${}^{75}\text{A}\text{s}$ NMR study of single crystals of the heavily overdoped pnictide superconductors ${\text{Ba}}_{1-x}{\text{K}}_x{\text{Fe}}_2{\text{As}}_2$ ($x=0.7$ and 1)

We performed ${}^{75}\text{A}\text{s}$ NMR studies on two overdoped high-quality ${\text{Ba}}_{1-x}{\text{K}}_x{\text{Fe}}_2{\text{As}}_2$ ($x=0.7$ and 1.0) single crystals. In the normal states, we found a dramatic increase in the spin-lattice relaxation $\left(1/{{}^{75}T}_1\right)$ from the $x=0.7$ to the $x=1.0$ samples. In ${\text{KFe}}_2{\text{As}}_2$, 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 another type of spin fluctuations in ${\text{KFe}}_2{\text{As}}_2$ which is accustomed to being treated as a simple Fermi liquid. In the superconducting state, the temperature scalings of $1/{{}^{75}T}_1$ below $T_c$ in the overdoped samples are significantly different from those in the under or optimally 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.

Figure 1

(Color online) The ${}^{75}\text{A}\text{s}$ NMR spectrum of the ${\text{Ba}}_{0.3}{\text{K}}_{0.7}{\text{Fe}}_2{\text{As}}_2$ single crystal at a fixed frequency 72.9 MHz. Inset: the dc susceptibility of the crystal under field cooling and ZFC with a 100 Oe field.

Figure 2

(Color online) The temperature dependence of ${}^{75}\text{A}\text{s}$ spin-lattice relaxation rate of the ${\text{Ba}}_{0.3}{\text{K}}_{0.7}{\text{Fe}}_2{\text{As}}_2$ single crystal measured at four different fields, $12\text{T}\parallel c$, $10\text{T}\parallel c$, $6.5\text{T}\parallel c$, and $12\text{T}\parallel ab$.

Figure 3

(Color online) The spin-lattice relaxation rate $1/{{}^{75}T}_1$ of the ${\text{KFe}}_2{\text{As}}_2$ under 12 T field with two orientation $H\parallel c$ and $H\parallel ab$. Inset: the Knight shift $K_n$ of the sample with both fields.

Figure 4

(Color online) The normal-state spin-lattice relaxation rate $1/{{}^{75}T}_1^c$ at 30 and 200 K in 1: $x=0.3$ (Ref. 16), 2: $x=0.4$ (Ref. 18), 3: $x=0.4$ (Ref. 17), 4: $x=0.7$ (current work), 5: $x=1.0$ (current work), and 6: $x=1.0$ (NQR, Ref. 26) ${\text{Ba}}_{1-x}{\text{K}}_x{\text{Fe}}_2{\text{As}}_2$ samples.

Figure 5

(Color online) The power-law exponent ${\alpha }_s$ of $1/T_1\sim T^{{\alpha }_s}$ of 1: $x=0.3$ (Ref. 16), 2: $x=0.4$ (Ref. 18), 3: $x=0.4$ (Ref. 17), 4: $x=0.7$ (current work), and 6: $x=1.0$ (NQR, Ref. 26) ${\text{Ba}}_{1-x}{\text{K}}_x{\text{Fe}}_2{\text{As}}_2$ samples.