Strong electronic two-dimensionality and close relationship between spin nematicity and superconductivity in soft-chemistry synthesized (Li,Fe)OHFeSe and FeSe single crystals

We have developed hydrothermal ion-exchange and ion-release routes to synthesize a series of big superconducting (Li,Fe)OHFeSe and FeSe single crystals out of insulating K$_{\mathrm{2}}$Fe$_{\mathrm{4}}$Se$_{\mathrm{5}}$ matrix. In (Li$_{\mathrm{0.84}}$Fe$_{\mathrm{0.16}})$OHFe$_{\mathrm{0.98}}$Se crystal, a common temperature scale $T$* $=$ 120 K has been established. Below $T$*, the normal state electronic behavior becomes highly two-dimensional prior to the superconducting transition and AFM spin fluctuations in the iron plane set in. In FeSe crystals, a spin-nematic order is identified by in-plane angular-dependent magnetoresistance and magnetism measurements, manifested as a rotational symmetry breaking and evident spin frustrations below a characteristic temperature $T_{\mathrm{sn}}$. Remarkably, a universal linear relationship between $T_{\mathrm{c}}$ and $T_{\mathrm{sn}}$ is observed on a series of crystal samples, indicating that the spin nematicity and the superconductivity in bulk FeSe have a common microscopic origin. \textbf{References:} [1] X. Dong et al., \textbf{\textit{JACS}} \textbf{137}, 66-69 (2015). [2] X. Dong et al., \textbf{\textit{PRB}} \textbf{92}, 064515 (2015). [3] D. Yuan et al., \textbf{\textit{Chin. Phys. B}} \textbf{25}, 077404 (2016). [4] D. Yuan et al.,, \textbf{\textit{PRB }}\textbf{94}, 060506R (2016).