Visualizing Electron-Pair Crystals in a Conducting Quantum Spin Liquid

A quantum spin liquid (QSL) state arises when magnetic ordering of strongly interacting quantum spins is overwhelmed by quantum fluctuations. QSLs should exhibit long-range quantum entanglement and fractionalized excitations, but lack spontaneously broken symmetries. Thus, Anderson’s pioneering theory that high temperature superconductors are electrically conducting (doped) QSLs, was thrown into doubt by the widespread discovery of their broken-symmetry states. Eventually, however, theorists realized that there was one hypothetical broken-symmetry state which could occur naturally in a doped QSL: the electron-pair density wave (PDW). This is a quantum crystal of electron-pairs whose density modulates periodically at wavevector QP. To search for a PDW state in cuprate superconductors we developed an atomic resolution Scanned Josephson Tunneling Microscope (SJTM). This quantum microscope images, not the conventional single-electron quasiparticles of metals and superconductors but the quantum coherent state of electron-pairs. By using SJTM we discovered the cuprate PDW state1. It exhibits periodic modulations at wave vector QP of the electron-pair density1, of the quasiparticle response to the electron-pair crystal2, and of the associated electron-pair binding energy3. Increasingly sophisticated SJTM techniques now reveal the atomic-scale interplay of the PDW state with other electronic states such as superconductivity2,4 and charge density wave4. Ultimately, if the remaining unidentified state (pseudogap) in the phase diagram of cuprate high temperature superconductors is a PDW, their final mystery will have been solved.
[1] Nature 532, 343 (2016)
[2] Science 364, 976 (2019)
[3] Nature 580, 6570 (2020)
[4] arXiv 2007.15228