Pressure dependence of coherence-incoherence crossover behavior in KFe2As2 observed by resistivity and As75 -NMR/NQR

We present the results of ⁷⁵As nuclear magnetic resonance (NMR), nuclear quadrupole resonance (NQR), and resistivity measurements in KF_e2 As₂ under pressure (p). The temperature dependence of the NMR shift, nuclear spin-lattice relaxation time (T₁), and resistivity show a crossover between a high-temperature incoherent, local-moment behavior and a low-temperature coherent behavior at a crossover temperature (T*). T* is found to increase monotonically with pressure, consistent with increasing hybridization between localized 3d orbital-derived bands with the itinerant electron bands. No anomaly in T* is seen at the critical pressure p_c= 1.8 GPa where a change of slope of the superconducting (SC) transition temperature T_c(p) has been observed. In contrast, T_c(p) seems to correlate with antiferromagnetic spin fluctuations in the normal state as measured by the NQR 1/T₁ data, although such a correlation cannot be seen in the replacement effects of A in the AFe₂As₂ (A=K,Rb,Cs) family. In the superconducting state, two T₁ components are observed at low temperatures, suggesting the existence of two distinct local electronic environments. The temperature dependence of the short T_1s indicates a nearly gapless state below T_c. On the other hand, the temperature dependence of the long component 1/T_1Limplies a large reduction in the density of states at the Fermi level due to the SC gap formation. These results suggest a real-space modulation of the local SC gap structure in KFe₂As₂ under pressure.