Multiple temperature scales of the periodic Anderson model: Slave boson approach

The thermodynamic and transport properties of intermetallic compounds with Ce, Eu, and Yb ions are discussed using the periodic Anderson model with an infinite correlation between f electrons. At high temperatures, these systems exhibit typical features that can be understood in terms of a single-impurity Anderson or Kondo model with Kondo scale T-K. At low temperatures, one often finds a normal state governed by the Fermi liquid (FL) laws with characteristic energy scale T-0. The slave boson solution of the periodic model shows that T-0 and T-K depend not only on the degeneracy and the splitting of the f states, the number of c and f electrons, and their coupling but also on the shape of the conduction-electrons density of states (c DOS) in the vicinity of the chemical potential mu. The ratio T-0/T-K depends on the details of the band structure which makes the crossover between the high- and low-temperature regimes system dependent. We show that the c DOS with a sharp peak close to mu yields T-0 < T-K, which explains the "slow crossover" observed in YbAl3 or YbMgCu4. The c DOS with a minimum or a pseudogap close to mu yields T-0 > T-K; this leads to an abrupt transition between the high- and low-temperature regimes, as found in YbInCu4-like systems. In the case of CeCu2Ge2 and CeCu2Si2, where T-0 similar or equal to T-K, we show that the pressure dependence of the T-2 coefficient of the electrical resistance, A=rho(T)/T-2, and the residual resistance are driven by the change in the degeneracy of the f states. The FL laws obtained for T < T-0 explain the correlation between the specific-heat coefficient gamma=C-V/T and the thermopower slope alpha(T)/T or between gamma and the resistivity coefficient A. The FL laws also show that the Kadowaki-Woods ratio, R-KW=A/gamma(2), and the ratio q=lim({T -> 0})alpha/gamma T assumes nonuniversal values due to different low-temperature degeneracies of various systems. The correlation effects can invalidate the Wiedemann-Franz law and lead to an enhancement of the thermoelectric figure of merit. They can also enhance (or reduce) the low-temperature response of the periodic Anderson model with respect to the predictions of a single-impurity model with the same high-temperature behavior as the periodic one.