High-Resolution Photoemission Study ofMgB2

We have performed high-resolution photoemission spectroscopy on ${\mathrm{MgB}}_2$ and observed opening of a superconducting gap with a narrow coherent peak. We found that the superconducting gap is $s$ like with the gap value ( $\Delta$) of $4.5\pm 0.3\mathrm{meV}$ at 15 K. The temperature dependence (15–40 K) of the gap value follows well the BCS form, suggesting that $2\Delta {/k}_B{T}_c$ at $T=0\mathrm{}$ is about 3. No pseudogap behavior is observed in the normal state. The present results strongly suggest that ${\mathrm{MgB}}_2$ is categorized into a phonon-mediated BCS superconductor in the weak-coupling regime.