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

TL;DR

This paper reports the discovery of isotropic massive Dirac fermions in LaAlSi via Landau level spectroscopy, revealing their topological nature and an unusually large oscillator strength in the zeroth Landau level transition, advancing understanding of relativistic quasiparticles in solids.

Contribution

It demonstrates the existence of isotropic 3D massive Dirac fermions in LaAlSi and explores their unique optical and topological properties using Landau level spectroscopy.

Findings

Identification of isotropic massive Dirac fermions in LaAlSi.

Observation of large oscillator strength in zeroth Landau level transition.

Evidence of topological nature through magneto-infrared response.

Abstract

Dirac fermions, characterized by their linear dispersion and relativistic nature, have emerged as a prominent class of quasiparticles in condensed matter physics. While the Dirac equation, initially developed in the context of high-energy physics, provides a remarkable framework for describing the electronic properties of these materials, the inherent symmetry constraints of condensed matter often lead to deviations from the idealized paradigm. In particular, three-dimensional Dirac fermions in solids often exhibit anisotropic behavior, challenging the notion of perfect symmetry inherent in the Dirac equation. Here, we report the observation of isotropic massive Dirac fermions in LaAlSi through Landau level spectroscopy. The presence of three-dimensional massive Dirac fermions across the Fermi energy is demonstrated by quantized and semiclassical analyses of the magnetic field evolution of Landau level transitions. The isotropic topological nature, Fermi velocity, and Dirac mass are evidenced by the identical magneto-infrared response among the Faraday and three Voigt geometries. Furthermore, we observe an unusually large oscillator strength in the zeroth Landau level transition of the Dirac fermion, compared to transitions with higher indices. This phenomenon, supported by model calculations, can be attributed to the combined effects of the partial excitation of Dirac fermion and the resonant dielectric coupling with the Weyl plasma. Our work provides a strategy for realizing ideal quasiparticle excitations and their coupling effects in condensed matter systems, offering a platform for exploring relativistic physics.