Doping dependence of phase-separation morphology in (Sr,K)Fe ₂As₂

The effects of the possible phase separation on macroscopic transport properties are explored in (Sr,K)Fe₂As₂. Three probes, the low-field shielding 4πχ_ZFC, the reversible diamagnetization M_SC at 5 T, and the specific heat C_p, with different characteristic length scales on the order of 103, 10, and 100 nm, respectively, were used to examine several (Sr_1-xK_x)Fe ₂As₂ samples. The data from different probes should be consistent if the sample can be regarded as mesoscopically homogeneous. While this seems to be the case for the optimally doped x=0.45 samples, the apparent transition temperature and the effective superfluid density strongly depend on the probes used for both the underdoped (x=0.3) and the overdoped (x=0.7) samples. Discrepancies of up to a factor of 10 are observed. The data further suggest that both the x=0.3 and x=0.7 samples are better described as Josephson-junction arrays formed over separated superconductive domains of 10-102 nm sizes. Such a morphology evolution may drastically affect the interpretation of the observed doping effects.