Zero-Bias Tunnel-Conductance Minima Due to the Excitation of Collective Modes in the Barrier

We evaluate the contribution to the tunnel conductance due to inelastic tunneling processes whereby the tunneling electron excites continuum modes in the barrier. For acoustical dispersion relations ω_→p=vₛp+ω₀ of the continuum modes, the coherent inelastic contribution is usually proportional to ${(|eV|{-}{}{{ω}}₀)}²{θ}(|eV|{-}{}{{ω}}₀){θ}({}{{ω}}D{-}|eV|)$, in which ${{ω}}D={v}ₛ{p}ₘₐₓ+{{ω}}₀$. Comparison with experimental data suggests that certain zero-bias minima may be attributed to phonon excitation via inelastic tunneling.