Electron-hole asymmetry in Co- and Mn-doped ${\text{SrFe}}_2{\text{As}}_2$

Phase diagrams of electron- and hole-doped ${\text{SrFe}}_2{\text{As}}_2$ single crystals are investigated using Co and Mn substitution at the Fe sites. We find that the spin-density-wave state is suppressed by both dopants but the superconducting phase appears only for Co (electron) doping, not for Mn (hole) doping. Absence of the superconductivity by Mn doping is in sharp contrast to the hole-doped system with K substitution at the Sr sites. First-principles calculations based on detailed structural investigations reveal that a distinct structural change, i.e., the increase in the Fe-As distance by Mn doping is the most decisive factor to induce a magnetic and semiconducting ground state in ${\text{SrFe}}_{2-x}{\text{Mn}}_x{\text{As}}_2$. The absence of electron-hole symmetry in the phase diagrams of the Fe-site doped ${\text{SrFe}}_2{\text{As}}_2$ suggests that the occurrence of high-$T_c$ superconductivity is sensitive to the structural modification rather than the carrier doping.

Figure 1

(Color online) Typical x-ray powder-diffraction patterns using $\text{Cu}K\alpha$ radiation for (a) undoped, (b) Mn doped $\left(x=0.15\right)$, and (c) Co doped $\left(x=0.15\right)$ ${\text{SrFe}}_2{\text{As}}_2$. The red (dark gray) and black solid lines represent the Rietveld refinement and experimental data, respectively. The difference between the observed and the calculated data is shown by the solid line below the row of vertical bars marking the angles of the Bragg reflections used to simulate the patterns. Doping dependence of lattice constants (d) $a$ and (e) $c$ for ${\text{SrFe}}_{2-x}{M}_x{\text{As}}_2$ (filled circle). For comparison, we also plot those of polycrystalline samples (Ref. 16) (cross).

Figure 2

(Color online) Temperature dependence of (a) the normalized resistivity $\rho \left(T\right)/\rho \left(300\text{K}\right)$ and (b) the magnetic susceptibility $\chi \left(T\right)$ at $H=7\text{T}$ along the $ab$ plane for Co- and Mn-doped ${\text{SrFe}}_2{\text{As}}_2$.

Figure 3

(Color online) Phase diagrams of ${\text{SrFe}}_{2-x}{M}_x{\text{As}}_2$ ($M=\text{Co}$ and Mn) as a function of the number of extra conduction electrons per Fe with respect to the undoped system. The solid symbols indicate the AFM transition temperatures while the open ones represent the superconducting transition temperatures. Note that Mn doping does not induce the superconductivity while K doping does (Ref. 23). The inset shows the doping dependence of $T_c$ for Co-doped ${\text{SrFe}}_2{\text{As}}_2$ single crystals (open circle). For comparison, together plotted are $T_c$ of polycrystalline ${\text{SrFe}}_{2-x}{\text{Co}}_x{\text{As}}_2$ in Ref. 14 (cross) and strain-induced ${\text{SrFe}}_2{\text{As}}_2$ in Ref. 27 (open diamond).

Figure 4

(Color online) Doping dependence of lattice parameters (a) $a$ and (b) $c$ for Co, Mn, and K doping. The negative (positive) value of $x$ indicates hole (electron) doping. The corresponding bond lengths of Fe with neighboring Fe and As and the bond angle around Fe with two neighboring As are shown (c) and (d), respectively. The local structure of the FeAs layer is shown in the inset of (d).

Figure 5

(Color online) The calculated stabilization energies of the SDW states with respect to the nonmagnetic state for (a) ${\text{SrFe}}_2{\text{As}}_2$ with the shift of the chemical potential only, (b) K-, Mn- and Co-doped ${\text{SrFe}}_2{\text{As}}_2$, keeping the crystal structure same as pristine ${\text{SrFe}}_2{\text{As}}_2$, (c) undoped ${\text{SrFe}}_2{\text{As}}_2$ but assuming the crystal structure taken from K-, Mn- and Co-doped compounds, and (d) K-, Mn- and Co-doped ${\text{SrFe}}_2{\text{As}}_2$. Note that the negative (positive) value of $x$ indicates hole- (electron) doping.

Figure 6

(Color online) The doping dependence of (a) the Fe-As distance, (b) the Fe-Fe distance, and (c) $c$-axis lattice parameter for ${\text{Sr}}_{1-x}{A}_x{\text{Fe}}_2{\text{As}}_2$ ($A=\text{K}$ and Cs) and ${\text{SrFe}}_{2-x}{M}_x{\text{As}}_2$ ($M=\text{Mn}$, Co, Ni, Rh, Pd, and Ir) as compared to those of pristine ${\text{SrFe}}_2{\text{As}}_2$. Note that except Mn (filled symbols), all other dopants (open symbols) induce superconductivity.