We discuss specific features of quasiparticles in a strong applied magnetic field and near the Mott-Hubbard localization: the strong spin dependence of the de Haasvan Alphen oscillations, the maximum in the field dependence of the linear specific-heat coefficient, and metamagnetic behavior. These properties are obtained within the approach involving auxiliary (slave boson) fields that provides both the Gutzwiller band narrowing and a nonlinear molecular field. The simultaneous observation of all three properties provides a consistent set of predictions of the mean-field approach to the almost-localized Fermi liquid. The situation for heavy fermion system CeRu2Siz is briefly discussed.Almost-localized systems of strongly correlated fermions comprise Mott-Hubbard systems [e.g., pure and doped Vz03 (Ref. 1) or La, , Sr"Ti03(Ref. 2)], heavy-fermion systems [such as UPt3, URu2Si2, or CeRu2Si2 (Ref. 3)], liquid He close to solidification, and high-temperature superconducting materials near the antiferromagnetic insulating state [e.g., La2 "Sr"CuO&for x-0.05 (Ref. 5) and YBa2Cu306+"for x-0.3 -0.4]. The first three classes of materials are frequently considered as Fermi liquids of almost localized quasiparticles, i.e. , the liquids bordering on a state with localized magnetic moments. The Fermi-liquid nature of their electronic or atomic (in the case of He) statesshould not be taken for granted, since close to the localization, regarded as a well-defined phase transition, one may encounter a soliton or other non-Fermi-liquid types of singleparticle excitations. The purpose of this paper is to propose a consistent set of experimentally verifiable predictions that determine the specific behavior of an almost-localized Fermi liquid in an applied magnetic field, treated within a simple single-particle approach. The lifetime effects for tempera-tures T~O, as well as the detailed applications to heavyfermion systems, will be discussed separately.In systems close to the Mott-Hubbard localization the band energy of quasiparticles is small (the effective mass m* -+~) and almost compensated by the short-range repulsive interaction among the carriers. In effect, the system is very susceptible to much weaker perturbations such as the exchange interactions (which lead to a spin-density wave formation on the itinerant side, and to antiferromagnetism on the insulating side), thermal noise (causing the disruption of a coherent band motion and a formation of localized moments at elevated temperature ), and applied magnetic field. The main goal of this paper is to show that the applied magnetic field induces a set experimentally verifiable new effects, namely, (i) a spectacular spin dependence of the effective mass as exhibited, e.g. , in de Haasvan Alphen oscillations, (ii) quasimetamagnetic behavior for the nonhalf-filled band case, and (iii) a strong and nonmonotonic magnetic field dependence of the linear specific-heat coefficient y. These effects should appear concurrently at low temperature.

We supplement (and critically overview) the existing extensive analysis of antiferromagnetic solution for the Hubbard model with a detailed discussion of two specific features, namely (i) the evolution of the magnetic (Slater) gap (here renormalized by the electronic correlations) into the Mott-Hubbard or atomic gap, and (ii) a rather weak renormalization of the effective mass by the correlations in the half-filled-band case, which contrasts with that for the paramagnetic case. The mass remains strongly enhanced in the non-half-filled-band case. We also stress the difference between magnetic and non-magnetic contributions to the gap. These results are discussed within the slave boson approach in the saddle-point approximation, in which there appears a non-linear staggered molecular field due to the electronic correlations that leads to the appearance of the magnetic gap. They reproduce correctly the ground-state energy in the limit of strong correlations. A brief comparison with the solution in the limit of infinite dimensions and the corresponding situation in the doubly-degenerate-band case with one electron per atom is also made.

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