The iron-based superconductor FeSe1−xSx provides a unique platform to study unconventional superconductivity without magnetic order. For x > 0.17, the system is in the tetragonal phase without a nematic transition, where a fourfold symmetric gap is expected. However, the superconducting gap structure remains controversial: angle-resolved photoemission spectroscopy (ARPES) has reported a twofold gap anisotropy, whereas scanning tunneling microscopy (STM) suggests a fourfold symmetry.

In this study, we performed angle-resolved specific heat measurements on an FeSe0.78S0.22 single crystal under in-plane rotating magnetic fields. As a bulk thermodynamic probe, specific heat provides direct access to the quasiparticle excitations in the superconducting state.

We observe a clearly resolved twofold anisotropy in the low-temperature specific heat, despite the tetragonal crystal structure of the sample. This result indicates a pronounced breaking of the underlying fourfold symmetry in the superconducting gap structure.

Our findings provide bulk evidence for the controversial gap symmetry in FeSe1−xSx and highlight the importance of further understanding its microscopic origin. Possible interpretations may include exotic superconducting states such as those associated with Bogoliubov Fermi surfaces.