Substrate-dependent quasiparticle recombination time in superconducting resonators

We demonstrate an increased quasiparticle recombination time in superconducting resonators on a SiN(x) membrane, compared to identical resonators on a SiN(x)/Si wafer. An interpretation is given using a thermal model of the membrane. Using an array of tunnel junctions to cool or heat the membrane, we show that the resonators on the membranes are extremely sensitive to small changes of the phonon temperature, which renders them excellent phonon thermometers with a noise level equivalent to 5 mu K/root Hz. The experimental set-up is in principle an ideal platform to study the interplay of quasiparticles and phonon populations in superconductors. (C) 2011 American Institute of Physics. [doi:10.1063/1.3624463]