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

TL;DR

The paper maps the $T$-$P$ phase diagrams of FeSe$_{1-x}$S$_x$ under pressure and reveals a crossover in superconductivity near the AFM end point, manifested as a two-step diamagnetic response indicating coexisting SC phases with different $T_c$. By combining dc magnetization and resistivity up to ~6 GPa for $x$ = 0.04, 0.08, 0.13, the authors identify three SC regions (SC1, SC2, SC3) and show that the SC fractions transfer continuously over a narrow pressure window, correlated with AFM emergence. The phase diagrams suggest SC2 lies between nematic and magnetic phases and may involve distinct pairing mechanisms, while SC1 and SC3 could stem from different origins inside versus outside the AFM phase. The work highlights the need for microscopic, high-pressure studies of gap structure and Fermi-surface evolution to pin down the pairing mechanisms driving superconductivity in this FeSe-derived system.

Abstract

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.

Figures (3)

• Figure 1: Temperature ($T$) dependence of zero-field-cooled dc magnetization ($M$) for FeSe$_{\rm 1-x}$S$_{\rm x}$ with $x$=0.04 (a), 0.08 (b) and 0.13 (c) measured by applying a magnetic field of 20 Oe at various pressures above 3-5 K. The data are intentionally shifted along the longitudinal axis for clarity. A jump in the $M$($T$) curves observed at low temperature for $x$=0.13 is due to SC transition of a Pb manometer. (d) $M$($T$) curve for x=0.08 at $P$=3.1 GPa consisting of two components $M_{\rm 1}$($T$) (red solid line) and $M_{\rm 2}$($T$) (blue solid line), each of which shows a diamagnetic behavior below $T_{\rm c1}^{\rm dia}$ and $T_{\rm c2}^{\rm dia}$, respectively. We used Eq. (1) as a fitting function for $M$($T$) data. Plots of diamagnetic amplitude $M_0$ for $M_{\rm 1}$($T$) and $M_{\rm 2}$($T$) versus pressure for x=0.04 (e), 0.08 (f) and 0.13 (g). The solid lines are guide for the eyes.
• Figure 2: Temperature dependence of electrical resistivity $\rho$ for FeSe$_{\rm 1-x}$S$_{\rm x}$ with $x$=0.04 (a), 0.08 (run 1) (b), 0.08 (run 2) and 0.13 (c) measured at various pressures above $\sim$4 K using glycerin as the PTM. The data are intentionally shifted along the longitudinal axis for clarity. The black upward triangles indicate zero-resistive temperature $T_{\rm c}^{\rm zero}$. The red and blue arrows indicate the nematic ($T_{\rm s}$) and magnetic ($T_{\rm m}$) transition temperatures, respectively.
• Figure 3: $T$-$P$ phase diagram of FeSe$_{\rm 1-x}$S$_{\rm x}$ with $x$=0.04 (a), 0.08 (b) and 0.13 (c). The broken lines are guide for the eyes. (d) Schematic view of the variation of the $T$-$P$ phase diagram of FeSe$_{\rm 1-x}$S$_{\rm x}$ with increasing $x$.