Kuang-An Chang

Giant Nonlinear Photon-Drag Currents in Centrosymmetric Moiré Bilayers

ArXiv.org · 2025-11-21

We present a unified microscopic theory of nonlinear photon-drag currents, formulated within a geometric-loop framework that provides both transparent quantum-geometric interpretation and numerical tractability. In this picture, the photon-drag shift current corresponds to the dipole moment of the geometric loop, while the photon-drag injection current arises from the same loop weighted by a band velocity difference. We apply the theory to an exact continuum model of twisted bilayer graphene (TB…

Unconventional Geometric Phase in Twisted Junction of Cuprates

ArXiv.org · 2025-11-21

Originally introduced in optics, the Pancharatnam-Berry phase is a general concept of geometric phase defined for any two interfering polarization states. In electronic systems, however, its counterpart has long been overlooked due to the absence of electron polarization. Here, using large-scale first-principles calculations, we investigate the electronic structure of twisted bilayer Bi2Sr2CaCu2O8. We find spontaneous spin polarization and the emergence of hidden flat bands at the interface betw…

Ultrafast manipulation of magnetic skyrmions by microwave fields

ArXiv.org · 2025-07-04

We theoretically investigate the inertial dynamics of magnetic skyrmions driven by circularly polarized microwave-induced inverse Faraday effect (MIFE). By incorporating an inertial mass term into the Thiele equation and analytically deriving the microwave-induced magnetic fields and forces, we demonstrate fundamentally distinct dynamical regimes under continuous-wave (CW) versus pulsed excitation. Skyrmion inertia qualitatively transforms trajectories from smooth spirals to polygonal orbits und…

All-Electrical Self-Switching of van der Waals Chiral Antiferromagnet

ArXiv.org · 2025-10-21

Antiferromagnets have garnered significant attention due to their negligible stray field and ultrafast magnetic dynamics, which are promising for high-density and ultrafast spintronic applications. Their dual functionality as both spin sources and information carriers could enable all-electrical self-induced switching of antiferromagnetic order, offering great potential for ultra-compact spintronic devices. However, related progress is still elusive. Here, we report the deterministic switching o…

Fractional Chern Insulators Transition in Non-ideal Flat Bands of Twisted Mono-bilayer Graphene

ArXiv.org · 2025-11-15

Fractional Chern insulators (FCIs) in ideal flat bands with Chern number $C$ are commonly understood as color-entangled states constructed from $C$ copies of the lowest Landau level. In realistic moiré systems, however, the band geometry is generally non-ideal, and the mechanism that stabilizes such FCIs remains unclear. Using twisted monolayer-bilayer graphene as a platform, we find two FCIs separated by a continuous transition driven by a geometric instability of the Bloch wave functions.Below…