In FeSe_1-xS_x, the superconducting state undergoes a significant transition across the nematic quantum critical point at x ≅ 0.17. Specifically, for highly substituted samples, both specific heat [1] and STM measurements [2] have revealed an anomalous zero-energy density of states within the superconducting phase. This phenomenon is theoretically explained by the formation of "Bogoliubov Fermi surfaces[3]" —two-dimensional gapless regions that emerge when time-reversal symmetry is broken [4]. The existence of this novel state has recently been further supported by ARPES experimental data [5].　

While the high-substitution region of FeSe_1-xS_x potentially hosts an entirely new type of superconducting state, current reports using the Chemical Vapor Transport (CVT) method have been limited to S substitution levels of approximately x ≅ 0.25. However, synthesizing crystals with higher S content is considered essential to further clarify the conditions for the existence and the emergence mechanism of Bogoliubov Fermi surfaces. In this presentation, we report on the synthesis of highly substituted FeSe_1-xS_x and present their characterization. Furthermore, we will discuss the results of our magnetic penetration depth measurements to provide insight into their superconducting states.

In FeSe_1-xS_x, the superconducting state undergoes a significant transition across the nematic quantum critical point at x ≅ 0.17. Specifically, for highly substituted samples, both specific heat [1] and STM measurements [2] have revealed an anomalous zero-energy density of states within the superconducting phase. This phenomenon is theoretically explained by the formation of "Bogoliubov Fermi surfaces[3]" —two-dimensional gapless regions that emerge when time-reversal symmetry is broken [4]. The existence of this novel state has recently been further supported by ARPES experimental data [5].　

While the high-substitution region of FeSe_1-xS_x potentially hosts an entirely new type of superconducting state, current reports using the Chemical Vapor Transport (CVT) method have been limited to S substitution levels of approximately x ≅ 0.25. However, synthesizing crystals with higher S content is considered essential to further clarify the conditions for the existence and the emergence mechanism of Bogoliubov Fermi surfaces. In this presentation, we report on the synthesis of highly substituted FeSe_1-xS_x and present their characterization. Furthermore, we will discuss the results of our magnetic penetration depth measurements to provide insight into their superconducting states.