Absence of low energy magnetic spin-fluctuations in isovalently and aliovalently doped LaCo₂B₂ superconducting compounds

Magnetization, resistivity and ¹¹B, ⁵⁹Co NMR measurements have been performed on the Pauli paramagnet $\text{LaC}{\text{o}}_2{\text{B}}_2$, and the superconductors $\text{L}{\text{a}}_{0.9}{\text{Y}}_{0.1}\text{C}{\text{o}}_2{\text{B}}_2$ ($T_{\text{c}}\approx 4.2$ K) and $\text{La}{\left(\text{C}{\text{o}}_{0.7}\text{F}{\text{e}}_{0.3}\right)}_2{\text{B}}_2$ ($T_{\text{c}}\approx 5.8$ K). The site selective NMR experiment reveals the multiband nature of the Fermi surface in these systems. The temperature independent Knight shift and 1/T ₁ T clearly indicate the absence of correlated low energy magnetic spin-fluctuations in the normal state, which is in contrast to other Fe-based pnictides. The density of states (DOS) of Co 3d electrons has been enhanced in superconducting $\text{L}{\text{a}}_{0.9}{\text{Y}}_{0.1}\text{C}{\text{o}}_2{\text{B}}_2$ and $\text{La}{\left(\text{C}{\text{o}}_{0.7}\text{F}{\text{e}}_{0.3}\right)}_2{\text{B}}_2$ with respect to the non superconducting reference compound $\text{LaC}{\text{o}}_2{\text{B}}_2$. The occurrence of superconductivity is related to the DOS enhancement. (paper)