  An experiment is proposed to stimulate a superconducting thin film with
terahertz (THz) acoustic waves, which is a regime not previously tested. For a
thin film on a piezoelectric substrate, this can be achieved by coupling the
substrate to a tunable coherent THz electromagnetic source. Suggested materials
for initial tests are a niobium film on a quartz substrate, with a BSCCO
intrinsic Josephson junction (IJJ) stack. This will create acoustic standing
waves on the nm scale in the thin film. A properly tuned standing wave will
enable electron diffraction across the Fermi surface, leading to electron
localization perpendicular to the substrate. This is expected to reduce the
effective dimensionality, and enhance the tendency for superconducting order
parallel to the substrate, even well above the superconducting critical
temperature. This enhancement can be observed by measuring the in-plane
critical current and the perpendicular tunneling gap. A similar experiment may
be carried out for a cuprate thin film, although the conduction electrons might
be more responsive to spin waves than to acoustic waves. These experiments
address a novel regime of large momentum transfer to the electrons, which
should be quite distinct from the more traditional regime of large energy
transfer obtained from direct electromagnetic stimulation. The experiments are
also motivated in part by novel theories of the superconducting state involving
dynamic charge-density waves and spin-density waves. Potential device
applications are discussed.
