Superconductivity at 22 K in Co-Doped ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ Crystals

Here we report bulk superconductivity in ${\mathrm{BaFe}}_{1.8}{\mathrm{Co}}_{0.2}{\mathrm{As}}_2$ single crystals below $T_c=22\mathrm{K}$, as demonstrated by resistivity, magnetic susceptibility, and specific heat data. Hall data indicate that the dominant carriers are electrons, as expected from simple chemical reasoning. This is the first example of superconductivity induced by electron doping in this family of materials. In contrast with cuprates, the ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ system appears to tolerate considerable disorder in the FeAs planes. First principles calculations for ${\mathrm{BaFe}}_{1.8}{\mathrm{Co}}_{0.2}{\mathrm{As}}_2$ indicate the interband scattering due to Co is weak.

Figure 1

Temperature dependence of molar susceptibility in zero-field-cooled ($zfc$) forms along the two crystallographic directions for (a) ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ in 1 kOe and (b) ${\mathrm{BaFe}}_{1.8}{\mathrm{Co}}_{0.2}{\mathrm{As}}_2$ in 1 T. (c) The temperature dependence of susceptibility in $zfc$ and field-cooled ($fc$) forms below 30 K, for ${\mathrm{BaFe}}_{1.8}{\mathrm{Co}}_{0.2}{\mathrm{As}}_2$ in 20 Oe along the $ab$ plane, assuming $\chi$ value of the perfect diamagnetism. (d) Field dependence of magnetization for ${\mathrm{BaFe}}_{1.8}{\mathrm{Co}}_{0.2}{\mathrm{As}}_2$ along the two crystallographic directions at 1.8 K; there are two peaks noted at $H_p$ and $H_x$. The demagnetization shape factor is accounted for in the ${\mathrm{BaFe}}_{1.8}{\mathrm{Co}}_{0.2}{\mathrm{As}}_2$ superconductivity data (see text).

Figure 2

(a) Temperature dependence of resistivity for ${\mathrm{BaFe}}_{1.8}{\mathrm{Co}}_{0.2}{\mathrm{As}}_2$, measured below 30 K, at 0 and 8 T applied magnetic fields. Inset is the temperature dependence of ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ and ${\mathrm{BaFe}}_{1.8}{\mathrm{Co}}_{0.2}{\mathrm{As}}_2$ up to room temperature at 0 T. (b) Hall coefficient $R_H$ versus temperature for ${\mathrm{BaFe}}_{1.8}{\mathrm{Co}}_{0.2}{\mathrm{As}}_2$. Inset is the field dependence of Hall resistivity at 30, 100, 200, and 300 K.

Figure 3

(a) Projections of the $d$ electron DOS, on a per atom basis, for the Fe and Co in a ${\mathrm{Ba}}_4{\mathrm{Fe}}_7{\mathrm{CoAs}}_8$ supercell. (b) LDA Fermi Surface of ${\mathrm{BaFe}}_{1.8}{\mathrm{Co}}_{0.2}{\mathrm{As}}_2$, for the LDA internal coordinates, shaded by band velocity (light blue is the low velocity).