Figure 1

(Color) (a) Temperature-dependent electrical resistivity $\left(\rho \right)$ at zero magnetic field for bulk ${\text{Cu}}_x{\text{Fe}}_{1-x}{\text{Se}}_{1-\delta }$ ($x=0$, 0.01, 0.02, 0.03, 0.05, and 0.1) samples. Inset shows the low-temperature resistivity plotted against $1/T^{1/4}$ (b) Temperature dependence of electrical resistivity for bulk ${\text{Mn}}_x{\text{Fe}}_{1-x}{\text{Se}}_{1-\delta }$ ($x=0$ and 0.022, in nominal composition) samples. Inset shows $Tc$ as a function of Mn substitution.Reuse & Permissions

Figure 2

(Color) (a) The bright field TEM image and the corresponding selected-area electron diffraction of ${\text{Cu}}_{0.04}{\text{Fe}}_{0.96}{\text{Se}}_{1-\delta }$ powder sample, aligned along the (001) direction revealing the fourfold symmetry of the tetragonal structure. The reflection marked by red indexes is expected to be very weak due to systematic abscence. At the right side we present the STEM/EDX elemental mappings of the area of interest marked by a red square using a $2\text{Å}$ electron probe, which demonstrate the random distribution of copper in the sample. (b) The x-ray absorption near-edge structure (XANES) in $\text{Fe}K$ edge for ${\text{Cu}}_x{\text{Fe}}_{1-x}{\text{Se}}_{1-\delta }\left(x=0–0.04\right)$, Fe foil (upper), and FeO (bottom). The inset plots the valence states of Fe, which are calculated from the first derivative of the XANES spectra.Reuse & Permissions

Figure 3

(Color) (a) Crystal structure of ${\text{Cu}}_x{\text{Fe}}_{1-x}{\text{Se}}_{1-\delta }$, sketched schematically with Fe in red, Cu in dark yellow, and Se in gray color. The pyramids chain and tetrahedron with respect to iron are shown to the right. (b) Temperature dependence of NPD for ${\text{Cu}}_{0.01}{\text{Fe}}_{0.9}{\text{Se}}_{1-\delta }$ (left) and ${\text{Cu}}_{0.1}{\text{Fe}}_{0.9}{\text{Se}}_{1-\delta }$ (right) bulk samples. (c) The typical refinement for NPD results of 10% Cu-doped FeSe sample. The calculated diffraction peaks for 10% Cu-doped FeSe is shown as the red line and the blue line shows the impurity phase pure Fe. The difference between the experimental data and the sum of calculated patterns is shown as cross mark. (d)The NPD of ${\text{Mn}}_{0.01}{\text{Fe}}_{0.99}{\text{Se}}_{1-\delta }$ (left) and ${\text{Mn}}_{0.022}{\text{Fe}}_{0.978}{\text{Se}}_{1-\delta }$ (right). Similar structural change is observed as evidenced by the peak splitting in (220), (221), and (114) reflections at $\sim 85\text{K}$ and $\sim 62\text{K}$, respectively.Reuse & Permissions

Figure 4

(Color) The lattice parameters and detailed structure information, including (a) volume (b) $a$, $c$, and (c) Fe-Se, Fe-Fe distances, as temperature dependence in superconducting FeSe with 1% Cu doping and nonsuperconducting FeSe with 10% Cu doping.Reuse & Permissions

Figure 5

(Color) [(a)–(c)] Lattice constants, Fe-Se and Fe-Fe bond lengths, Se-Fe-Se bond angles, and $Tc$ of ${\text{Cu}}_x{\text{Fe}}_{1-x}{\text{Se}}_{1-\delta }$ as a function of Cu substitution, $x$. (d) The structural phase diagrams of ${\text{Cu}}_x{\text{Fe}}_{1-x}{\text{Se}}_{1-\delta }$ and ${\text{Mn}}_x{\text{Fe}}_{1-x}{\text{Se}}_{1-\delta }$ determined from neutron and synchrotron x-ray powder diffraction data and resistivity data. The solid triangles and circles indicate the onset of tetragonal to monoclinic structural distortion, $Ts$, and the hollow triangles and circles designate the onset of superconductivity, $Tc$.Reuse & Permissions