Two-Dimensional Superconducting Fluctuations Associated with Charge-Density-Wave Stripes in La 1.87 Sr 0.13 Cu 0.99 Fe 0.01 O 4

The presence of a small concentration of in-plane Fe dopants in ${\mathrm{La}}_{1.87}{\mathrm{Sr}}_{0.13}{\mathrm{Cu}}_{0.99}{\mathrm{Fe}}_{0.01}{\mathrm{O}}_{4}$ is known to enhance stripelike spin and charge density wave (SDW and CDW) order and suppress the superconducting ${T}_{c}$. Here, we show that it also induces highly two-dimensional superconducting correlations that have been argued to be the signatures of a new form of superconducting order, the so-called pair density wave (PDW) order. In addition, using resonant soft x-ray scattering, we find that the two-dimensional superconducting fluctuation is strongly associated with the CDW stripe. In particular, the PDW signature first appears when the correlation length of the CDW stripe grows over eight times the lattice unit ($\ensuremath{\sim}8a$). These results provide critical conditions for the formation of the PDW order.