Heavy-fermion behaviour

The NpT2X2 compounds are usually treated as materials with localized 5f states, and crystalline electric-field effects are held responsible for the depression of the magnetic moments (2.4/tB is expected for the free-ion moment of Np). However, the heavy-fermion behaviour (found in NpCu2Si2) is able to introduce an instability of the localized 5f states. In this sense the reduction in hyperfine field at the Np nucleus could be understood as being due to a partial loss of the orbital moments resulting from 5f-electron delocalization. 

The dihydride of Yb (like that of Eu) is non-metallic, since the Yb (and the Eu) ion is found in its divalent form. For higher hydrides, YbH2+j with x 0.4, fluctuating valencies seem to prevail leading to heavy-fermion behaviour and high yei-values (Drulis et al. 

In the case of Cej Y Pd2Si2 (O x l) system the results of heat capacity, resistivity and susceptibility measurements (Besnus et al. 1987) show that a partial substitution of Ce by Y leads to a rapid decrease in the Neel temperature. Spontaneous magnetism and the Kondo effect coexist up to y = 0.4 (T = 3 K) where is estimated as 3 For higher Ce dilution the magnetic moment of the ground state doublet has become completely quenched and a genuine heavy fermion behaviour is evident, for example, y = 0.875 J/K Ce atom and = 10 K at Y = 0.5 and y = 0.58 J/K Ce atom and = 15 K at y = 0.75. 

An evolution of a heavy-fermion behaviour in CeCUjSij from strong mixed valence in CeNijSij is demonstrated in the CeCu2, Ni Si2 system. Heavy-fermion behaviour, as probed by the low-temperature susceptibility, y(0) is destroyed at X = 0.65 (Sampathkumaran and Vijayaraghavan 1986). 

Historically, the first and most frequent experiments performed on lanthanides and actinides and their compounds have been measurements of their magnetic properties (magnetochemistry). The magnetic properties of the deep-lying (isolated) 4f and 5f shells were first thought to be easily interpretable. However, as we shall see, many complications are going to be encountered, such as complex structures, mixed valencies and heavy-fermion behaviour. 

The susceptibility measurements of Ott et al. (1982) are plotted in fig. 80. Ott et al. (1982) were the first authors to realize that at room temperature Yb behaves like a trivalent ion, but at low temperatures order did not set in. They concluded that the Yb pnictides are heavy-fermion materials, a break-through was the detection of order (at very low temperatures) by specific-heat measurements (Ott et al. 1985). Mossbauer experiments by Bonville et al. (1987) confirmed the heavy-fermion behaviour of YbP below 0.41 K. The ordered moment was estimated to be 0.8/tb, even lower than the moment of the crystalline electric field Tg ground state, which yields a moment of 1.33/xb- The moment depression is attributed to hybridization. Order was confirmed by NMR measurements (Takagi et al. 1988). 

In the RM3 and AnM3 intermetallics, where M is Sn or In, intermediate-valence and/or heavy-fermion behaviour is realized. 

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