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Abstract:
Magnetization, resistivity and 11 B, 59 Co NMR measurements have been performed on the Pauli paramagnet IMG [http://ej.iop.org/images/0953-8984/28/34/345701/cmaa2a4dieqn001.gif] ̑ext{LaC}{{̑ext{o}}_{2}}{{̑ext{B}}_{2}}} , and the superconductors IMG [http://ej.iop.org/images/0953-8984/28/34/345701/cmaa2a4dieqn002.gif] {̑ext{L}{{̑ext{a}}_{0.9}}{{̑ext{Y}}_{0.1}}̑ext{C}{{̑ext{o}}_{2}}{{̑ext{B}}_{2}}} ( IMG [http://ej.iop.org/images/0953-8984/28/34/345701/cmaa2a4dieqn003.gif] {{{T}_{̑ext{c}}}≈ 4.2} K) and IMG [http://ej.iop.org/images/0953-8984/28/34/345701/cmaa2a4dieqn004.gif] {̑ext{La}{{łeft(̑ext{C}{{̑ext{o}}_{0.7}}̑ext{F}{{̑ext{e}}_{0.3}}˚ight)}_{2}}{{̑ext{B}}_{2}}} ( IMG [http://ej.iop.org/images/0953-8984/28/34/345701/cmaa2a4dieqn005.gif] {{{T}_{̑ext{c}}}≈ 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 3 d electrons has been enhanced in superconducting IMG [http://ej.iop.org/images/0953-8984/28/34/345701/cmaa2a4dieqn006.gif] {̑ext{L}{{̑ext{a}}_{0.9}}{{̑ext{Y}}_{0.1}}̑ext{C}{{̑ext{o}}_{2}}{{̑ext{B}}_{2}}} and IMG [http://ej.iop.org/images/0953-8984/28/34/345701/cmaa2a4dieqn007.gif] {̑ext{La}{{łeft(̑ext{C}{{̑ext{o}}_{0.7}}̑ext{F}{{̑ext{e}}_{0.3}}˚ight)}_{2}}{{̑ext{B}}_{2}}} with respect to the non superconducting reference compound IMG [http://ej.iop.org/images/0953-8984/28/34/345701/cmaa2a4dieqn008.gif] {̑ext{LaC}{{̑ext{o}}_{2}}{{̑ext{B}}_{2}} . The occurrence of superconductivity is related to the DOS enhancement.
