We describe the synthesis of nitrides of iridium and palladium using the laser-heated diamond anvil cell. We have used the in situ techniques of x-ray powder diffraction and Raman scattering to characterize these compounds and have compared our experimental findings where possible to the results of first-principles theoretical calculations. We suggest that palladium nitride is isostructural with pyrite, while iridium nitride has a monoclinic symmetry and is isostructural with baddeleyite.
We investigate the formation of self-trapped holes (STH) in three prototypical perovskites (SrTiO3, BaTiO3, PbTiO3) using a combination of density functional theory (DFT) calculations with local potentials and hybrid functionals. First we construct a local correction potential for polaronic configurations in SrTiO3 that is applied via the DFT+U method and matches the forces from hybrid calculations. We then use the DFT+U potential to search the configuration space and locate the lowest energy STH configuration. It is demonstrated that both the DFT+U potential and the hybrid functional yield a piece-wise linear dependence of the total energy on the occupation of the STH level suggesting that self-interaction effects have been properly removed. The DFT+U model is found to be transferable to BaTiO3 and PbTiO3, and STH formation energies from DFT+U and hybrid calculations are in close agreement for all three materials. STH formation is found to be energetically favorable in SrTiO3 and BaTiO3 but not in PbTiO3, which can be rationalized by considering the alignment of the valence band edges on an absolute energy scale. In the case of PbTiO3 the strong coupling between Pb 6s and O 2p states lifts the valence band minimum (VBM) compared to SrTiO3 and BaTiO3. This reduces the separation between VBM and STH level and renders the STH configuration metastable with respect to delocalization (band hole state). We expect that the present approach can be adapted to study STH formation also oxides with different crystal structures and chemical composition.
Using quantum mechanical perturbation theory (PT) we analyze how the energy of perturbation of different orders is renormalized in solids. We test the validity of PT analysis by considering a specific case of spin-orbit coupling as a perturbation. We further compare the relativistic energy and the magnetic anisotropy from the PT approach with direct density functional calculations in FePt, CoPt, FePd, MnAl, MnGa, FeNi, and tetragonally strained FeCo. In addition using decomposition of anisotropy into contributions from individual sites and different spin components we explain the microscopic origin of high anisotropy in FePt and CoPt magnets. The magnetocrystalline anisotropy is a central magnetic property for both fundamental and practical reasons.1-3 It can depend sensitively on many quantities such as dopants or small changes in lattice constant.4 While control of this sensitive quantity can be crucial in many applications, e.g. permanent magnetism 5 , magnetooptics 6 and magnetoresistive random-access memory devices 7 , it is often unclear what mechanisms are responsible for these anisotropy variations, even from a fundamental point of view. It was understood long ago 8,9 that the magnetic anisotropy energy (MAE) K in bulk materials is a result of simultaneous action of spin-orbit coupling (SOC) and crystal field (CF). While in general this statement is still valid, existing microscopic methods do not accurately describe K in the majority of materials. One can calculate MAE using ab-initio electronic structure methods based on density functional theory, however quantitative agreement is often rather poor. In any case such methods are usually not well equipped to resolve it into components that yield an intuitive understanding, to enable its manipulation and control. Sometimes K is analyzed in terms of SOC matrix elements of ξl·s, where ξ is the SOC constant. However, this perturbation also induces changes in other terms contributing the total energy, which can affect the MAE as well. Below we show how the actual atomic SOC is 'screened' in crystals and study spin decomposition of SOC and MAE in real world magnets.Let us write the total Hamiltonian of magnetic electronic system aswhere H 0 is the non-relativistic Hamiltonian (sum of kinetic and potential energies of electrons) and V = ξl·s is * Materials of this paper have been presented at the 58th Annual the SOC Hamiltonian. We assume that ξ is small relative to CF and spin splittings. The change in the total energy of the system when SOC is added (below we call it relativistic part of the total energy) can be written aswhere E so is the matrix element of SOC with full perturbed wavefunction and ∆E 0 is the induced energy change of the scalar-relativistic Hamiltonian ( sum of kinetic and potential energies) due to the SOC perturbation. Using standard quantum mechanical perturbation theory (PT) each quantity |φ = |n , E = E (n) and E so = V (n) (wave function, total energy and perturbation V ) can be expressed as a sum over orders n: V (n) is proportional to...
A broad visible luminescence band and characteristic IR luminescence of Yb(3+) ions are observed under UV excitation in ytterbium-doped aluminosilicate glass. Samples made under both oxidizing and reducing conditions are analyzed. A strong charge-transfer absorption band in the UV range is observed for glass samples containing ytterbium. Additional absorption bands are observed for the sample made under reducing conditions, which are associated with f-d transitions of divalent ytterbium. The visible luminescence band is attributed to 5d-4f emission from Yb(2+) ions, and the IR luminescence is concluded to originate from a relaxed charge-transfer transition. The findings are important to explain induced optical losses (photodarkening) in high-power fiber lasers.
Using first-principles theory, we have calculated the energy of Gd as a function of spin direction, theta, between the c and a axes and found good agreement with experiment for both the total magnetic anisotropy energy and its angular dependence. The calculated low temperature direction of the magnetic moment lies at an angle of 20 degrees to the c axis. The calculated magnetic anisotropy energy of Gd metal is due to a unique mechanism involving a contribution of 7.5 microeV from the classical dipole-dipole interaction between spins plus a contribution of 16 microeV due to the spin-orbit interaction of the conduction electrons. The 4f spin polarizes the conduction electrons via exchange interaction, which transfers the magnetic anisotropy of the conduction electrons to the 4f spin.
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