Crossover of Superconductivity across the antiferromagnetic end point in FeSe$_{ m 1-x}$S$_{ m x}$ under pressure

Temperature-pressure ($T$-$P$) phase diagrams of FeSe${\rm 1-x}$S${\rm x}$ were investigated by the measurements of dc magnetization ($M$) and electrical resistivity ($ρ$) under pressure, using single crystal specimens with $x$=0.04, 0.08 and 0.13. For all specimens, the $M$($T$) curves under pressure near the end point of the antiferromagnetic (AFM) phase are found to show a two-step diamagnetic response, which can be described as the sum of two diamagnetic components $M_1$($T$) and $M_2$($T$), indicating that two superconducting (SC) phases with different $T_{\rm c}$ values coexist within a pressure range of $Δ$$P$$\sim$1 GPa. Moreover, the pressure dependence of the amplitudes of $M_1$($T$) and $M_2$($T$) indicates a continuous transfer of the volume fraction between the two SC phases. These behaviors suggest that a crossover of superconductivity occurs in conjunction with the emergence of AFM phase and imply that the SC phases inside and outside the AFM phase could have different origins.

💡 Research Summary

In this work the authors investigate the pressure‑temperature phase diagram of the iron‑based superconductor FeSe₁₋ₓSₓ for three sulfur concentrations (x = 0.04, 0.08, 0.13) using single‑crystal specimens. High‑pressure dc magnetization measurements were performed in a miniature diamond‑anvil cell (DAC) combined with a SQUID magnetometer, while four‑probe electrical resistivity was measured in an opposed‑anvil cell up to about 6 GPa. The pressure‑transmitting media were argon for x = 0.08 and 0.13 and glycerin for x = 0.04, ensuring quasi‑hydrostatic conditions in the relevant pressure range.

The magnetization data reveal a striking two‑step diamagnetic response that appears once the applied pressure exceeds roughly 2 GPa. The first step, occurring at a temperature Tdia₁, corresponds to the conventional superconducting transition (SC1) already known for FeSe‑based compounds. The second step, at a lower temperature Tdia₂, emerges as a distinct diamagnetic component (SC2). By fitting each step with the phenomenological form M(T)=−M₀