Phonon Cooling of Nanomechanical Beams with Tunnel Junctions

We demonstrate electronic cooling of 1D phonon modes in suspended nanowires for the first time, using normal-metal-insulator-superconductor (N-I-S) tunnel junctions. Simultaneous cooling of both electrons and phonons to a common temperature was achieved. In comparison with nonsuspended devices, better cooling performance is achieved in the whole operating range of bath temperatures between 0.1-0.7 K. The observed low-temperature thermal transport characteristics are consistent with scattering of ballistic phonons at the nanowire-bulk contact as being the mechanism limiting thermal transport. At the lowest bath temperature of the experiment similar to 100 mK, both phonons and electrons in the beam were cooled down to 42 mK, which is below the refrigerator base temperature.