Heavy-fermion state

HFS heavy-fermion state RRR residual resistance ratio... 

Fig. 12. Total heat capacity of a single crystal of PrFe4Pi2 vs. temperature in various applied magnetic fields (a) low fields and (b) high fields. The dashed fines in (b) correspond to the best fit of the heavy fermion state to die resonant level model (Crlm)-Cph is the estimate of die phonon contribution to the heat capacity (Aoki et al., 2002).

Fig. 13. Magnetic field vs. temperature phase diagram of single crystal PrFe4Pi2 with the magnetic fields applied along the (100) direction. The labels ODS and HFS refer to ordered state and heavy fermion state, respectively. The ordered state is probably due to quadrupolar ordering of the Pr 4f ground state. The solid and broken lines represent second-order and first-order phase boundaries, respectively (Aoki et al., 2002).

At low temperatures, there are at least two ways in which the f electrons, and their magnetic moments, behave either they order spontaneously in ferromagnetic, antiferromagnetic or complicated magnetic structures, or the f electrons can form a heavy fermion state, strongly correlated with the conduction electrons. 

The effect of chemical pressure on YbPtBi single crystals was studied by heat-capacity measurements on yttrium- and lutetium-doped samples (Lacerda et al. 1993). According to these measurements, the heavy-fermion state of this compound seems to be imchangeable by a relatively large amoimt of nonmagnetic doping (yttrium or lutetium). Furthermore, the heat capacity measurements reveal only a small pressure dependence when compared with other heavy-fermion materials. 

The heavy-fermion state with its high density of quasiparticle states and large Griineisen parameters has a profound influence on sound propagation and thermal properties. Here we mention some typical effects (Yoshizawa et al. 1986, Liithi and Yoshizawa 1987, Becker and Fulde 1986, 1987a, b). 

The underlying normal heavy fermion state of UPts, which was the first system where heavy quasiparticles have directly been seen in dHvA oscillations (Taillefer and Lonzarich, 1988), has been described above. It was argued that the picture of heavy quasiparticle mass generation in 5f-metals has to be revised. This is due to a considerably different degree of localisation of 5f-electrons in different orbital states as opposed to the simple LDA picture which assumes complete delocalisation for all 5f-orbitals. 

The UCu4fiAl8 i system has been obtained in the form of amorphous thin film. From ac resistivity measurements it follows that compared to the results in the crystalline bulk alloys, the onset of magnetic order is suppressed at low Cu concentrations, while the onset of a coherent heavy-fermion state is suppressed at high x. The system reveals a single-ion Kondo behavior down to the lowest temperatures, but significant deviations were detected from the behavior of dipolar Kondo system (Lunkenheimer et al. 1994). 

However, later on Cox (1987) suggested that nuclear resonance is a more effective method to probe the heavy-fermion state than electron-spin resonance, simply because the probe nuclei usually is closer to the heavy-electron sites so that the strong range... 

We have ended this review with heavy-fermion systems. In fact, in terms of their hybridization, they lie intermediate between the materials discussed in sect. 4, and those discussed in sect. 5. What is extraordinaiy about the heavy-fermion compounds, and has been responsible for the great interest in them, has been the formation of a correlated state at low temperature. That the superconductivity appearing in some of these systems is intimately connected, if not driven by, the magnetic interactions is beyond doubt, although a complete theory is still lacking. The heavy-fermion state involves both the localized-like f electrons and the conduction-electron states. For example, extremely laige masses are seen in the de Haas-van Alphen experiments (Reinders et al. 1986, Taillefer and Lonzarich 1988), but these do not account for all the susceptibility a large... 

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