Charge density wave, superconductivity and broken inversion symmetry at the surface of 2H-NbSe₂

A pair density wave (PDW) state may emerge when superconductivity develops in the charge density wave (CDW) system. A recent experiment using Josephson-current scanning tunneling microscopy (STM) has discovered a PDW state in 2H-NbSe₂, where a 3×3 CDW order coexists with superconductivity [1]. Notably, the phases of the PDW and CDW modulations differ by 2π/3. In this study, we aim to understand the origin of the phase difference by examining the details of the superconducting gap using ultralow-temperature STM. We have identified at least three characteristic energies in the superconducting-gap spectrum, which may indicate the multiband and anisotropic nature of the superconducting gap. While these energies are spatially homogeneous, their spectral weights display spatial modulations with a 2π/3 phase difference with respect to the CDW, as in the case of PDW. We interpret the phase difference as a result of the intra-CDW-unit-cell modulation of the superconducting properties influenced by the broken in-plane space inversion symmetry at the surface.

[1] X. Liu et al., Science 372, 1447 (2021).