Enhancement of transition temperature in iron based superconductor KFe$_2$As$_2$ under pressure

Superconducting pairing symmetry in the iron pnictides is one of the key issues to clarify the origin of high T$_c$ superconductivity. The versatile pairing symmetry, including sign-reversed full gap, symmetry-imposed, or accidental nodal states, has been proposed theoretically and experimentally [1,2], and it can undergo a transition from one to another by chemical substitution or pressure. For instance, recent pressure study on heavily hole-doped KFe$_2$As$_2$ may imply a possible symmetry change accompanied by sudden reversal in pressure dependence of T$_c$ [3]. To explore the phase diagram of KFe$_2$As$_2$ in a wider pressure range, we here report low-temperature transport study under high pressure up to 33 GPa by utilizing a designer diamond anvil cell. We will discuss the evolution of the superconducting and structural properties of this material, highlighting novel changes in the system at high pressure. [1] J.P. Reid et al., PRL 109, 087001 (2012). [2] T. Shibauchi et al., Annu. Rev. Condens. Matter Phys. 5, 113 (2014). [3] F. Tafti et al., Nat. Phys. 6, 349 (2013).