Pressure-induced high-$T_c$ superconducting phase in FeSe: Correlation between anion height and $T_c$

In this study, we performed high-pressure electrical resistivity measurements of polycrystalline FeSe in the pressure range of 1–16.0 GPa at temperatures of 4–300 K. A precise evaluation of $T_c$ from zero-resistivity temperatures revealed that $T_c$ shows a slightly distorted dome-shaped curve with maximum $T_c^{\text{offset}}$ (30 K) at 6 GPa. With the application of pressure, the temperature dependence of resistivity above $T_c$ changes dramatically to a linear dependence, that the high-$T_c$ phase appears above 3 GPa. We found a striking correlation between $T_c$ and the Se height: the lower the Se height, the more enhanced is $T_c$. Moreover, this relation is broadly applicable to other iron pnictides, strongly indicating that high-temperature superconductivity can appear only around the optimum anion height $\left(\sim 1.38\text{Å}\right)$. On the basis of these results, we suggest that the anion height should be considered as a key determining factor of $T_c$ of iron-based superconductors containing various anions.

Figure 1

(Color online) Temperature dependence of magnetic susceptibility (top main panel) and electrical resistivity (bottom main panel) of polycrystalline FeSe at ambient pressure. The cross point of two extrapolated lines denotes $T_c^{\text{onset}}$. The top and bottom insets show the crystal structure of FeSe and the setting of the sample in the high-pressure apparatus (see text for details), respectively.

Figure 2

(Color online) Temperature dependence of resistivity at ambient and several other pressures (top panel: 0–8 GPa and bottom panel: 9–16 GPa) for polycrystalline sample of FeSe.

Figure 3

(Color online) Pressure dependence of $T_c^{\text{onset}}$ (open circle) $T_c^{\text{offset}}$ (closed circle, top main panel) and width of superconducting transition $\Delta T_c\left(=T_c^{\text{onset}}-T_c^{\text{offset}}\right)$ (bottom main panel). The solid lines are obtained by connecting the data points.

Figure 4

(Color online) Enlarged view of resistivity below 120 K between 1 and 6 GPa. The dotted lines are guides to the eye, showing the dependence of $T$ and $T^2$. For simplicity, we have not shown the data at 1.5 GPa.

Figure 5

(Color online) Pressure dependence of $T_c^{\text{offset}}$ and Se height $h_{\text{Se}}$ (inversely scaled), as obtained from Ref. 6. The inset shows $T_c^{\text{offset}}$ as a function of the Se height. The dotted line is a guide to the eye.

Figure 6

(Color online) $T_c$ as a function of anion height $\left(h_{\text{anion}}\right)$ for various iron (and nickel)-based superconductors, as obtained from Ref. 28 (triangle: FeSe, circle: other pnictides). Lanthanides $\left(Ln\right)$ indicate $Ln\text{FeAsO}$ (1111 system). 111, 122, and 42226 represent LiFeAs, ${\text{Ba}}_{0.6}{\text{K}}_{0.4}{\text{Fe}}_2{\text{As}}_2$, and ${\text{Sr}}_4{\text{Sc}}_2{\text{Fe}}_2{\text{P}}_2{\text{O}}_6$ (Ref. 29), respectively. The yellow line shows the fitting result by the Lorenz function. The inset shows a schematic view of $h_{\text{anion}}$.