New superconducting scanning phonon spectroscopy

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Abstract
A new phonon-detection technique utilizing superconducting tunnel junctions is introduced. In this technique the high-frequency phonons emitted from a generator are detected by a superconducting sensor placed apart from it by a thin layer (≲100 μm) of liquid helium. The sensor is either a single-tunnel junction or a double-tunnel junction. The phonon signal increases nonlinearly with an increase of generator current. In the case that a double-tunnel-junction sensor is used, the information on the energy spectrum of the transmitted phonons is obtained by varying the detectable phonon cutoff energy by quasiparticle injection. For the phonons emitted from a superconducting tunnel junction, a clear change at the phonon cutoff energy equal to the generator gap is observed. The spatial scanning spectroscopy with a spatial resolution of 20 μm is performed by moving a sensor relative to the generator. The observed phonon spatial distribution has a periodically modulated structure for the superconducting tunnel generator accompanying a diffusive instability. The limits and applications of this technique are also discussed.