Singlet Orbital Ordering in Bilayer Sr_{3}Cr_{2}O_{7}.

We perform an extensive study of Sr_{3}Cr_{2}O_{7}, the n=2 member of the Ruddlesden-Popper Sr_{n+1}Cr_{n}O_{3n+1} system. An antiferromagnetic ordering is clearly visible in the magnetization and the specific heat, which yields a huge transition entropy, Rln(6). By neutron diffraction as a function of temperature we have determined the antiferromagnetic structure that coincides with the one obtained from density functional theory calculations. It is accompanied by anomalous asymmetric distortions of the CrO_{6} octahedra. Strong coupling and Lanczos calculations on a derived Kugel-Khomskii Hamiltonian yield a simultaneous orbital and moment ordering. Our results favor an exotic ordered phase of orbital singlets not originated by frustration.

Lifshitz transition in Kondo alloys.

We study the low-energy states of Kondo alloys as a function of the magnetic impurity concentration per site x and the conduction electron average site occupation n(c). Using two complementary approaches, the mean-field coherent potential approximation and the strong-coupling limit, we identify and characterize two different Fermi-liquid regimes. We propose that both regimes are separated by a Lifshitz transition at x=n(c). Indeed, we predict a discontinuity of the number of quasiparticles that are enclosed in the Fermi surface. This feature could provide a scenario for the non-Fermi liquid properties that were recently observed in Kondo alloy systems around x=n(c).

Metamagnetic transition in the twoforbitals Kondo lattice model.

In this work, we study the effects of a transverse magnetic field in a Kondo lattice model with twoforbitals interacting with the conduction electrons. Thefelectrons that are present on the same site interact through Hund's coupling, while on neighboring sites they interact through intersite exchange. We consider here that part offelectrons are localized (orbital 1) while another part (orbital 2) are delocalized, as it is frequent in uranium systems. Then, only electrons in the localized orbital 1 interact through exchange interaction with the neighboring ones, while electrons in orbital 2 are coupled with conduction electrons through a Kondo interaction. We obtain a solution where ferromagnetism and Kondo effect coexist for small values of an applied transverse magnetic field forT→0. Increasing the transverse field, two situations can be obtained when Kondo coupling vanishes: first, a metamagnetic transition occurs just before or at the same time of the fully polarized state, and second, a metamagnetic transition occurs when the spins are already pointing out along the magnetic field.

Breakdown of coherence in Kondo alloys: crucial role of concentration versus band filling.

We study the low energy states of the Kondo alloy model (KAM) as function of the magnetic impurity concentration per site, x, and the conduction electron average site occupation, n c . In previous works, two different Fermi liquid regimes had been identified at strong Kondo coupling [Formula: see text], that may be separated by a transition at x = n c . Here, we analyze the KAM for finite [Formula: see text] on a Bethe lattice structure. First, using the mean-field coherent potential approximation (DMFT-CPA) which is exact at lattice coordination [Formula: see text], we show that the real part of the local potential scattering may be located outside the conduction electron band, revealing a possible breakdown of Luttinger theorem for intermediate values of impurity concentration x. Unusual physical signatures are expected, e.g. in ARPES experiments. In order to take into account fluctuations associated with finite dimensionality, i.e. finite Z, we extend this analysis by studying the KAM with an adaptation of the statistical-DMFT method that was developped elsewhere. We review the distributions of local potential scattering and their evolution with model parameters: concentration, strength of Kondo coupling, coordination number, local site neighborhood, connection with percolation issue. Relevence for Kondo alloys material with f -electrons is also discussed.

Crossover from weak to strong coupling superconductivity in multi-band systems.

The study of superconductivity in correlated systems is an exciting area of condensed matter physics. In this paper we consider superconducting ground states in systems described by two-band models with different effective masses. These two bands are coupled through an effective hybridization that can be directly tuned by pressure. We consider the cases of s-wave superconductivity associated with the electrons in a narrow band and also with inter-band pairing. To study the system in the strong coupling regime we introduce the s-wave scattering length a(s), and obtain the superconducting order parameters and the chemical potential as functions of the interaction strength 1/k(F)a(s) along the BCS-BEC crossover at T = 0. Finally, we discuss the phase diagram of this model as a function of external pressure and how our results can be applied for two-band systems as Fe pnictides or heavy fermions. The main result of this study is the occurrence of a superconducting quantum critical point (SQCP) in this two-band model.