Coexistence of a charge density wave and superconductivity in the cluster compound K2Mo15Se19

The competition between charge density wave (CDW) and superconductivity is a central theme in condensed-matter physics with ramifications to correlated electron systems and high-temperature superconductivity. While the emergence of superconductivity is often observed upon suppressing the CDW transition by tuning the chemical composition or the external pressure, their coexistence has been reported in only a handful of materials, with the cuprates being the most prominent example. Here, we demonstrate that both cooperative electronic phenomena coexist in the cluster compound K2Mo15Se19 . The CDW transition sets in at T-CDW = 114 K, accompanied by a commensurate periodic modulation of the crystal lattice along the c axis evidenced by electron diffraction. Bulk type-II superconductivity develops upon further cooling below T-c = 2.8 K. The presence of similar signatures of CDW ordering in other A(2)Mo(15)Se(19) compounds shows that this electronic instability may be ubiquitous in these compounds, providing a novel family where the interplay between CDW and superconductivity may be systematically investigated.