arXiv subjects

A. M. Llois

Publications and source records attributed to A. M. Llois.

23 records · Page 2Linked to original sources

arXiv2007

Kondo temperature and crystal field levels of Ce systems within a finite U Non-Crossing Approximation

We calculate the Kondo temperature ($T_K$) and crystal-field levels of strongly correlated multiorbital systems solving the Anderson Impurity Model with the finite U Non-Crossing Approximation (UNCA) in its simplest scheme, that is, considering the self energies at lowest order in the 1/N diagrammatic expansion. We introduced an approximation to the vertex function that includes the double energy dependence and investigate its effect on the values of $T_K$ for simple electronic models. We also analyze the competition between the two spin flip mechanisms, involving virtual transitions to empty and doubly occupied states, in the determination of the ground state symmetry by including an extra diagram of higher order in $1/N.$ We finally combine the resulting simple formalism with {\it ab initio} calculated electronic structures to obtain $T_K$'s, ground states, and crystal field splittings in excellent agreement with experimental results for two particular Ce compounds, namely CeIn$_3$ and CeSn$_3$.

cond-mat.str-el↗

arXiv2006

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.

cond-mat.str-el↗

arXiv2002

Impurity and band effects competition on the appearence of Inverse Giant Magnetoresistence in Cu/Fe multilayers with Cr

We have studied the dependence of impurity vs. band effects in the appearance of inverse giant magnetoresistance (IGMR) in Cu/Fe superlattices whit Cr. Current in plane (CIP) and current perpendicular to the plane (CPP) geometries are considered. For the calculation of the conductivities we have used the linearized Boltzmann equation in the relaxation time approximation. Cr impurity effects are taken into account through the spin dependent relaxation times and the band effects through the semiclassical velocities obtained from the LDA calculated electronic structure. The larger the Cr/Fe hybridization strength, the bigger is the tendency towards IGMR. In particular, in CIP geometry roughness at these interfaces increases the IGMR range. The results are compared with experiments and we conclude that the experimental GMR curves can only be explained if Cr bands are present.

cond-mat.mtrl-sci↗

arXiv2000

Adsorbed 3d transition metal atoms and clusters on Au(111):Signatures derived from one electron calculations

The spectroscopic characteristics of systems with adsorbed d impurities on noble metal surfaces should depend on the number and geometric arrangement of the adsorbed atoms and also on their d band filling. Recent experiments using scanning tunneling microscopy have probed the electronic structure of all 3d transition metal impurities and also of Co dimers adsorbed on Au(111), providing a rich variety of results. In this contribution we correlate those experimental results with ab-initio calculations and try to establish necessary conditions for observing a Kondo resonance when using the single impurity Anderson model. We find that the relevant orbitals at the STM tip position, when it is on top of an impurity, are the dThe spectroscopic characteristics of systems with adsorbed d impurities on noble metal surfaces should depend on the number and geometric arrangement of the adsorbed atoms and also on their d band filling. Recent experiments using scanning tunneling microscopy have probed the electronic structure of all 3d transition metal impurities and also of Co dimers adsorbed on Au(111), providing a rich variety of results. In this contribution we correlate those experimental results with ab-initio calculations and try to establish necessary conditions for observing a Kondo resonance when using the single impurity Anderson model. We find that the relevant orbitals at the STM tip position, when it is on top of an impurity, are the d orbitals with m=0 and that the energy of these levels with respect to the Fermi energy determines the possibility of observing a spectroscopic feature due to the impurity. orbitals with m=0 and that the energy of these levels with respect to the Fermi energy determines the possibility of observing a spectroscopic feature due to the impurity.

cond-mat.mtrl-sci↗

arXiv2000

Study of Ce intermetallic compounds: an LDA classification and hybridization effects

Spin-polarized calculations within the LDA approximation have been done with the goal of characterizing Ce intermetallic compounds. Symmetry and chemical environment effects on 4f hybridization and, thereafter, on magnetization effects have been studied.

cond-mat↗