Coevolution of superconductivity and Hall coefficient with anisotropic lattice shrinkage in compressed KCa 2 Fe 4 As 4 F 2

The stability of superconductivity in superconductors is widely recognized to be determined by various factors, including charge, spin, orbit, lattice, and other related degrees of freedom. Here, we report our findings on the pressure-induced coevolution of superconductivity and Hall coefficient in KCa 2 Fe 4 As 4 F 2 , an iron-based superconductor possessing a hybrid crystal structure combining KFe 2 As 2 and CaFeAsF. Our investigation, involving high-pressure resistance, Hall effect and x-ray diffraction (XRD) measurements, allows us to observe the connection of the superconductivity and Hall coefficient with the anisotropic lattice shrinkage. We find that its ambient-pressure tetragonal (T) phase presents a collapse starting at around 18 GPa, where the sign of the Hall coefficient ( R H ) changes from positive to negative. Upon further compression, both superconducting transition temperature ( T c ) and R H exhibit a monotonous decrease. At around 41 GPa, the superconductivity is completely suppressed ( T c = 0), where the parameter a begins to decline again and the Hall coefficient remains nearly unchanged. Our experiment results clearly demonstrate that the pressure-induced anisotropic lattice collapse plays a crucial role in tuning the interplay among multiple degrees of freedom in the superconducting system and, correspondingly, the stability of the superconductivity.