51 research outputs found
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
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
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