Semiconductor physics has been focusing on doping for a long time, and we have developed over the years the "modern theory of Doping" [Alex Zunger and Oleksandr I. Malyi, "" Chem. Rev.听2021, 121, 5, 3031-3060听(2021)]. This approach describes formation energies of defects as a function of Fermi energy and chemical potentials; calculates Fermi energies as a function of temperature and chemical potentials. We are under the impression that doping theory of quantum materials is more naively executed and hence wish to treat some of the main issues in this field with more adequate methodology.
Specific proposed projects include:
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Exploration A: Identifying the role of individual symmetry breaking modes in creating an electronic landscape in undoped 'quantum oxides' classified with band edge orbital character (BEOC): Here, we will test ways to analyze the electronic structure of undoped ABO3 compounds calculated with symmetry-broken DFT supercells finding the contribution of different symmetry breaking modes to basic properties such as band gap and carrier masses. Compounds will be classified via the BEOC as traditionally done for correlated compounds. We will analyze the electronic landscape of the undoped QMa family following different forms of symmetry breaking and analyze insulation in (d,d), (p,p) QMa's as a result of symmetry breaking. We will utilize DFT supercell calculations with allowed internal symmetry breaking to describe the electronic properties of QMa's and group them according to their BEOC. The primary focus here will be given to exploring the roles of internal symmetry breaking on the electronic properties of QMa's and revealing how they control doping.
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Exploration B: Metallization without doping and the insulator to metal transition: Here we will explore the insulator to metal transitions between different bonafide crystallographic/magnetic phases that offer another route to metallization than doping. We will focus on compounds characterized by two types of valence and conduction band edges: (i) (p, p*) negative charge transfer insulators such as YNiO3 and (ii) (d, d*) Mott insulator compounds such as LaVO3. We will analyze if metallization is related to loss of positional structural symmetry breaking present in the insulating phase of the systems. This will be investigated by finite-T Molecular Dynamics (MD) and pressure searching for a transition. We will focus on the electronic and magnetic properties鈥攕uch as density of states, unfolded band structure, magnetic moment configurations鈥攊n parallel with the total energy change associated with the structural/magnetic microscopic degrees of freedom (m-DOF). This exploration will include internal energy as well as the free energy from molecular dynamics, both to be done with DFT in sufficiently large supercells that allows the flexibility of symmetry breaking. Our study will encompass many fundamental types of symmetry breaking quantum materials, including (i) Metals before symmetry breaking become (modified) metals after symmetry breaking such as LiOsO3 and SrHgPb, (ii) Metals before symmetry breaking become insulators after symmetry breaking such as VO2, NbO2, and YNiO3, (iii) Insulators before symmetry breaking becoming other insulators after symmetry breaking such as BaTiO3, (iv) Insulators before symmetry breaking become metals after symmetry breaking. 听Note that all above examples are 鈥渂ulk鈥� (stoichiometric) metal/nonmetal. Another different category is 鈥渘onstoichiometric (traditional) doping鈥� meaning small shifts in the Fermi energy, or replacing in a supercell just one or two atoms by a dopant. For example, we will dope the insulator LaFeO3 with unoccupied split-off flat bands first virtually (shifting the Fermi energy) and then chemically (by a real atom such as Mn and Co).
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Exploration C: Creation of carriers via spontaneous generation of structural defects and non-stoichiometry without deliberate doping (this means creating a vacancy in a supercell): While formation of large defect concentration in traditional insulators occurs at rather high temperature, oxide QMa's can be different. The existence of non-stoichiometry in QMa's is often thought to be a growth effect rather than a specific electronic instability. We will examine if intrinsic degenerate gapped metals, i.e., compounds that have the Fermi level inside the conduction band (CB) as in BaNbO3 or valence band (VB) as in Ba4Bi3, are exceptions to the Daltonian view that compounds maintain integer stoichiometry at low temperatures since forming stoichiometry-violating defects costs energy. We will study how the presence of electrons (holes) in conduction (valence) bands promotes an instability towards creating structural defects in degenerate gapped metals, leading to gap levels that can reduce the formation energy of acceptors (donors). We will focus on a family of degenerate gapped metals (e.g., select 1-2 from NaWO3, SrNbO3, paramagnetic (PM) SrVO3, PM BaMoO3, and PM SrFeO3) which belong to metallic (d, d*) compounds prone, in our view, to create self-doping structural defects. We will explore if such Fermi level-induced spontaneous non-stoichiometry can lead to the formation of crystallographically ordered vacancy compounds (OVCs).
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Exploration D: Doping irregularities in ABO3 compounds reflecting the electronic landscapes of the undoped phases: We will examine the doping trends among different QMa's prototypes (Mott-like; Charge transfer, negative charge transfer鈥攕ee Exploration A) using the doping methodology we have developed and tested over the years. We will thus uncover when the doping results in: (i) antidoping behavior; (ii) formation polaron states; (iii) regular doping response common to traditional wide band gap insulators. This study will reveal the basic trends and regularities of doping within the family of "quantum oxides", completing the current equivalent knowledge of doping trends among the traditional IV-IV, III-V, and II-VI semiconductors.