Non BCS-like superfluid density in a disordered charge density wave material: Pd-intercalated ErTe₃

The rare-earth tri-tellurides RTe₃ (R=La-Pr, Sm, Gd-Tm) are quasi-2D metals that show unidirectional incommensurate charge density wave (CDW) states. Pd intercalation introduces significant disorder to the crystal lattice, suppressing the CDW formation and leading to a superconducting ground state. The material presents an opportunity to explore in detail the interplay between superconductivity (SC) and CDW formation in the presence of disorder. Here we measured the superfluid response as a function of Pd concentration in Pd_xErTe₃ (0<x<0.06) by using a scanning SQUID microscope with sub-micron spatial resolution. We observed an anomalous increase of superfluid density near the critical temperature, which cannot be explained by the Bardeen-Cooper-Schrieffer (BCS) theory. We also quantify the inhomogeneity of the emergent superconductivity as the CDW is progressively disordered by the Pd intercalation. Our results clarify how the superconducting state emerges from the disordered CDW.