The tetragonal phase of FeSe_1-xS_x hosts a highly unusual pairing state characterized by numerous unpaired electrons, as reported by specific heat, thermal conductivity, and scanning tunneling spectroscopy measurements [1-3]. Although several theoretical scenarios have been proposed - including the formation of a Bogoliubov Fermi surface [4,5], the influence of soft nematic fluctuations [6], and competition with magnetism [7] - the origin of this ultra-nodal pairing state remains largely elusive. Here, using high-quality single crystals of tetragonal FeSe_1-xS_x, we investigate bulk-sensitive, momentum-resolved quasiparticle excitations via thermal transport and thermal Hall measurements. By analyzing the thermal Hall response, we obtain the first momentum-resolved insight into the superconducting gap structure on both electron and hole pockets. Importantly, our results demonstrate that the superconducting gap on the electron pocket is substantially larger than that on the hole pocket, providing strong evidence for the key role of electron–electron scattering and for the dominance of d-wave pairing interactions in this system. Moreover, the thermal Hall response reveals the presence of ultra-nodal quasiparticles in both the hole and electron pockets, with a noticeable imbalance between them. These results highlight the importance of frustration between the electron–electron and electron–hole pairing interactions as a key ingredient for stabilizing the exotic ultra-nodal superconducting state.

[1] Y. Sato et al., Proc. Natl. Acad. Sci. U.S.A. 115, 1227 (2018).

[2] T. Hanaguri et al., Sci. Adv. 4, eaar6419 (2018).

[3] Y. Mizukami et al., Commun. Phys. 6, 183 (2023).

[4] C. Setty et al., Nat. Commun. 11, 523 (2020); Phys. Rev. B 102, 064504 (2020).

[5] Y. Cao et al., Phys. Rev. B 108, 224506 (2023).

[6] K. R. Islam and A. Chubukov, npj Quantum Mater. 9, 28 (2024).

[7] H. Wu et al., Phys. Rev. B 109, L220501 (2024).