Superconductivity in (NH${}_3$)${}_y$Cs${}_{0.4}$FeSe

Alkali-metal-intercalated FeSe materials, (NH${}_3$)${}_{\mathrm{y}}$$M_{0.4}$FeSe ($M$: K, Rb, and Cs), have been synthesized using the liquid NH${}_3$ technique. (NH${}_3$)${}_{\mathrm{y}}$Cs${}_{0.4}$FeSe shows a superconducting transition temperature ($T_{\mathrm{c}}$) as high as 31.2 K, which is higher by 3.8 K than the $T_{\mathrm{c}}$ of nonammoniated Cs${}_{0.4}$FeSe. The $T_{\mathrm{c}}$s of (NH${}_3$)${}_{\mathrm{y}}$K${}_{0.4}$FeSe and (NH${}_3$)${}_{\mathrm{y}}$Rb${}_{0.4}$FeSe are almost the same as those of nonammoniated K${}_{0.4}$FeSe and Rb${}_{0.4}$FeSe. The $T_{\mathrm{c}}$ of (NH${}_3$)${}_{\mathrm{y}}$Cs${}_{0.4}$FeSe shows a negative pressure dependence. A clear correlation between $T_{\mathrm{c}}$ and lattice constant $c$ is found for ammoniated metal-intercalated FeSe materials, suggesting a correlation between Fermi-surface nesting and superconductivity.

Figure 1

(a) X-ray diffraction pattern and (b) $\mathit{M}$/$H$-$T$ plots of the FeSe sample. In (a), the experimental x-ray diffraction pattern (blue circles) can be reproduced by the pattern (red line) calculated from the two phases β-FeSe (80$\%$) and α-FeSe (20$\%$); the black ticks show the 2θ positions predicted for β-FeSe (top) and α-FeSe (bottom). The lower red line shows the difference between experimental and calculated patterns.

Figure 2

(a) X-ray diffraction pattern, (b) schematic crystal structure, (c) $\mathit{M}$/$H$-$T$ plots, and (d) ρ-$T$ plot of the (NH${}_3$)${}_{\mathrm{y}}$Cs${}_{0.4}$FeSe sample. In (a), the experimental x-ray diffraction pattern (blue circles) can be reproduced by the pattern (red line) calculated from the three phases (NH${}_3$)${}_{\mathrm{y}}$Cs${}_{0.255\left(5\right)}$FeSe (37$\%$), β-FeSe (37$\%$), and α-FeSe (26$\%$); the black ticks show the 2θ positions predicted for (NH${}_3$)${}_{\mathrm{y}}$Cs${}_{0.255\left(5\right)}$FeSe (top), β-FeSe (middle), and α-FeSe (bottom). The lower red line shows the difference between experimental and calculated patterns. In (b), NH${}_3$ molecules are not shown. In (d), a picture of the pellet used for ρ measurements is inset. Its $L$, $W$, and $d$ were 0.108, 0.122, and 0.017 cm, respectively.

Figure 3

(a) χ′-$T$ plots of (NH${}_3$)${}_{\mathrm{y}}$Cs${}_{0.4}$FeSe sample under different pressures, and (b) pressure dependence of $T_{\mathrm{c}}$. In (b), red circles indicate the $T_{\mathrm{c}}$ determined from χ′-$T$ plot. Blue and green triangles indicate the $T_{\mathrm{c}}$ from $\mathit{M}$/$H$-$T$ plots measured in increasing and decreasing pressure, respectively. The line was fitted to the points, excluding the brief upward deviation enlarged in the inset, which shows data below 1 GPa in greater detail.

Figure 4

$\mathit{M}$/$H$-$T$ plots of (a) (NH${}_3$)${}_{\mathrm{y}}$K${}_{0.4}$FeSe and (b) (NH${}_3$)${}_{\mathrm{y}}$Rb${}_{0.4}$FeSe samples. Relationship (c) between $T_{\mathrm{c}}$${}^{\mathrm{onset}}$ and ionic radii of intercalated metal atoms and (d) between $T_{\mathrm{c}}$${}^{\mathrm{onset}}$ and $c$. In (c) and (d), the star-shaped symbols are plotted based on the data from this paper. Solid circle and triangle symbols are taken from Refs. 3 and 4, respectively.