Energy Gap of SuperconductingNb3Sn

The superconducting energy gap $2\Delta$ of vapor-deposited ${\mathrm{Nb}}_3$Sn has been determined by far-infrared reflectivity measurements. The energy gap of virgin (as-deposited) ${\mathrm{Nb}}_3$Sn was $(3.77\mathrm{}\pm 0.16)k_B{T}_c$. After the same sample was mechanically polished and thereby disturbed or damaged, a gap of $1.9k_B{T}_c$ was observed. The virgin-state energy gap was restored by etching away the damaged surface. We conclude that the bulk energy gap of ${\mathrm{Nb}}_3$Sn is $(3.77\mathrm{}\pm 0.16)k_B{T}_c$. The smaller energy gap is discussed in terms of surface effects and the results of other measurements.