Many-electron effects in transition metal and rare earth compounds : Electronic structure, magnetic properties and point defects from first principles

Abstract : The topic of this thesis is the first-principles theory of the electronic structure of materials with strong electronic correlations. Tremendous progress has been made in this field thanks to modern implementations of Density Functional Theory (DFT). However, the DFT framework has some limits. First, it is designed to predict ground state but not excited state properties of materials, even though the latter may be just as important for many applications. Second, the approximate functionals used in actual calculations have more limited validity than conceptually exact DFT: in particular, they are not able to describe those materials where many-electron effects are most important.Since the 1990's, different many-body theories have been used to improve or complement DFT calculations of materials. One of the most significant non-perturbative methods is Dynamical Mean-Field Theory (DMFT), where a lattice model is self-consistently mapped onto an impurity model, producing good results if correlations are mostly local. We briefly review these methods in the first part of this thesis. Recent developments on DMFT and its extensions were aimed at better describing non-local effects, understanding out-of-equilibrium properties or describing real materials rather than model systems, among others. Here, we focus on the latter aspect.In order to describe real materials with DMFT, one typically needs to start with an electronic structure calculation that treats all the electrons of the system on the same footing, and apply a many-body correction on a well-chosen subspace of orbitals near the Fermi level. Defining such a low-energy subspace consistently requires to integrate out the motion of the electrons outside this subspace. Taking this into account correctly is crucial: it is, for instance, the screening by electrons outside the subspace strongly reduces the Coulomb interaction between electrons within the subspace. Yet it is a complex task, not least because DFT and DMFT are working on different observables. In the second part of this thesis, we discuss low-energy models in the context of the recently proposed Screened Exchange + DMFT scheme. In particular, we study the importance of non-local exchange and dynamically-screened Coulomb interactions. We illustrate this by discussing semi-core states in the d10 metals Zn and Cd.In the third and last part, we use the methods described above to study the electronic structure of three fundamentally and technologically important correlated materials. First, we discuss the physics of point defects in the paramagnetic phase of bcc Fe, more precisely the simplest of them: the monovacancy. Surprisingly for such a simple point defect, its formation energy had not yet been reported consistently from calculations and experiments. We show that this is due to subtle but nevertheless important correlation effects around the vacancy in the high-temperature paramagnetic phase, which is significantly more strongly correlated than the ferromagnetic phase where DFT calculations had been done.Second, we study the metal-insulator phase transition in the metastable VO2 B phase. We show that this transition is similar to that between the conventional rutile and M2 VO2 phases, involving both bonding physics in the dimer and an atom-selective Mott transition on the remaining V atoms. Motivated by recent calculations on SrVO3, we study the possible effect of oxygen vacancies on the electronic structure of VO2.Finally, we propose a scheme beyond DFT for calculating the crystal field splittings in rare earth intermetallics or oxides. While the magnitude of this splitting for the localized 4f shell of lanthanides does not typically exceed a few hundred Kelvin, it is crucial for their hard-magnetic properties. Using a modified Hubbard I approximation as DMFT solver, we avoid a nominally small but important self-interaction error, stressing again the importance of carefully tailored low-energy models.
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Pascal Delange. Many-electron effects in transition metal and rare earth compounds : Electronic structure, magnetic properties and point defects from first principles. Strongly Correlated Electrons [cond-mat.str-el]. Université Paris-Saclay, 2017. English. ⟨NNT : 2017SACLX040⟩. ⟨tel-01686495⟩

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