Cooper-Pair Density Waves
In collaboration with Prof. Dr. Andrew Mackenzie
We recently introduced nanometer resolution Scanned Josephson Tunneling Microscopy (SJTM), a technique allowing imaging of Cooper-pair tunneling from a superconducting STM tip to the Cooper-pair condensate of a superconductor. The SJTM operates at millikelvin temperatures and sequentially forms an array of 65,500 nanoscale Josephson junctions, whose Josephson critical current Ic is then measured to form the condensate image (Nature 532, 343 (2016)). For the first time in superconductivity research, one can visualize the Cooper-pair condensate itself (Fig. 1A).
Fig. 1A Josephson critical current Ic(r) images with atomic-resolution in 75nmX75nm FOV; B) Fourier transform of A shows the existence of 4a0-periodic modulations of Cooper-pair density due to the PDW state; C) Schematic representation of a biaxial d-symmetry PDW
SJTM is a very promising new approach to research into all kinds of heterogeneous superconductivity. Projects of immediate research interest include:
a) The Cooper-pair density wave (PDW) state occurs when the density of Cooper-pairs modulates periodically in space at wavevector QP . Only one instance has ever been detected (Nature 532, 343 (2016)). Now we plan a search for new PDW states in several classes of materials. Transition metal dichalcogenides appear ideal, because they often host both superconductivity (SC) and charge density waves (CDW); Ginzburg-Landau theory predicts that a PDW state must be induced by the interactions between the SC and CDW states. Heavy-fermion superconductors e.g. CeCoIn5 at high fields are also reported to host PDW states. Copper-based high temperature superconductors (CuHTS) materials, e.g. YBa2Cu3O7 and La2BaCuO4, are widely predicted to host a strong-coupling PDW state.
b) In CuHTS, an exceptional new electronic phase appears at highest magnetic fields. It supports unexplained quantum oscillations and an unidentified density wave (DW) state. Although generally referred to as a CDW, theory indicates that this could actually be a PDW state. Because this field-induced DW state is accessible in the “halo” surrounding quantized vortex cores (Science 295, 466 (2002)), we now plan to image this “halo” DW using SJTM to determine directly if it is a PDW.