${\mathrm{LaCo}}_2{\mathrm{B}}_2$ with a ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$-type (so-called “122”) structure composed of alternately stacked La and CoB layers exhibits metallic electrical conductivity and Pauli paramagnetic behavior down to 2 K. Bulk superconductivity with a $T_c$ of $\sim 4\mathrm{K}$ emerges upon substituting with dopant elements, i.e., isovalent substitution to form $({\mathrm{La}}_{1-x}{\mathrm{Y}}_x){\mathrm{Co}}_2{\mathrm{B}}_2$, or aliovalent substitution to form $\mathrm{La}({\mathrm{Co}}_{1-x}{\mathrm{Fe}}_x{)}_2{\mathrm{B}}_2$. According to the density functional theory calculations, highly covalent bonding between Co $3d$ dominating the Fermi level and $B$ $2p$ levels deeper than the Fermi level removes magnetic ordering from Co $3d$ electrons even in the undoped samples. This is a first Co-based superconductor among the 122-type family.

• Received 6 February 2011

DOI:https://doi.org/10.1103/PhysRevLett.106.237001