Colloquia

Inspired by investigations of Dirac, Majorana and Weyl fermions in the context of particle, condensed matter and atomic physics, we propose theoretical and experimental platforms to create and detect massless helical boson modes that we call Weyl bosons, the bosonic cousins of Weyl fermions [1]. We show that these exotic excitations arise naturally in one-dimensional interacting Fermi gases, when spinorbit coupling and Rabi fields are present, through the mixing charge and spin degrees of freedom [2]. We obtain the phase diagram of chemical potential versus Rabi fields for given spin-orbit coupling and interactions, showing regions where zero, one or two types of Weyl bosons exist. We find that, when two types of Weyl bosons emerge, they must propagate with different velocities. Furthermore, we show that the disappearance of any Weyl boson species is described by a topological quantum phase transition of the Lifshitz type, where the velocity of the disappearing Weyl boson vanishes, and the velocity of the surviving Weyl boson develops a cusp at the transition boundary. Lastly, to detect the existence of Weyl bosons, we propose measurements of the dynamic structure factor tensor (charge-charge, charge-spin and spin-spin), where the energy dispersions, spectral weights and helicities of the emergent Weyl bosons may be experimentally extracted in ultracold Fermi systems such as 6Li, 40K, 173Yb and 87Sr.

[1] “Creating and detecting Weyl bosons with ultracold Fermions”, Xiaoyong Zhang and C. A. R. Sá de Melo, Phys. Rev. A 113, L011303 (2026); https://doi.org/10.1103/4dj8-6wbb

[2] “Effects of spin-orbit coupling and Rabi fields in Tomonaga-Luttinger liquids: current status and open questions”, Xiaoyong Zhang and C. A. R. Sá de Melo, Comptes Rendus Physique (French Academy of Sciences), Vol. 26, p. 483-514 (2025); https://doi.org/10.5802/crphys.254