Bulk heavy-fermion compounds

In this chapter we want to focus on the electron-phonon effects in the mixed-valence and heavy-fermion compounds. In the mixed-valence compounds one usually observes an extremely low bulk modulus Cg (table 6). As an example the case of Sm Yi S is given in fig. 44, see Penney and Melcher (1976). In addition, the so-called Poisson ratio v is negative in these compounds (see table 6). Analogously, certain phonon branches exhibit anomalies. A detailed discussion and explanation of these facts and related experiments will be given in sect. 4.5. 

UPt3 is considerably less compressible than the AnBejs compounds studied. This parallels the situation found in the bulk moduli of the Laves-phase intermetallic compounds of uranium (see table 2). In both cases, the intermetallics with a transition metal (Fe, Co, Mn, Ir for the Laves phases, Pt for the heavy-fermion compounds) are considerably stiffer than those incorporating a metal of one of the main groups of the periodic table (Al for the Laves-type compounds, Be for the heavy-fermion compounds). In interpreting compression data of these types of compounds, one should think of a possible stiffening effect linked to the existence of partly filled shells. 

In Yb compounds the phonon broadening was estimated from a measurement of pure Yb metal, which, as we have already seen in fig. 24, displays 4f spectra similar to those of the heavy fermion compounds despite the position of the 4f7/2 peak at —1 eV below E. The temperature dependence of these levels, now due entirely to phonon broadening, is shown in fig. 29. The data were fitted at each temperature with Gaussian broadened Doniach-Sunjic (1970) lineshapes and separated into bulk and surface components. The fits are overlaid in the upper frame, normalized to equal integrated intensities. A substantial apparent temperature dependence is observed even in the absence of the Fermi function cut-off, and in the absence of any spectral weight loss. The 300 K spectra are broadened by 130meV over the 25 K spectra (added as root mean square). 

Another U compound that shows superconductivity and magnetic order is UPtg. Although with y=450 mJ mol it belongs to the real heavy-fermion systems, it is comparable to URu2Si2 in many of its physical properties. The static bulk susceptibility... 

For heavy fermions at low temperatures, > 50 dominates Q and C /T 1 J/mol dominates Cy so this correction can be quite large. For example, for UPts at lOK, where Q = 40, V = 42.4 cm /mol and C =3.25 J/mol K the correction is of order 12kbar, which is a large fraction of the total anomaly in the bulk modulus. (For mixed-valent compounds with To > lOOK this is less of a problem because the thermodynamic Gruneisen parameter is dominated by the phonon term, which is small.) Note that for heavy fermions, when Q Qe and C Cg, we have, by eqs. (8) and (13),... 

Although the bulk properties of 5f-electron heavy fermions are entirely similar to those of 4f-electron heavy fermions (see, for example, Stewart 1984), the respective photoelectron spectra appear substantially different (an excellent review of the early work is given by Allen 1992). Unlike the triple-peaked 4f spectra in Ce compounds (the /° and / states within the SIM interpretation), measurements at the 6d absorption edge where the 5f signal is enhanced, generally yield a rather broad, triangular shaped spectrum... 

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