Dynamic Stimulation of Superconductivity With Resonant Terahertz Ultrasonic Waves

Abstract

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.