Pressure-induced dependence of transport properties and lattice stability of CaK(Fe1-xNix)4As4 (x=0.04 and 0) superconductors with and without a spin-vortex crystal state

Here we report the investigation on correlation between the transport properties and the corresponding stability of the lattice structure for CaK(Fe1-xNix)4As4 (x = 0.04 and 0), a new type of putative topological superconductors with and without a spin-vortex crystal (SVC) state in a wide pressure range involving superconducting (SC) to nonsuperconducting (NSC) transition and the half to full collapse of tetragonal [half-collapsed tetragonal (h-cT) and full-collapsed tetragonal (f-cT)] phases by the complementary measurements of high-pressure resistance, Hall coefficient, and synchrotron x-ray diffraction. The three critical pressures are identified: Pon ch that is the turn-on critical pressure of the h-cT phase transition and it coincides with the critical pressure for the sign change of Hall coefficient from positive to negative, a manifestation of the Fermi surface reconstruction; Poff ch that is the turn-off pressure of the h-cT phase transition; and Pcf that is the critical pressure of the f-cT phase transition. By comparing the high-pressure results measured from the two kinds of samples, we find a distinct "left-shift" of the Pon ch for the doped sample, at the pressure of which its SVC state is fully suppressed, however, the Poff ch and the Pcf remain the same as that of the undoped one. Our results not only provide a consistent understanding on the results reported before, but also demonstrate the importance of the Fe-As bonding in stabilizing the superconductivity of the iron pnictide superconductors through the "pressure window."