Temperature-Induced Change in the Magnitude of the Effective Density of States: A NQR/NMR Study of the A-Site-Ordered Perovskite System CaCu₃Ru₄O₁₂

Abstract

NQR/NMR measurements, including spin–lattice relaxation rate (1/ T 1 ) and Knight shift ( K ) evaluations, have been carried out for Cu and Ru nuclear spins in the A -site ordered perovskite system CaCu 3 Ru 4 O 12 , which exhibits a mass-enhancement-like behavior although it possesses no f electrons. The temperature evolution of T 1 for Cu, which was measured up to 700 K, exhibits 1/ T 1 T = constant behavior at high enough temperatures. This clearly denies the existence of a Cu localized moment, which was previously suggested as the origin of the mass enhancement in a dense Kondo scenario. Furthermore, K and (1/ T 1 T ) for Cu exhibit significant temperature dependences with two characteristic temperatures T F =20 K and T X =180 K, suggesting a temperature-induced transition of microscopic electronic character at T F and T X . We propose a two–band model, where the effective density of states for one of the bands shows a significant temperature dependence in T F < T < T X . Possible origins are discussed.