New Alkali-Metal- and 2-Phenethylamine-Intercalated Superconductors   $A_x$(C$_8$H$_{11}$N)$_y$Fe$_{1-z}$Se ($A$ = Li, Na) with the Largest   Interlayer Spacings and $T_\mathrm{c}$ $\sim$ 40 K
  New FeSe-based intercalation superconductors, $A_x$(C$_8$H$_{11}$N)$_y$Fe$_{1-z}$Se ($A$ = Li, Na), with $T_\mathrm{c}$ = 39-44 K have been successfully synthesized via the intercalation of alkali metals and 2-phenethylamine into FeSe. The interlayer spacings, namely, the distances between neighboring Fe layers, $d$, of $A_x$(C$_8$H$_{11}$N)$_y$Fe$_{1-z}$Se ($A$ = Li, Na) are 19.04(6) and 18.0(1) {\AA}, respectively. These $d$ values are the largest among those of the FeSe-based intercalation compounds and are understood to be due to the intercalation of two molecules of 2-phenethylamine in series perpendicular to the FeSe layers. It appears that the relationship between $T_\mathrm{c}$ and $d$ in the FeSe-based intercalation superconductors is not domic but $T_\mathrm{c}$ is saturated at ~ 45 K, which is comparable to the $T_\mathrm{c}$ values of single-layer FeSe films, for $d$ $\geq$ 9 {\AA}. 
Takehiro Hatakeda, Takashi Noji, Kazuki Sato, Takayuki Kawamata,   Masatsune Kato, Yoji Koike
cond-mat.supr-con