Electron-phonon coupling and phonon generation in normal-metal microbridges

The second derivative of current—voltage characteristic, $\text{d}{}^2\text{I}\text{d}\text{V}{}^2$, of a small orifice connecting two pieces of normal metals is shown to be proportional to the function $\text{G(ω) =}\text{α}\text{?}{}^2\text{(ω)F(ω)}$ at ω = eV, where F(ω) is the phonon density of states, and α̃² (ω) the square of the electron—phonon matrix element averaged over the Fermi surface and multiplied by the additional structure factor taking into account the geometry of the orifice. The constriction is shown to work, in a current-carrying state, as a source of non-equilibrium phonons emitted in the immediate vicinity of the orifice.