A 75 s NMR study of single crystals of the heavily overdoped pnictide superconductors Ba1−xKxFe2As2 (x=0.7 and 1)

We performed A 75 s NMR studies on two overdoped high-quality Ba1−xKxFe2As2 (x=0.7 and 1.0) single crystals. In the normal states, we found a dramatic increase in the spin-lattice relaxation (1/T 75 1) from the x=0.7 to the x=1.0 samples. In KFe2As2, the ratio of 1/T 75 1TKn2, where K 75 n is the Knight shift, increases as temperature drops. These results indicate the existence of another type of spin fluctuations in KFe2As2 which is accustomed to being treated as a simple Fermi liquid. In the superconducting state, the temperature scalings of 1/T 75 1 below Tc in the overdoped samples are significantly different from those in the under or optimally doped ones. A power-law scaling behavior 1/T 75 1T∼T0.5 is observed, which indicates universal strong low-energy excitations in the overdoped hole-type superconductors.