Microwave flux-flow Hall measurements in FeSe_1-xS_x systems using a circularly polarized dielectric cavity

 The BCS-BEC crossover in FeSe systems has been explored by various methods. Notably, ARPES studies suggested that S-substitution enables access to a deeper crossover regime [1]. The TDGL model predicts that the flux-flow Hall angle increases when transitioning from the BCS to the crossover regime, suggesting it as an effective probe [2]. However, in the flux flow state, low-frequency current-driven measurements are significantly hindered by vortex pinning.

 Therefore, we focused on emerging circularly polarized microwave technology. We employed our developed circularly polarized microwave dielectric resonator to measure the flux-flow Hall effect [3]. This method measures the microwave Hall effect via the circular dichroism of the circularly polarized standing wave modes established within the resonator.

 We discuss our measurement results of the flux-flow Hall effect in relation to the pseudogap phase and giant superconducting fluctuations. We also discuss the connection between systematic S-substitution and the BCS-BEC crossover.

References

[1] T. Hashimoto et al., Sci. Adv. 6, eabb9052 (2020). [2] M. Heyl et al., Nat. Commun. 13, 6986 (2022). [3] M. Roppongi, …Y. Yoshino et al., Rev. Sci. Instrum. 95, 12 (2024).