Too cool for blackbody radiation: Overbias photon emission in ambient STM due to multielectron processes

Light emission from tunnel junctions are a potential photon source for nanophotonic applications. Surprisingly, the photons emitted can have energies exceeding the energy supplied to the electrons by the bias. Three mechanisms for generating these so-called overbias photons have been proposed, but the relationship between these mechanisms has not been clarified. In this work, we argue that multielectron processes provide the best framework for understanding overbias light emission in tunnel junctions. Experimentally, we demonstrate for the first time that the superlinear dependence of emission on conductance predicted by this theory is robust to the temperature of the tunnel junction, indicating that tunnel junctions are a promising candidate for electrically driven broadband photon sources.