Low-Energy Magnetic Excitations in the Antiferromagnetically Ordered State of an Incipient Heavy-Electron Compound UP as Studied by ³¹P NMR

The microscopic magnetic properties of UP with the cubic NaCl-type structure have been investigated by 31P NMR in the magnetically ordered state. 1 /T1 is extraordinarily large in the single-g type-I phase (T0 ≅& 22.5 K< T< TN≅ 122 K), and shows about two orders of magnitude abrupt decrease through the single-Q ↔ double-Q phase transition at T0. Below about 4K in the double-Q type-I phase (T<T0), 1/T1 shows a Korringa-like temperature-dependence, which corresponds qualitatively to a 5f-derived γ term in the specific heat. We discuss this behavior as a clear indication of the existence of 5f-derived very-low-energy magnetic excitations even in the ordered state, and an incipient heavy-electron character of the compound is concluded. We also present a simple qualitative model of a system of exchange-coupled Kondo impurities under internal exchange (or molecular) field in order to discuss the origin of these excitations.