Pressure Induced Static Magnetic Order in Superconducting ${\mathrm{FeSe}}_{1-x}$

We report on a detailed investigation of the electronic phase diagram of ${\mathrm{FeSe}}_{1-x}$ under pressures up to 1.4 GPa by means of ac magnetization and muon-spin rotation. At a pressure $\simeq 0.8\mathrm{GPa}$ the nonmagnetic and superconducting ${\mathrm{FeSe}}_{1-x}$ enters a region where static magnetic order is realized above $T_c$ and bulk superconductivity coexists and competes on short length scales with the magnetic order below $T_c$. For even higher pressures an enhancement of both the magnetic and the superconducting transition temperatures as well as of the corresponding order parameters is observed. These exceptional properties make ${\mathrm{FeSe}}_{1-x}$ to be one of the most interesting superconducting systems investigated extensively at present.

Figure 1

(a) Temperature dependence of ${\chi }_{\mathrm{ac}}$ of ${\mathrm{FeSe}}_{0.94}$ measured at (from the top to the bottom) $p=0.0$, 0.32, 0.7, 0.93, 1.18, and 1.39 GPa. The transition temperature $T_c$ is determined from the intersection of straight line fits to the data above and below the transition. (b) Dependence of $T_c$ on $p$ of ${\mathrm{FeSe}}_{0.94}$ and ${\mathrm{FeSe}}_{0.98}$. (c) Pressure dependence of the normalized ac susceptibility $-{\chi }_{\mathrm{ac}}(1-N)$ at $T=6\mathrm{K}$.

Figure 2

(a) Zero-field $\mu \mathrm{SR}$ time spectra of ${\mathrm{FeSe}}_{0.94}$ measured at (from the top to the bottom) $p=0.0$, 0.76, 0.94, 1.12, and 1.39 GPa. Dependence of the internal field at the muon stopping site $B_{\mathrm{int}}$ which is proportional to the magnetic order parameter (b), and the magnetic volume fraction (c), on temperature at various pressures. The solid lines in panel (b) are the fit of $B_{\mathrm{int}}(T)$ in the region $T_c(p)\le T\le T_N$ to $B_{\mathrm{int}}(T)=B_{\mathrm{int}}(0)[1-(T/T_N{)}^{\alpha }{]}^{\beta }$ ($\alpha$ and $\beta$ are the power exponents).

Figure 3

(a) Pressure dependence of the superconducting transition temperature $T_c$, the magnetic ordering temperature $T_N$, and the internal field $B_{\mathrm{int}}$ (magnetic order parameter) obtained in ac susceptibility and muon-spin rotation experiments. (b) Pressure dependence of $T_N$, $T_c$, and the magnetic volume fraction. The $T_c(p)$ and $T_N(p)$ lines are guides for the eye. The closed and the open symbols refer to ${\mathrm{FeSe}}_{0.94}$ and ${\mathrm{FeSe}}_{0.98}$ sample. SC, M, and PM denote the superconducting, magnetic, and nonmagnetic (paramagnetic) states of the sample.