron coupling to bond-selective photochemistry, providing a mechanistic framework for understanding and controlling nonequilibrium reactivity in molecular crystals.

Majorana zero modes, which obey non-Abelian exchange statistics, are promising candidates for topological quantum computation due to their robustness against environmental perturbations. The iron-based superconductor Fe(Te,Se) has been identified as an intrinsic topological superconductor, possibly hosting Majorana zero modes. In this paper, we report the observation of near-zero-energy localized states at multiple structural defects on the Fe(Te,Se) surface, which could be misidentified as Majorana zero modes without additional verification. By using spin-polarized scanning tunneling spectroscopy, we demonstrate that the near-zero-energy localized states on step edges and line defects originate from topologically trivial Yu-Shiba-Rusinov states. In addition, zero-energy bound states are also observed for regions without surface defects. A combined spatial and magnetic field dependent analysis of the spin-resolved tunneling spectra in these regions reveals that this type of zero-energy states cannot be attributed to the presence of Majorana bound states. These findings emphasize the importance of spin-dependent studies of low-energy states for pursuing Majorana zero modes.

The divergence of the electron density of states (DOS) plays an important role in enhancing many-body interactions and inducing various quantum phases in low-dimensional systems. However, such unique electronic structures remain experimentally elusive in three-dimensional (3D) systems, particularly those with strong spin-orbit coupling (SOC). Using angle-resolved photoemission spectroscopy and first-principles calculations for a Laves-phase superconductor CsBi 2 , which features a Bi-pyrochlore 3D network with strong SOC, we identify two characteristic electronic structures with a large DOS. One is a quasiflat band within a topologically nontrivial band sector with p -orbital character, locally formed around the U-K line, which enhances DOS near the Fermi level. The other involves type-I and type-II saddle points connected by a quasiflat band, which cooperatively produce an enhancement in the DOS. Our findings lay a foundation for exploring exotic phenomena driven by the interplay of multiple singularities with a large DOS, nontrivial topology, and strong SOC in 3D pyrochlores.

In this work, we reveal giant magneto‐optical responses in two‐dimensional (2D) antiferromagnets with nearly flat electronic bands, based on first‐principles calculations and group‐theoretical analysis. We identify a record‐large second‐order magneto‐optical Schäfer–Hubert (SH) effect—featuring a polarization rotation angle of —in monolayer antiferromagnetic , driven by flat‐band–enhanced interband optical transitions. Both the valence and conduction bands exhibit pronounced directional flatness, giving rise to highly anisotropic optical absorption and broadband hyperbolic frequency windows spanning the entire visible spectrum. This anisotropy leads to an exceptionally strong linear dichroism (LD) reaching 50%, far exceeding values reported in other 2D magnetic systems. Remarkably, the giant SH effect and LD appear at distinct photon energies, reflecting a momentum–direction–dependent crossover between flat and dispersive bands. Both responses are further amplified with increasing film thickness. Our results establish flat‐band antiferromagnets as a fertile platform for realizing giant nonlinear magneto‐optical effects and open new avenues for 2D opto‐spintronic device applications.

Reconfigurable infrared plasmons are central to emerging nanophotonic technologies, yet their practical impact remains constrained by limited tuning bandwidth, poor reversibility, and material-specific spectral windows. Here we demonstrate ultrabroadband and fully reversible tuning of infrared plasmons in metallic van der Waals NbSe2 films enabled by gate-controlled lithium-ion intercalation. Ion intercalation provides a unique pathway to simultaneously modulate free-carrier density and screening from high-energy interband transitions, jointly reshaping both plasmon frequency and oscillator strength. As a result, plasmon resonances can be continuously swept over an unprecedented spectral span exceeding 5500 cm–1 within a single material platform, covering the telecom, mid-infrared and thermal far-infrared region. Leveraging this broad and reversible tunability, we realize electrically tunable wavelength-selective thermal emitters and thermal display patterns through plasmon-mediated emissivity modulation. Our results establish ion-intercalated van der Waals metals as a versatile and programmable platform for ultrabroadband tunable infrared nanophotonics and reversible thermal-radiation control.

The precise spatial engineering of quantum phases stands as a major challenge in condensed matter physics, with profound implications for fundamental science and quantum technologies. While moiré heterostructures have enabled remarkable tunability on the correlated states, the deterministic spatial manipulation of emergent quantum phases like superconductivity and magnetism remains an open challenge. Here, we investigate hexagonal boron nitride (hBN)-aligned twisted bilayer graphene (TBG), in which fabrication-induced uniaxial heterostrain gives rise to a periodic quasi-one-dimensional (1D) super-moiré supercell. This supercell imprints a striking chain-like architecture onto the electronic states. Using atomic-scale spectroscopy and point-contact Andreev reflection, we demonstrate that intrinsic superconductivity arising from TBG emerges exclusively within these nanoscale channels, a phenomenon unprecedented in other quantum materials. Furthermore, we show that the superconducting state is governed by the local inversion symmetry, which can be deliberately tuned via the moiré registry with the hBN substrate. This work establishes a structural approach to atomically pattern quantum phases, bridging the gap between nanoscale symmetry control and macroscopic quantum functionality. Our results position super-moiré lattices as a programmable platform for quantum phase design, opening routes to explore exotic dimensionality crossovers in correlated electron systems.

Axion quasiparticles introduce magnetoelectric coupling in topological materials, yet their optical signatures are intrinsically weak, severely limiting experimental detection and practical exploitation. Here, we demonstrate maximal optical chirality of axion quasiparticles by coupling them with degenerate quasibound states in the continuum (qBICs). By integrating three-dimensional topological insulators (3DTIs) with achiral photonic crystal slabs (PCSs), we reveal that axion quasiparticles induce an effective Zeeman effect that lifts the degeneracy of achiral qBICs in PCSs, splitting them into two nondegenerate high-Q chiral qBICs with maximally opposite chirality. These result in a more than 10^{3}- and 200-fold increase in the polarization conversion efficiency and Faraday rotation angle, respectively, compared to the intrinsic chiroptical response of bare 3DTI films. Moreover, circular dichroism and nonreciprocal optical responses are revealed as new key signatures for probing axion quasiparticles in 3DTI-PCS coupled systems. Our results provide a definitive route for observing axion quasiparticles and topological physics, while establishing a promising platform for controlling optical polarization, spin, phase, and nonreciprocal responses.