Charge density wave fluctuations, heavy electrons, and superconductivity in KNi2S2

Understanding the complexities of electronic and magnetic ground states in solids is one of the main goals of solid-state physics. Materials with the canonical ThCr₂Si₂-type structure have proved particularly fruitful in this regard, as they exhibit a wide range of technologically advantageous physical properties described by “many-body physics,” including high-temperature superconductivity and heavy fermion behavior. Here, using high-resolution synchrotron x-ray diffraction and time-of-flight neutron scattering, we show that the isostructural mixed valence compound KNi₂S₂ displays a number of highly unusual structural transitions, most notably the presence of charge density wave fluctuations that disappear on cooling. This behavior occurs without magnetic or charge order, in contrast to expectations based on other known materials exhibiting related phenomena. Furthermore, the low-temperature electronic state of KNi₂S₂ is found to exhibit many characteristics of heavy-fermion behavior, including a heavy electron state (m^*/m_e∼ 24), with a negative coefficient of thermal expansion, and superconductivity below T_c=0.46(2) K. In the potassium nickel sulfide, these behaviors arise in the absence of localized magnetism, and instead appear to originate in proximity to charge order.