arXiv subjects

L. Petit

Publications and source records attributed to L. Petit.

33 records · Page 2Linked to original sources

arXiv2006

First Principles Electronic Structure of Mn doped GaAs, GaP, and GaN semiconductors

We present first-principles electronic structure calculations of Mn doped III-V semiconductors based on the local spin-density approximation (LSDA) as well as the self-interaction corrected local spin density method (SIC-LSD). We find that it is crucial to use a self-interaction free approach to properly describe the electronic ground state. The SIC-LSD calculations predict the proper electronic ground state configuration for Mn in GaAs, GaP, and GaN. Excellent quantitative agreement with experiment is found for magnetic moment and p-d exchange in (GaMn)As. These results allow us to validate commonly used models for magnetic semiconductors. Furthermore, we discuss the delicate problem of extracting binding energies of localized levels from density functional theory calculations. We propose three approaches to take into account final state effects to estimate the binding energies of the Mn-d levels in GaAs. We find good agreement between computed values and estimates from photoemisison experiments.

cond-mat.mtrl-sci↗

arXiv2006

Pressure Induced Valence Transitions in f-Electron Systems

A review is given of pressure induced valence transitions in f-electron systems calculated with the self-interaction corrected local spin density (SIC-LSD) approximation. These calculations show that the SIC-LSD is able to describe valence changes as a function of pressure or chemical composition. An important finding is the dual character of the f-electrons as either localized or band-like. A finite temperature generalisation is presented and applied to the study of the p-T phase diagram of the alpha to gamma phase transition in Ce.

cond-mat.supr-con↗

arXiv2006

Mott Transition of MnO under Pressure: Comparison of Correlated Band Theories

The electronic structure, magnetic moment, and volume collapse of MnO under pressure are obtained from four different correlated band theory methods; local density approximation + Hubbard U (LDA+U), pseudopotential self-interaction correction (pseudo-SIC), the hybrid functional (combined local exchange plus Hartree-Fock exchange), and the local spin density SIC (SIC-LSD) method. Each method treats correlation among the five Mn 3d orbitals (per spin), including their hybridization with three O $2p$ orbitals in the valence bands and their changes with pressure. The focus is on comparison of the methods for rocksalt MnO (neglecting the observed transition to the NiAs structure in the 90-100 GPa range). Each method predicts a first-order volume collapse, but with variation in the predicted volume and critical pressure. Accompanying the volume collapse is a moment collapse, which for all methods is from high-spin to low-spin (5/2 to 1/2), not to nonmagnetic as the simplest scenario would have. The specific manner in which the transition occurs varies considerably among the methods: pseudo-SIC and SIC-LSD give insulator-to-metal, while LDA+U gives insulator-to-insulator and the hybrid method gives an insulator-to-semimetal transition. Projected densities of states above and below the transition are presented for each of the methods and used to analyze the character of each transition. In some cases the rhombohedral symmetry of the antiferromagnetically ordered phase clearly influences the character of the transition.

cond-mat.str-el↗

arXiv2006

Ground-state valency and spin configuration of the Ni-ions in nickelates

The ab initio self-interaction-corrected local-spin-density approximation is used to study the electronic structure of both stoichiometric and non-stoichiometric nickelates. From total energy considerations it emerges that, in their ground-state, both LiNiO2 and NaNiO2 are insulators, with the Ni ion in the Ni3+ low spin state (6t2g 1eg) configuration. It is established that a substitution of a number of Li/Na atoms by divalent impurities drives an equivalent number of Ni ions in the NiO2 layers from the JT-active trivalent low-spin state to the JT-inactive divalent state. We describe how the observed considerable di_erences between LiNiO2 and NaNiO2 can be explained through the creation of Ni2+ impurities in LiNiO2. The indications are that the random distribution of the Ni2+ impurities might be responsible for the destruction of the long-range orbital ordering in LiNiO2.

cond-mat.str-el↗

arXiv2006

Electronic structure of normal and inverse spinel ferrites from first principles

We apply the self-interaction corrected local spin density %(SIC-LSD) approximation to study the electronic structure and magnetic properties of the spinel ferrites MnFe$_{2}$O$_{4}$, Fe$_{3}$O$_{4}$, CoFe$_{2}$O$_{4}$, and NiFe$_{2}$O$_{4}$. We concentrate on establishing the nominal valence of the transition metal elements and the ground state structure, based on the study of various valence scenarios for both the inverse and normal spinel structures for all the systems. For both structures we find all the studied compounds to be insulating, but with smaller gaps in the normal spinel scenario. On the contrary, the calculated spin magnetic moments and the exchange splitting of the conduction bands are seen to increase dramatically when moving from the inverse spinel structure to the normal spinel kind. We find substantial orbital moments for NiFe$_{2}$O$_{4}$ and CoFe$_{2}$O$_{4}$.

cond-mat.mtrl-sci↗

arXiv2006

Valency Configuration of Transition Metal Impurities in ZnO

We use the self-interaction corrected local spin-density approximation to investigate the ground state valency configuration of transition metal (TM = Mn, Co) impurities in n- and p-type ZnO. We find that in pure Zn1-xTMxO, the localized TM2+ configuration is energetically favored over the itinerant d-electron configuration of the local spin density (LSD) picture. Our calculations indicate furthermore that the (+/0) donor level is situated in the ZnO gap. Consequently, for n-type conditions, with the Fermi energy eF close to the conduction band minimum, TM remains in the 2+ charge state, while for p-type conditions, with eF close to the valence band maximum, the 3+ charge state is energetically preferred. In the latter scenario, modeled here by co-doping with N, the additional delocalized d-electron charge transfers into the entire states at the top of the valence band, and hole carriers will only exist, if the N concentration exceeds the TM impurity concentration.

cond-mat.mtrl-sci↗

arXiv2006

Electronic structure of rare-earth impurities in GaAs and GaN

The electronic structures of substitutional rare-earth (RE) impurities in GaAs and cubic GaN are calculated. The total energy is evaluated with the self-interaction corrected local spin density approximation, by which several configurations of the open 4f shell of the rare-earth ion may be investigated. The defects are modelled by supercells of type REGa$_{n-1}$As$_n$, for n=4, 8 and 16. The preferred defect is the rare-earth substituting Ga, for which case the rare-earth valency in intrinsic material is found to be trivalent in all cases except Ce and Pr in GaN. The 3+ --> 2+ f-level is found above the theoretical conduction band edge in all cases and within the experimental gap only for Eu, Tm and Yb in GaAs and for Eu in GaN. The exchange interaction of the rare-earth impurity with the states at both the valence band maximum and the conduction band minimum is weak, one to two orders of magnitude smaller than that of Mn impurities. Hence the coupling strength is insufficient to allow for ferromagnetic ordering of dilute impurities, except at very low temperatures.

cond-mat.mtrl-sci↗

arXiv2005

First principles study of rare-earth oxides

The self-interaction-corrected local-spin-density approximation is used to describe the electronic structure of dioxides, REO$_2$, and sesquioxides, RE$_2$O$_3$, for the rare earths, RE=Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy and Ho. The valencies of the rare earth ions are determined from total energy minimization. We find Ce, Pr, Tb in their dioxides to have the tetravalent configuration, while for all the sesquioxides the trivalent groundstate configuration is found to be the most favourable. The calculated lattice constants for these valency configurations are in good agreement with experiment. Total energy considerations are exploited to show the link between oxidation and $f$-electron delocalization, and explain why, among the dioxides, only the CeO$_2$, PrO$_2$, and TbO$_2$ exist in nature. Tetravalent NdO$_2$ is predicted to exist as a metastable phase - unstable towards the formation of hexagonal Nd$_2$O$_3$.

cond-mat.str-el↗

arXiv2004

Electronic structure of transition metal impurities in p-type ZnO

The self-interaction corrected local spin-density approximation is used to investigate the ground-state valency configuration of transition metal (TM=Mn, Co) impurities in p-type ZnO. Based on the total energy considerations, we find a stable localised TM$^{2+}$ configuration for a TM impurity in ZnO if no additional hole donors are present. Our calculations indicate that the (+/0) donor level is situated in the band gap, as a consequence of which the TM$^{3+}$ becomes more favourable in p-type ZnO, where the Fermi level is positioned at the top of the valence band. When co-doping with N, it emerges that the carrier-mediated ferromagnetism can be realized in the scenario where the N concentration exceeds the TM impurity concentration. If TM and N concentrations are equal, the shallow acceptor levels introduced by N are fully compensated by delocalised TM d-electrons.

cond-mat.str-el↗

arXiv2004

Electronic Structure of Samarium Monopnictides and Monochalcogenides

The electronic structures of SmX (X=N, P, As, Sb, Bi, O, S, Se, Te, Po)compounds are calculated using the self-interaction corrected local-spin density approximation. The Sm ion is described with either five or six localized $f$-electrons while the remaining electrons form bands, and the total energies of these scenarios are compared. With five localized $f$-electrons a narrow $f$-band is formed in the vicinity of the Fermi level leading to an effective intermediate valence. This scenario is the ground state of all the pnictides as well as SmO. With six localized $f$-electrons, the chalcogenides are semiconductors, which is the ground state of SmS, SmSe and SmTe. Under compression the Sm chalcogenides undergo first order transitions with destabilization of the $f$ states into the intermediate valence state, the bonding properties of which are well reproduced by the present theory.

cond-mat.str-el↗

arXiv2004

Ab initio study of charge order in Fe3O4

We present a self-interaction corrected local spin density (SIC-LSD) study of the electronic structure and possible charge order of magnetite, Fe$_{3}$O$_{4}$. The issue of charge order in magnetite is explored in both cubic and orthorhombic structures, the latter being an approximation to the true, low temperature, monoclinic structure. We find that the Verwey charge ordered phase is not the groundstate solution for this compound neither in cubic nor orthorhombic structure.

cond-mat.str-el↗

arXiv2003

Self-interaction Corrected Calculations of Correlated f-electron Systems

The electronic structures of several actinide solid systems are calculated using the self-interaction corrected local spin density approximation. Within this scheme the $5f$ electron manifold is considered to consist of both localized and delocalized states, and by varying their relative proportions the energetically most favourable (groundstate) configuration can be established. Specifically, we discuss elemental Pu in its $δ$-phase, PuO$_2$ and the effects of addition of oxygen, the series of actinide monopnictides and monochalcogenides, and the UX$_3$, X= Rh, Pd, Pt, Au, intermetallic series.

cond-mat.str-el↗

arXiv2003

Ab initio charge, spin and orbital energy scales in LaMnO3

The first-principles SIC-LSD theory is utilized to study electronic, magnetic and orbital phenomena in LaMnO$_{3}$. The correct ground state is found, which is antiferro orbitally ordered with the spin magnetic moments antiferromagnetically aligned. Jahn-Teller energies are found to be the largest energy scale. In addition it is the Jahn-Teller interaction which is the dominant effect in realising orbital order, and the electronic effects alone do not suffice.

cond-mat.str-el↗

arXiv2003

Electronic structure of half-metallic double perovskites

We present the self-interaction corrected local spin density (SIC-LSD) electronic structure and total energy calculations, leading also to valencies of the ground state configurations, for the half-metallic double perovskites such as Sr$_{2}$FeMoO$_{6}$, Ba$_{2}$FeMoO$_{6}$, Ca$_{2}$FeMoO$_{6}$, and Ca$_{2}$FeReO$_{6}$. We conclude that the Fe and Mo (or Re) spin magnetic moments are anti-parallel aligned, and the magnitude of the hybridization induced moment on Mo does not vary much between the different compounds. The hybridization spin magnetic moment on Re is of the order of -1.1 $μ_{B}$, while that on Mo is about -0.4 $μ_{B}$, independently of the alkaline earth element. Also the electronic structure of all the compounds studied is very similar, with a well defined gap in the majority spin component and metallic density of states for the minority spin component, with highly hybridized Fe, Mo (or Re), and oxygen bands.

cond-mat.str-el↗

arXiv2003

Ab-initio determination of the localized/delocalized f-manifold in UPd_2Al_3

The electronic structure of UPd_2Al_3 is described using the self-interaction corrected local-spin-density approximation to density functional theory. The groundstate is found to be characterized by the coexistence of localized (f^2) and delocalized U f electrons, in agreement with experimental evidence. We observe significant difference in electronic structure between UPd_2Al_3 and the previously studied UPt_3 compound. Even though a trend towards localization exists in UPt_3, the total energies and the density of states at the Fermi level favor a groundstate with localized f^1, rather than f^2 U ions.

cond-mat.str-el↗