A research team, headed by Fangsen Li from the Nano Vacuum Interconnection Experimental Station at the Suzhou Institute of Nano-tech and Nano-bionics (SINANO), Chinese Academy of Sciences, joined forces with research teams led by Canli Song, Xucun Ma, and Qi-Kun Xue from the Department of Physics at Tsinghua University, as well as the research group helmed by Ziqiang Wang from Boston College. They employed advanced equipment, including vacuum-interconnected extremely low-temperature scanning tunneling microscopes and molecular beam epitaxy systems, to directly observe, for the first time ever, the competitive-coupled coexistence of sublattice Cooper pair density waves (PDW) and charge density waves (CDW) in single-layer 1T′-MoTe₂ superconducting thin films.

The study revealed that PDW displays a triple lattice period structure along the direction of Mo atomic chains. In the perpendicular direction, it exhibits spatial modulation with a half-unit cell period. This represents the smallest spatially periodic pairing modulation that can be experimentally resolved to date.

Further research demonstrated that the formation of biaxial pairing density waves is closely tied to finite momentum pairing in multiband systems. Their modulation amplitude gradually diminishes as the temperature rises or an applied magnetic field increases, which is in line with the characteristics of pairing density waves. Significantly, the PDW wave vector is markedly different from the CDW wave vector, uncovering the intrinsic properties of biaxial pairing density waves.

This remarkable achievement fills a void in research on the microscopic mechanisms of PDW. It establishes a conceptual bridge between 1T′-MoTe₂ and strongly correlated superconductors like iron chalcogenides. Moreover, it offers new experimental evidence and a physical perspective for a more in-depth understanding of pairing density waves and their microscopic mechanisms in unconventional superconductors. The research findings were published in Physical Review Letters.