  LaCo2B2 with a ThCr2Si2-type structure composed of alternately stacked La and
CoB layers exhibits metallic electrical conductivity and Pauli paramagnetic
behavior down to 2K. Bulk superconductivity with a Tc of ~4K emerges upon
substitution with dopant elements; i.e., isovalent substitution to form
(La1-xYx)Co2B2, or aliovalent substitution to form La(Co1-xFex)2B2. Highly
covalent bonding between Co 3d and B 2p levels in the CoB layers, which is
caused by the B 2p level being shallower than the Fermi level, removes magnetic
ordering from Co 3d electrons even in the undoped samples.
