Giant and broadband circular dichroism from particle–hole symmetry breaking in Weyl semimetals

Circular dichroism originates from symmetry breaking in a material structure and leads to differential absorption of left-handed and right-handed circularly polarized light. However, circular dichroism in most materials is inherently weak and spectrally narrow, especially in the mid-to-far infrared. Here we uncover giant infrared circular dichroism in the magnetic-field-forced Weyl semimetal Mn(Bi,Sb)2Te4 driven by extreme particle–hole symmetry breaking. Helicity-resolved magneto-infrared spectroscopy reveals circular dichroism exceeding 3,000 mdeg (~130 mdeg nm−1) with an above-degree response extending over the 6–13 μm spectral range. The optical resonances are enhanced by a strong band nesting effect intrinsic to the Landau levels of type-II Weyl dispersion. A symmetry-based k·p model reproduces these magneto-infrared responses and demonstrates that magnetization-induced asymmetric spin–orbit coupling generates particle–hole symmetry breaking, which suppresses spin-up, parity-even wavefunction components in the valence Landau band and thereby produces pronounced optical helicity-selectivity. Our findings establish particle–hole symmetry breaking as an effective route towards helicity-resolved optical control in quantum materials. The different Fermi velocities of particle and hole bands result in a giant mid-infrared circular dichroism in the magnetic Weyl semimetal Mn(Bi1−xSbx)2Te4.