Electron-phonon coupling induced pseudogap and the superconducting transition in ${\mathrm{Ba}}_{0.67}{\mathrm{K}}_{0.33}{\mathrm{BiO}}_3$

We study the single-particle density of states (DOS) across the superconducting transition ${(T}_c=31\mathrm{}\mathrm{K})$ in single-crystal ${\mathrm{Ba}}_{0.67}{\mathrm{K}}_{0.33}{\mathrm{BiO}}_3$ using ultrahigh resolution angle-integrated photoemission spectroscopy. The superconducting gap opens with a pileup in the DOS, $\Delta \left(5.3\mathrm{}\mathrm{K}\right)=5.2\mathrm{}\mathrm{meV}$ and $2\Delta {\mathrm{}\left(0\right)/k}_B{T}_c=\mathrm{3.9.}\mathrm{}$ In addition, we observe a pseudogap below and above $T_c,$ occurring as a suppression in intensity over an energy scale up to the breathing mode phonon (∼70 meV). The results indicate electron-phonon coupling induces a pseudogap in ${\mathrm{Ba}}_{0.67}{\mathrm{K}}_{0.33}{\mathrm{BiO}}_3.$