Evidence for a Multiple Superconducting Gap in ${\mathrm{MgB}}_2$ from High-Resolution Photoemission Spectroscopy

We study the new binary intermetallic superconductor ${\mathrm{MgB}}_2$ using high-resolution photoemission spectroscopy. The superconducting-state spectrum measured at 5.4 K shows a coherent peak with a shoulder structure, in sharp contrast to that expected from a simple isotropic-gap opening. The spectrum can be well reproduced using the weighted sum of two Dynes functions with the gap sizes of 1.7 and 5.6 meV. Temperature-dependent study shows that both gaps close at the bulk transition temperature. These results provide spectroscopic evidence for a multiple gap of ${\mathrm{MgB}}_2$.