Kurashiki City College

It is shown that collective modes can emerge in the Kondo insulator which is described as an ordered state of conduction-f electron-hole (c̅-f) pair fields in the sym. case of the Kondo lattice model (KLM).The one-loop approximation is adopted in a functional integral formalism similar to that developed by Popov for the s-state superfluid Fermi system (SFS).The massless Nambu-Goldstone (NG) mode due to the phase fluctuation of c̅-f pair fields possesses a quadric dispersion, differently from a linear one for the SFS, and the massive Higgs mode due to the amplitude fluctuation has a gap 2Δ, twice the fermionic gap, similarly to the SFS.Because of the quadric dispersion of the NG mode, similar to the ferromagnetic spin wave mode, the sp. heat of the KLM is expected not to be an activation-type but to exhibit a T^3/2-dependence, which may be confirmed exptl.

We develop a generalized Ginzburg-Landau-Wilson theory of the Kondo lattice, taking account of dynamical quantum fluctuations of Kondo bosons by means of the functional integral method with the one-loop and mean mode-mode coupling approximationsWe consider the disorder state of Kondo bosons, where the thermodn. quantities exhibit relevant behaviors in a wide temperature (T) range.The dynamical quantum fluctuations cancel the static fluctuations, thereby eliminating the redundant Dulong-Petit term of the sp. heat existed in the static approximation (SA), thereby the entropy tending to Nk_B ln 2 for T → ∞ appropriately.The local moments of f electrons are formed asymptotically with increasing T, somewhat faster than for the SA and for the impurity Kondo system.The elec. resistivity of conduction electrons is shown to exhibit a behavior similar to the impurity Kondo system at high Ts.

We investigate the characteristics of itinerant to localized transition of f electrons in heavy fermion systems in terms of the generalized Ginzburg-Landau (GL) theory based on the Kondo lattice model, in which the static thermal fluctuations of Kondo bosons are taken into account by the mean mode-mode coupling approximationFor a finite gradient term in the GL expansion corresponding with heavy fermions, we obtain a first-order phase transition from the mean-field heavy-fermion state at low temperatures (T) and low magnetic fields (H) to the asymptotically free local moment regime at high T and high H.The asymptotic freedom of the local moments at high T is represented by a gradual logarithmic function of InT similarly to the impurity Kondo problem, whereas that at high H by a relatively sharp function of H^-1 different from In H, giving rise to a metamagnetism similar to the exptl. results of heavy fermion materials.