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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We have determined an instability of the Fermi-liquid state of almost localized fermions in an applied magnetic ffeld. It is proposed that a transition to a strongly correlated fermions (statistical-spin-liquid) state takes place at that point. The resultant magnetization curve and the field dependence of the specific heat are calculated and compared with those for CeRu 2 Si2 .PACS numbers: 71.27.+a, 71.30.+h, 71.10.FóThe metallic state close to Mott-Hubbard localization is commonly regarded as a Fermi-liquid state of correlated fermions [1]. The quasiparticles in this liquid have spin-dependent effective masses (if the band fllling n # 1), and experience a nonlinear molecular field in the spin polarized state [2]. The principal question is: what happens for n # 1 if we apply a magnetic field and the number of double occupancy d2 --> 0, i.e. magnetization m Ξ n↑ -n ↓ --> n? Does it transform gradually into a gas of fermions with one spin direction up or a non-Fermi liquid state comes into play before the system saturates magnetically?The purpose of this paper is to demonstrate that at the point of instability of the almost localized Fermi liquid (ALFL) a transition to the spin liquid state takes place. Consequently, we calculate the magnetization curve exhibiting a mixed itinerant-localized behavior, as well as the applied field dependence of the linear specific heat coeímcient γ Ξ C/T across the transition. In Fig. 1a we have displayed the field dependence of the double occupancy d2 Ξ (ni↑ni↓) in the narrow band of correlated fermions, for three different relative temperatures t Ξ kBT/W (W is the bare band width, U/Uc is the relative magnitude of interaction, U = 2W and n is the band filling). At the points of instability (marked by the dashed lines) there is a metamagnetic transition, associated with a jump in d 2 and magnetization m. For the fields h = μΒ Η a /W beyond this point (h = hc) w e o b t a i n a n o n p h y s i c a l values of d2 < 0, so the ALFL state is unstable. On the physical grounds one can visualize the state d2 = 0 as a liquid of itinerant spins, since the hopping takes place only via empty lattice sites. Therefore, we proposed [3] that such a state (323)

We summarize the main novel features of almost localized fermions in tle presence of au applied magnetic field: (i) the spin dependence of their effective mass, wlich leads to quantum beats in the de Haas-van AIphen effect and (ii) the presence of a nonlinear molecular fleld and related metamagnetic behavior.