Thermal transport measurements have been made on the Fe-based superconductor ${\mathrm{Lu}}_2{\mathrm{Fe}}_3{\mathrm{Si}}_5$ ($T_c\sim 6\mathrm{K}$) down to a very low temperature $T_c/120$. The field and temperature dependences of the thermal conductivity confirm the multigap superconductivity with fully opened gaps on the whole Fermi surfaces. In comparison to ${\mathrm{MgB}}_2$, ${\mathrm{Lu}}_2{\mathrm{Fe}}_3{\mathrm{Si}}_5$ reveals a remarkably enhanced quasiparticle heat conduction in the mixed state. The results can be interpreted as a consequence of the unequal weight of the Fe $3d$-electron character among the distinct bands.

• Received 13 September 2010

DOI:https://doi.org/10.1103/PhysRevLett.106.107002