Isotropic Dirac fermion and anomalous oscillator strength of the zeroth Landau level transition in LaAlSi

Dirac fermions, with linear dispersion and relativistic nature, are a prominent class of quasiparticles in condensed matter physics. While the Dirac equation provides a remarkable framework for describing these quasiparticles, symmetry constraints in condensed matter often cause deviations from the idealized paradigm. In particular, three-dimensional Dirac fermions in solids typically exhibit anisotropy, challenging the perfect symmetry inherent in the Dirac equation. Here, we report isotropic massive Dirac fermions in LaAlSi revealed by Landau level spectroscopy. Quantized and semiclassical analyses of Landau level transitions demonstrate the presence of three-dimensional massive Dirac fermions. The isotropic topological nature, Fermi velocity, and Dirac mass are evidenced by identical magneto-infrared response in the Faraday and three Voigt geometries. Furthermore, the zeroth Landau level transition exhibits unusually large oscillator strength compared to higher-index transitions. Model calculations suggest this arises from partial excitation of Dirac fermions and resonant dielectric coupling with the Weyl plasma. Our work provides a strategy for realizing ideal quasiparticle excitations and their coupling effects in condensed matter, offering a platform for exploring relativistic physics.