Bulk Superconductivity at 10 K in the Layered Compounds $Y_2{C}_2{I}_2$ and $Y_2{C}_2{\mathrm{Br}}_2$

Resistivity, susceptibility, and specific heat measurements on the new layered superconductors ${\mathrm{Y}}_2{\mathrm{C}}_2{\mathrm{I}}_2$ and ${\mathrm{Y}}_2{\mathrm{C}}_2{\mathrm{Br}}_2$ have been performed to characterize their superconducting state. Our experiments indicate bulk superconductivity with $T_c=9.97\mathrm{}\mathrm{K}$ for ${\mathrm{Y}}_2{\mathrm{C}}_2{\mathrm{I}}_2$ and $T_c=5.04\mathrm{}\mathrm{K}$ for ${\mathrm{Y}}_2{\mathrm{C}}_2{\mathrm{Br}}_2$. An analysis of the specific heat anomalies at $T_c$ in terms of the weak coupling BCS and the two-fluid model, as well as estimates based on McMillan's formula, reveals strong electron-phonon coupling in the present systems, in contrast to the related three-dimensional superconductor ${\mathrm{YC}}_2$ with $T_c\approx 4.0\mathrm{K}$.