Superconductors are quantum fluids and pair density waves (PDW) are quantum crystals. Both are macroscopic quantum states of coherently condensed electron pairs. To visualize and explore these states directly at atomic scale, scanned Josephson tunneling microscopes (SJTM) are now used. Such instruments can image both the single-electron quasiparticles and, in a different mode, the quantum condensate of electron-pairs. Moreover, these instruments can, in the Andreev quasiparticle retroreflection regime, be operated as scanned Andreev tunneling microscopes (SATM). Emerging research areas include: visualization of PDW states in conventional spin-singlet superconductors; in cuprate d-wave high-temperature superconductors using d-wave scan tips; and in putative spin-triplet topological superconductors. PDW studies using SJTM and SATM thus open a rich new vein of macroscopic quantum physics, with great potential for discovery.
Wang Lab of Quantum Matter 