Huge enhancement of superconductivity in the collapsed tetragonal KFe$_{2}$As$_{2}$

Recent work (F. F. Tafti, \textit{et al. Nature Phys.}\textbf{9}, 349 (2013)) on hole-overdoped iron pnictide KFe$_{2}$As$_{2}$ indicated a paring symmetry change at pressure of around 1.7 GPa. The investigation for the low-pressure region (below 7 GPa) revealed oscillation of T$_{\mathrm{c}}$ with pressure. Here we report results of high-pressure transport and XRD measurements on KFe$_{2}$As$_{2}$ single crystals at high pressures up to 30 GPa. We map out the phase diagram of KFe$_{2}$As$_{2}$ and find a huge enhancement of $T_{\mathrm{c}}$ in the collapsed tetragonal phase. The correlation between $T_{\mathrm{c}}$, electronic and crystal structures is discussed. The strong electronic correlations are proposed to account for such an unexpected $T_{\mathrm{c}}$ enhancement in KFe$_{2}$As$_{2}$.