Unusually large $^{12}$C/$^{13}$C carbon isotope effect in the quasi one-dimensional superconductor Sc$_3$CoC$_4$

The substitution series of the quasi one-dimensional superconductor Sc$_3$Co($^{12}$C$_{1-x}$$^{13}$C$_x$)$_4$ with $x$ = 0 $-$ 1 was synthesized by arc melting. The sample homogeneity is confirmed by powder X-ray diffraction and $^{13}$C solid-state NMR spectroscopy. An unusually large $^{12}$C/$^{13}$C carbon isotope shift of the superconducting onset temperature, $T_{\rm c}^{\rm onset}$, is observed in the temperature dependent volume susceptibility, ${\chi}$($T$), and the electrical resistivity, ${\rho}$($T$), of poly (pc)- and single crystalline (sc) samples. The obtained isotope coefficients, ${\alpha_{\chi,pc}}$ = 6.2(7), ${\alpha_{\rho,pc}}$ = 7.5(6) and ${\alpha_{\rho,sc}}$ = 7.0(6), represent the highest positive values observed so far.