Mixed-valence-driven heavy-fermion behavior and superconductivity in KNi2Se2

Based on specific heat and magnetoresistance measurements, we report that a "heavy" electronic state exists below T approximate to 20 K in KNi2Se2, with an increased carrier mobility and enhanced effective electronic band mass, m* = 6m(b) to 18m(b). This heavy state evolves into superconductivity at T-c = 0.80(1) K. These properties resemble that of a many-body heavy-fermion state, which derives from the hybridization between localized magnetic states and conduction electrons. Yet, no evidence for localized magnetism or magnetic order is found in KNi2Se2 from magnetization measurements or neutron diffraction. Instead, neutron pair-distribution-function analysis reveals the presence of local charge-density-wave distortions that disappear on cooling, an effect opposite to what is typically observed, suggesting that the low-temperature electronic state of KNi2Se2 arises from cooperative Coulomb interactions and proximity to, but avoidance of, charge order.