paper

75As NMR study of single crystals of the heavily overdoped pnictide superconductors Ba{1-x}KxFe2As2 (x = 0.7 and 1)

arXiv:0908.4369 · doi:10.1103/PhysRevB.81.012503

Abstract

We performed $^{75}$ As NMR studies on two overdoped high-quality Ba $_{1 - x}$ K $_{x}$ Fe $_{2}$ As $_{2}$ (x=0.7 and 1.0) single crystals. In the normal states, we found a dramatic increase of the spin-lattice relaxation ( $1 /^{75} T_{1}$ ) from the x=0.7 to the x=1.0 samples. In KFe $_{2}$ As $_{2}$ , the ratio of $1 /^{75} T_{1} T K_{n}^{2}$ , where $^{75} K_{n}$ is the Knight shift, increases as temperature drops. These results indicate the existence of a new type of spin fluctuations in KFe $_{2}$ As $_{2}$ which is accustomed to being treated as a simple Fermi liquid. In the superconducting state, we observe a step-like feature in the temperature dependence of the spin-lattice relaxation of the x=0.7 sample, which supports a two-gap superconductivity as the underdoped materials. However, the temperature scalings of $1 /^{75} T_{1}$ below Tc in the overdoped samples are significantly different from those in the under or optimal doped ones. A power-law scaling behavior $1 /^{75} T_{1} T \sim T^{0.5}$ is observed, which indicates universal strong low energy excitations in the overdoped hole-type superconductors.