Anisotropic plasmons in threefold Hopf semimetals

Abstract Threefold Hopf semimetals are a novel type of topological semimetals that possess an internal anisotropy characterized by a dipolar structure of the Berry curvature and an isotropic energy band structure consisting of a Dirac cone and a flat band. In this study, we theoretically investigate the impact of internal anisotropy on plasmons in threefold Hopf semimetals using random-phase approximation. In contrast to the classical intuition that isotropy of the energy band dispersion leads to isotropic plasmons in the classical regime ( i.e. , in the wavelength limit), we find that plasmons in threefold Hopf semimetals exhibit notable anisotropy even in the long-wavelength limit. We derive an explicit analytical form of the long-wavelength plasmon frequency, and numerically demonstrate the validity of our results in a wide range of situations. Our work reveals that the anisotropy of long-wavelength plasmons can reach 25%, making it experimentally observable.