Summary: The behavior of electrons' mass within a solid has a significant impact on the flow of electronic and heat energy. Massless Weyl fermions in Weyl semimetals are considered potential game-changers in quantum technologies due to their ability to enable frictionless transmission of quantum information. However, recent research challenges the widely accepted notion that Weyl fermions are truly massless.
For truly massless fermions, electron currents generated at the same Weyl node by fields of opposite helicity should exhibit perfect mirror symmetry. But new findings reveal unexpected deviations from this symmetry, even at wavelengths of laser fields close to the Weyl point and weak intensities. These deviations are influenced by the band structure's non-zero mass approximation, leading to a magnified optical response.
Circularly polarized light has played a pivotal role in understanding and probing exotic properties of Weyl semimetals, particularly circularly polarized light-driven selective excitations in the vicinity of Weyl nodes. The excitation process depends on the chirality of Weyl fermions and the helicity of circularly polarized light. Selective excitations in broken inversion-symmetric Weyl semimetals lead to population asymmetry around the Weyl nodes and the circular photogalvanic effect, which generates current upon irradiation with circular light.
Broken inversion-symmetry in Weyl semimetals is crucial to avoid cancellation of contributions from pairs of chiral Weyl nodes. Therefore, measurements of coupling between massless fermions and circularly polarized light yield non-zero results only in inversion-broken Weyl semimetals.
The research raises a question about the assumption of perfectly massless Weyl fermions, specifically how quickly this assumption is violated as one moves away from the exact location of the node. Deviations from linear dispersion near the degenerate point imply that even for gapless nodes, the mass becomes nonzero away from the point.
Addressing this question, the research confirms that circularly polarized light with opposite helicity can generate non-mirror-symmetric excitations in inversion-symmetric Weyl semimetals when considering the nonlinearity of the band structure, even near the Weyl nodes.