Phonon structure in the tunneling conductance of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$

Clear phonon structures were observed in the tunneling conductance of a ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$-GaAs junction. The spectral function of the electron-phonon interaction gives a value of 87.3 K for the critical temperature and 22.2 meV for half a gap. There is no particularly large phonon structure, and the high ${\mathit{T}}_{\mathit{c}}$ cannot be attributed to a particular phonon mode in the electron-phonon mechanism. The gap edge structure is sharp, and an s-wave state is probable. However, if the angular distribution of the tunneling current is highly anisotropic we cannot definitely exclude a d-wave state.