Exploration of superconductivity in light element compounds has drawn considerable attention because those materials can easily realize the high $T_{ c}$ superconductivity, such as ${ \mathrm{ LnNi}}_{ 2}{ \mathrm{ B}_{ 2}}{ \mathrm{ C}}$ ($T_{ c}$ =17 K), ${ \mathrm{ Mg}}{ \mathrm{ B}}_{ 2}$ ($T_{ c}$ =39 K), and very recently super-hydrides under pressure ($T_{ c}$ =250 K). Here we report the discovery of bulk superconductivity at 7.8 K in scandium borocarbide ${ \mathrm{ Sc}}_{ 20}{ \mathrm{ B}}{ \mathrm{ C}}_{ 27}$ with a tetragonal lattice which structure changes based on the compound of ${ \mathrm{ Sc}}_{ 3}{ \mathrm{ C}}_{ 4}$ with very little B doping. Magnetization and specific heat measurements show bulk superconductivity. An upper critical field of Hc2(0) ~ 8 T is determined. Low temperature specific-heat shows that this system is a BCS fully gapped s-wave superconductor. Electronic structure calculations demonstrate that compared with ${ \mathrm{ Sc}}_{ 3}{ \mathrm{ C}}_{ 4}$ there are more orbital overlap and hybridization between Sc 3d electrons and 2p electrons of C-C(B)-C fragment in ${ \mathrm{ Sc}}_{ 20}{ \mathrm{ B}}{ \mathrm{ C}}_{ 27}$, which form a new electric conduction path of Sc-C(B)-Sc. Those changes influence the band structure at the Fermi level and may be the reason of superconductivity in ${ \mathrm{ Sc}}_{ 20}{ \mathrm{ B}}{ \mathrm{ C}}_{ 27}$.
Categories: cond-mat.supr-con