Superconductivity and magnetism in Rb${}_{0.8}$Fe${}_{1.6}$Se${}_2$ under pressure

High-pressure magnetization, structural, and ${}^{57}$Fe Mössbauer studies were performed on superconducting Rb${}_{0.8}$Fe${}_{1.6}$Se${}_{2.0}$ with $T_c$ $=$ 32.4 K. The gradual decrease of $T_c$ with increasing pressure up to 5 GPa is followed by a step-like suppression of superconductivity at higher pressures. No structural phase transition in the Fe vacancy-ordered superstructure is observed in synchrotron x-ray diffraction (XRD) studies up to 15.6 GPa. The Mössbauer spectra above 5 GPa reveal the appearance of a new paramagnetic phase, exhibiting magnetic order below 80 K, coinciding with the irreversible disappearance of superconductivity. We interpret these changes as due to a pressure-induced diffusion of Rb ions between the two phases, responsible for the antiferromagnetism and superconductivity in Rb${}_{0.8}$Fe${}_{1.6}$Se${}_{2.0}$.

Figure 1

(a) Temperature dependence of the ZFC magnetization $M$ of Rb${}_{0.8}$Fe${}_{1.6}$Se${}_2$ at different pressures measured in a magnetic field of 20 Oe, the magnetization was normalized to the values at 5 K. Inset: variation of the superconducting transition temperature $T_c$ under pressure. (b) Powder diffraction patterns of Rb${}_{0.8}$Fe${}_{1.6}$Se${}_2$ at different pressures. Inset: the superstructure reflections (110), (020), and (220) demonstrate the $\sqrt{5}$ × $\sqrt{5}$ superstructure persist up to the highest pressure of 15.6 GPa.

Figure 2

Left panel: Room temperature ${}^{57}$Fe-Mössbauer spectra of Rb${}_{0.8}$Fe${}_{1.6}$Se${}_2$ measured at different pressures. Subspectra of the magnetic Fe sites are marked in blue (AFM phase I), subspectra of nonmagnetic Fe sites (phase II) are shown in green. At 5.2 GPa a new PM phase III (doublet shown in red) emerges from the AFM phase I. Right panel: Second run of Mössbauer measurements of Rb${}_{0.8}$Fe${}_{1.6}$Se${}_2$ performed at different pressures and temperatures. Sequence of measurements is presented from the top to the bottom. Inset: distribution function $P$($H_{\mathrm{hf}}$) of magnetic hyperfine fields on Fe atoms in Rb${}_{0.8}$Fe${}_{1.6}$Se${}_2$ measured at $p$ $=$ 7.6 GPa (dashed line) and $p$ $=$ 0.3 GPa (solid line) at $T$ $=$ 4.2 K. The Mössbauer spectrum measured at $p$ $=$ 0.3 GPa and $T$ $=$ 295 K is dominated by the PM doublet shown in red. Subspectrum of the magnetic Fe sites is marked in blue.

Figure 3

(a) Pressure dependence of the relative fractions of the PM component II (up triangles), the PM component III (solid circles), and the AFM fraction I (squares). Down triangles result from rescaling the amount of the PM fraction II. This procedure makes evident that under pressure the PM phase II behaves similarly to the AFM phase I, and that a new component III develops at the expense of both phases I and II. (b) Left scale: normalized variation of $T_{\mathrm{c}}/T_c$${}_{\max }$ (open circles) in Rb${}_{0.8}$Fe${}_{1.6}$Se${}_2$. Right scale: pressure dependence of the magnetic hyperfine field $H_{\mathrm{hf}}$(I) in the AFM phase I. The pressure dependence of the phase fractions and of $H_{\mathrm{hf}}$ is described by sigmoidal curves (dashed lines).