The competition between ground states is a central topic in modern condensed matter physics. It is of common belief that unconventional superconductivity (SC) emerges from a precursor state, such as pseudogap or charge order, which is suppressed upon the appearance of the superconducting condensate.
Since the discovery of a Charge Density Wave (CDW) modulation (1976) and Cu doping induced SC (2006) in 1T-TiSe2, this system has become prototypical for studying the interplay between these two states by means of Scanning Tunneling Microscopy and Spectroscopy (STM/STS).
While the SC in 1T-CuxTiSe2 seems to follow a standard BCS s-wave behaviour, the microscopic origin of the CDW is still under debate. In order to address this issue, we performed a detailed STM/STS study of the impact of Cu intercalation on the CDW in 1T-CuxTiSe2 .
Density Functional Theory modelling allowed us to identify Cu atoms, which are found to intercalate randomly on the octahedral site in the van der Waals gap. Tunneling spectroscopy shows Cu to shift the chemical potential and dope delocalized electrons near the Fermi level. The CDW modulation is found to break up in a complicated energy and position-dependent domain structure, which can be explained in terms of charge inhomogeneities induced by Cu intercalation. Under certain conditions, the CDW is visible and well developed in all the crystals investigated, including the superconducting ones, suggesting a possible coexistence of SC and CDW order. These findings further invalidate both Fermi surface nesting and excitonic pairing as the primary CDW formation mechanism in this material.
 A.M. Novello et al. Phys. Rev. Lett. 118, 017002