Heavy-fermion compounds

In a heavy fermion compound Yb MnSbn, the dephasing rate of the coherent optical phonons decreased with lowering temperature above Curie temperature Tc, but increased below Tc- The results were attributed to the coupling between an optical phonon mode and the Kondo effect [100]. 

The Hall effect has been studied only for some borocarbides. The normal state Hall coefficients / h were found to be negative and only weakly temperature dependent for polycrystalline borocarbides based on R = Y (Fisher et al. 1995 Narozhnyi et al. 1996 Mandal and Winzer 1997), La (Fisher et al. 1995), Ho (Fisher et al. 1995 Mandal and Winzer 1997) and Gd (Mandal and Winzer 1997). A negative but strongly temperature dependent R was found for the heavy-fermion compound YbNi2B2C (Narozhnyi et al. 1999b). 

The pressure-dependent electrical resistivity of the heavy-fermion compound YbNi2B2C (see also Section 4.12) could be explained by competing contributions from crystal-electric-field splitting and Kondo effect (Oomi et al., 2006). The pressure-dependent room-temperature thermoelectric power of YNi2B2C exhibits a peak around 2 GPa, which was explained by changes in the Fermi-surface topology (Meenakshi et al., 1998). A possible correlation with a small peak in the temperature-dependent thermopower around 200 K (Fisher et al., 1995 Section 3.4.3) needs further investigation. 

Keywords quantum critical phenomena, heavy-fermion compound, U(Pt,Pd)3 system... 

UPt3 belongs to the class of uranium-based heavy-fermion compounds. That classification is due to the large value for y in the expression for the specific heat at low temperature ... 

For the compound UPt3, the value for y amounts to 420 mJ/mol K2, whereas the values for A and X depend on the direction in which the resistivity and the susceptibility have been measured, although these results do not differ more than roughly a factor of two. To illustrate these findings, the specific heat of an artificial heavy-fermion compound is shown in fig. 1 in a plot of c/T versus T2 and compared with the result for, again, an artificial normal metal. For the effective Fermi velocity, vF, one deduces values of order 5x103 m/sec and for the effective Fermi temperature values between 10 and 100 K. 

In the resistivity curve for x = 0.05, the Cr-type of anomaly, pointing to the possible presence of a spin density wave below 5.8. K, is clearly visible. At higher x-values, the resistivity increases towards lower temperatures, with, for the x = 0.15 compound, a maximum around 5 K which is, with reference to other heavy-fermion compounds like, for instance, UBei3, considered to be caused by coherence effects. 

Likewise the Hubbard model the periodic Anderson model (PAM) is a basic model in the theory of strongly correlated electron systems. It is destined for the description of the transition metals, lanthanides, actinides and their compositions including the heavy-fermion compounds. The model consists of two groups of electrons itinerant and localized ones (s and d electrons), the hybridization between them is admitted. The model is described by the following parameters the width of the s-electron band W, the energy of the atomic level e, the on-site Coulomb repulsion U of d-electrons with opposite spins, the parameter V of the... 

This is drastically different for heavy Fermion compounds where extreme enhancement factors are known. Therefore, the organic superconductors were compared and related to this latter unique class of metals [284]. 

In the meantime some new routes towards high temperature and possibly exotic superconductivity have been investigated. This is the case for the heavy fermion compounds in which the close interplay between local magnetic moments and the spin of delocalized electrons has led to the possibility of a nonphonon mediated mechanism for electron pairing [4]. A very successful route towards high-T, s has been followed with conducting layered cuprates after the discovery of superconductivity in (La, Sr)2Cu04 [5]. 

The relativistic LMTO and LAPW methods were used to calculate [77-80] the Fermi surface of UPta. This is a heavy fermion compound, and its physical properties axe strongly influenced the presence of the narrow U-/ bands at the Fermi level. The shape of the Fermi surface is then sensitive to relativistic effects, in particular the SO-coupling. The results of the calculations [78] were surprising since they showed that the topology of the Fermi surface was well described by these band structures although they were obtained within the LDA. A similar precision was not found for the effective cyclotron masses which were off by up to a factor of 30 when compared to experiments. The crystal potential enters in the LMTO via the potential parameters [30,73] for each (or each j in the relativistic version [4]), including the mass parameters fi (eq.(49)). A convenient way... 

An even broader response than found in U heavy-fermion compounds was observed in the spin-fluctuator UA12, where a significant intensity extends to energies above 100 meV. The residual line width was deduced to be equal to 25 meV (Loong et al. 1986). 

Fig. 2.3. A and y2 relationship for typical heavy-fermion compounds, including semi-heavy fermion compounds (Kadowaki and Woods 1986).

From C(T) measurements a value of y = 167 mJ/mol K2 was obtained after extrapolation from temperatures below the magnetic transition. The y value that can approximately be derived from the data above is even substantially higher and of the order of 300 mJ/mol K2 (Sechovsky et al. 1986a,b, 1988a, Palstra et al. 1987) and classifies UNiAl as a middle weight heavy-fermion compound. 

The compounds that do not order magnetically are represented by a triangle (7 ) on a logarithmic scale because of their wide range of values (from a few degrees for the heavy-fermion compounds to a thousand for the intermediate-valence compounds). The Greek letters a, P and y in fig. 1 represent the respective allotropic phases of Ce metal for comparison. 

IV) Within this region are included all the Ce-X compounds that do not order magnetically - heavy-fermion compounds and intermediate-valence compounds. 

In fig. 19 we show an example of an angularly dependent Knight shift. The sample is a single crystal of paramagnetic CeBe (a heavy-fermion compound) which has a... 

The value of C T for YbNiSb extrapolated to OK is 150mJ/molK. Together with the high value of the specific resistivity, YbNiSb may be classified as a low-carrier heavy-fermion compound (Dhar et al. 1993). 

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