LaCo2B2: a Co-based layered superconductor with a ThCr2Si2-type structure.

LaCo(2)B(2) with a ThCr(2)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 ∼4  K emerges upon substituting with dopant elements, i.e., isovalent substitution to form (La(1-x)Y(x))Co(2)B(2), or aliovalent substitution to form La(Co(1-x)Fe(x))(2)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.