Hongki Min

We provide a systematic framework to investigate magnetoplasmons in multilayer two-dimensional electron systems using the the Kac-Murdock-Szegő (KMS) Toeplitz matrix to describe interlayer Coulomb interactions. In the absence of interlayer tunneling, we show that the single-layer magnetoplasmon branch splits into N collective modes-one in-phase mode and N-1 out-of-phase modes-and derive their asymptotic behaviors in the long-wavelength limit, as well as in the large layer separation and weak magnetic-field limit. When interlayer tunneling is present, we clarify the magnetoplasmon dispersion, both qualitatively and quantitatively, by identifying the magnetoplasmon mode associated with each interband transition, including tunneling magnetoplasmons arising from interband transitions with the same Landau level index. Our study reveals the symmetry-governed hybridization between the identified modes, along with an enhanced tunneling magnetoplasmon gap exceeding the associated interband gap. The KMS-based analytic formalism thus provides a comprehensive physical understanding of magnetoplasmons in multilayer structures.

Extending moiré graphene beyond twisted bilayers, helical trilayer graphene has emerged as a supermoiré platform hosting topological bands and correlated electronic states. Here we establish a domain-resolved framework for helical multilayer graphene that links supermoiré relaxation to low-energy electronic structure and valley topology. Real-space lattice calculations show that relaxation reconstructs the system into locally periodic single-moiré domains, enabling a local continuum description. Within each domain, downfolding the first-shell model yields effective Hamiltonians near the Dirac points that reveal how the low-energy spectrum decomposes into folded Dirac sectors. The resulting valley Chern topology is domain-dependent and includes gate-tunable responses, as independently corroborated by real-space lattice calculations. Our results identify folded Dirac sectors as the organizing principle of thicker helical graphene stacks, with local stacking families determining the band character and valley topology.

We investigate the quasiparticles of a single nodal ring semimetal SrAs_{3} through axis-resolved magneto-optical measurements. We observe three types of Landau levels scaling as ϵ∼sqrt[B], ϵ∼B^{2/3}, and ϵ∼B that correspond to Dirac, semi-Dirac, and classical fermions, respectively. Through theoretical analysis, we identify the distinct origins of these three types of fermions present within the nodal ring. In particular, semi-Dirac fermions-a novel type of fermion that can give rise to a range of unique quantum phenomena-emerge from the end points of the nodal ring where the energy band disperses linearly along one direction and quadratically along the perpendicular direction, a feature not achievable in nodal point or line structures. The capacity of the nodal ring to simultaneously host multiple fermion types, including semi-Dirac fermions, establishes it as a valuable platform to expand the understanding of topological semimetals.

Twisted van der Waals heterostructures have led to emerging layer-dependent correlated physics in moiré potentials. While optoelectronic controls over interlayer electronic coupling have been reported, the concomitant interlayer vibration has not yet been controlled. Here, we report experimental evidence of ultrafast optical control over the amplitude and oscillation period of interlayer breathing phonons in WSe₂/WS₂ heterobilayers. Femtosecond optical excitation above the Mott density in gate-tuned devices shows as large as 10% changes of stiffening and softening amplitude of coherent phonons. A theoretical model, incorporating both Buckingham and Hartree energies, is presented to elucidate the impact of charge-separated carriers generated by photoexcitation on phonon dynamics. This work, therefore, provides insights for extending optoelectronic engineering into the coherent phonons in moiré systems.

The light dark matter mass regime has emerged as the next frontier in the direct detection experiment due to the lack of any detection signal in the higher mass range. In this paper, we propose a new detector material, a bilayer stack of graphene to detect sub-MeV dark matter. Its voltage-tunable low energy sub-eV electronic band gap makes it an excellent choice for the detector material of a light dark matter search experiment. We compute its dielectric function using the random phase approximation and estimate the projected sensitivity for sub-MeV dark matter-electron scattering and sub-eV dark matter absorption. We show that a bilayer graphene dark matter detector can have competitive sensitivity as other candidate target materials, like a superconductor, but with a tunable threshold energy in this mass regime. The dark matter scattering rate in bilayer graphene is also characterized by a daily modulation from the rotation of the Earth which may help us mitigate the backgrounds in a future experiment. We also outline a detector design concept and provide noise estimates that can be followed to set up an experiment in the future. Published by the American Physical Society 2024