Crystal-field splittings in CeX (X= N, P, As, Sb, Bi) compounds

The unusual and interesting physical properties of rare earth intemetallic compounds have their origin in the combination of strongly correlated 4f states and their hybridization with the conduction electron sea, which gives rise to their complex low temperature Kondo behavior. In particular, Ce compounds are very sensitive to the crystalline and chemical environment, as compared to other rare earth systems. The interaction of the 4f state with the conduction band plays an important role in the determination of the different magnetic, structural and transport properties of these systems. Among the cerium compounds, those of the type CeX, which crystallize in the rock salt structure, exhibit extremely unusual magnetic properties. By making use of the mixed LDA-NCA calculation technique we analyse the crystal-field splittings of CeX compounds (X=N, P, As, Sb, Bi). The obtained ab-initio hybridization functions are taken as imputs to calculate the crystal-field splittings within NCA (non crossing approximation) and the tendencies are contrasted with experiments. KEY WORDS: Highly correlated systems, crystal fields, p-electron.Comment: 8 pages, 2 figure

Spin density wave instabilities in the NbS2 monolayer

In the present work, we study the magnetic properties of the NbS2 monolayer by first-principles calculations. The transition metal dichalcogenides (TMDC) are a family of laminar materials presenting exciting properties such as charge density waves (CDW), superconductivity and metal-insulating transitions among others. 2H-NbS2 is a particular case within the family, because it is the only one that is superconductor without exhibiting a CDW order. Although no long range magnetic order was experimentally observed in the TMDC, we show here that the single monolayer of NbS2 is on the verge of a spin density wave (SDW) phase. Our calculations indicate that a wave-like magnetic order is stabilized in the NbS2 monolayer in the presence of magnetic defects or within zig-zag nanoribbons, due to the presence of unpaired electrons. We calculate the real part of the bare electronic susceptibilty and the corresponding nesting function of the clean NbS2 monolayer, showing that there are strong electronic instabilities at the same wavevector asociated with the calculated SDWs, also corresponding with one of the main nesting vectors of the Fermi surface. We conclude that the physical mechanism behind the spin-wave instabilities are the nesting properties, accentuated by the quasi 2D character of this system, and the rather strong Coulomb interactions of the 4d band of the Nb atom. We also estimate the amplitude of the spin-fluctuations and find that they are rather large, as expected for a system on the verge of a quantum critical transition

On the nature of the Mott transition in multiorbital systems

We analyze the nature of Mott metal-insulator transition in multiorbital systems using dynamical mean-field theory (DMFT). The auxiliary multiorbital quantum impurity problem is solved using continuous time quantum Monte Carlo (CTQMC) and the rotationally invariant slave-boson (RISB) mean field approximation. We focus our analysis on the Kanamori Hamiltonian and find that there are two markedly different regimes determined by the nature of the lowest energy excitations of the atomic Hamiltonian. The RISB results at $T\to0$ suggest the following rule of thumb for the order of the transition at zero temperature: a second order transition is to be expected if the lowest lying excitations of the atomic Hamiltonian are charge excitations, while the transition tends to be first order if the lowest lying excitations are in the same charge sector as the atomic ground state. At finite temperatures the transition is first order and its strength, as measured e.g. by the jump in the quasiparticle weight at the transition, is stronger in the parameter regime where the RISB method predicts a first order transition at zero temperature. Interestingly, these results seem to apply to a wide variety of models and parameter regimes.Comment: Accepted for publication in Physical Review

Co-doped Ceria: Tendency towards ferromagnetism driven by oxygen vacancies

We perform an electronic structure study for cerium oxide homogeneously-doped with cobalt impurities, focusing on the role played by oxygen vacancies and structural relaxation. By means of full-potential ab-initio methods, we explore the possibility of ferromagnetism as observed in recent experiments. Our results indicate that oxygen vacancies seem to be crucial for the appearance of a ferromagnetic alignment among Co impurities, obtaining an increasing tendency towards ferromagnetism with growing vacancy concentration. The estimated couplings cannot explain though, the experimentally observed room-temperature ferromagnetism. In this systematic study, we draw relevant conclusions regarding the location of the oxygen vacancies and the magnetic couplings involved. In particular, we find that oxygen vacancies tend to nucleate in the neighborhood of Co impurities and we get a remarkably strong ferromagnetic coupling between Co atoms and the Ce^{3+} neighboring ions. The calculated magnetic moments per cell depend on the degree of reduction which could explain the wide spread in the magnetization values observed in the experiments