  In his seminal work published in Acta Physica Sinica in 1965, Yu Lu pointed
out that the superconducting gap exhibits weak modulations near the
pair-breaking magnetic impurity in a superconductor. In the past ten years, a
series of high-resolution scanning tunneling microscopy works reported weak
superconducting gap modulations in certain superconductors and explained these
phenomena as pair density waves. In line with Yu's discovery, Lee DH et al.
pointed out that in many cases, the interference effect of pair-breaking
scattering can also lead to superconducting gap modulations in space. We will
discuss the distinction and unification of these two kinds of mechanisms, as
well as their relevance to recent experimental observations.
