OBSERVATION OF PHOTON-ASSISTED SHOT-NOISE IN A PHASE-COHERENT CONDUCTOR

We observe a novel signature of coherent transport in the nonequilibri um current fluctuations of a diffusive metallic conductor shorter than the electron phase-breaking length. Under illumination with monochrom atic microwaves, the shot noise develops features at voltages correspo nding to the photon energies, V = nhv/e, which are oscillatory functio ns of the microwave power, while the conductance and the I-V curve are unaffected. The observed effect bears a strong resemblance to photon- assisted tunneling, although this marks the first demonstration in a l inear system, and in a system without an explicit tunnel barrier.