Different response of superconductivity to the transition-metal impurities in K${}_{0.8}$Fe${}_{2-y-x}{M}_x$Se${}_2$ ($M$ $=$ Cr, Mn, Co, Zn)

In this paper, we perform Fe-site substitution experiments on K${}_{0.8}$Fe${}_{2-y}$Se${}_2$ superconductor by choosing Cr, Mn, Co, and Zn as dopants. It is found that the doping of Cr, Co, and Zn leads to a drastic depression of superconductivity, while the introduction of Mn does not decrease the superconducting transition temperature ($T_c$) when $x⩽0.067$. Electron-spin-resonance measurements reveal the existence of strong magnetic fluctuation in both the parent compound and the doped samples. In the Cr-, Co-, and Zn-doped samples, the magnetic pair-breaking effect severely affects the superconductivity, resulting in drastic depression of superconductivity. On the other hand, in the Mn-doped case, it does not break the cooper pairs and thus hardly affects the $T_c$ value.

Figure 1

(a) Powder x-ray diffraction patterns for the K${}_{0.8}$Fe${}_{2-y-x}$Co${}_x$Se${}_2$ samples. The red curve is the single-crystal XRD pattern of a K${}_{0.8}$Fe${}_{2-y}$Se${}_2$ sample. On the top, a picture of the as-grown single crystal is given. (b) and (c) The lattice constants of the transition-metal-doped samples.

Figure 2

Temperature dependence of in-plane resistivity for (a) K${}_{0.8}$Fe${}_{2-y-x}$Cr${}_x$Se${}_2$, (b) K${}_{0.8}$Fe${}_{2-y-x}$Mn${}_x$Se${}_2$, (c) K${}_{0.8}$Fe${}_{2-y-x}$Co${}_x$Se${}_2$, and (d) K${}_{0.8}$Fe${}_{2-y-x}$Zn${}_x$Se${}_2$.

Figure 3

(a) The plot of the $T_c$ value vs $x$ for the K${}_{0.8}$Fe${}_{2-y-x}$$M_x$Se${}_2$ ($M=\text{Cr}$, Mn, Co, and Zn) samples. (b) The plot of the residual resistivity vs $x$.

Figure 4

The temperature dependence of magnetic susceptibility of the K${}_{0.8}$Fe${}_{2-y-x}$$M_x$Se${}_2$ ($M=\text{Cr}$, Mn, Co, and Zn) samples near the superconducting transition temperature. (The data are collected under zero-field-cooling condition with external magnetic field of 100 gauss.) In the inset, the comparison of magnetic susceptibility between ZFC condition and FC condition for the K${}_{0.8}$Fe${}_{2-y}$Se${}_2$ parent sample is given.

Figure 5

The temperature dependence of magnetic susceptibility of the K${}_{0.8}$Fe${}_{2-y-x}$$M_x$Se${}_2$ ($M=\text{Cr}$, Mn, Co, and Zn) samples at high temperatures.

Figure 6

(a) Electron-spin-resonance spectra at different temperatures for K${}_{0.8}$Fe${}_{2-y}$Se${}_2$ sample. (b) The temperature dependence of the measured resonance magnetic field for the K${}_{0.8}$Fe${}_{2-y}$Se${}_2$ parent compound and the K${}_{0.8}$Fe${}_{2-y-x}$$M_x$Se${}_2$ ($M=\text{Cr}$, Mn, Co, and Zn) samples. (c) and (d) The temperature dependence of relative intensity and the ESR linewidth, respectively.

Figure 7

Electron-spin-resonance spectra at different temperatures for (a) K${}_{0.8}$Fe${}_{2-y-0.1}$Cr${}_{0.024}$Se${}_2$, (b) K${}_{0.8}$Fe${}_{2-y-0.1}$Mn${}_{0.067}$Se${}_2$, (c) K${}_{0.8}$Fe${}_{2-y-0.1}$Co${}_{0.023}$Se${}_2$, and (d) K${}_{0.8}$Fe${}_{2-y-0.1}$Zn${}_{0.029}$Se${}_2$.