The generalization of the Local Density Approximation (LDA) method for the systems with strong Coulomb correlations is presented which gives a correct description of the Mott insulators. The LDA+U method is based on the model hamiltonian approach and allows to take into account the non-sphericity of the Coulomb and exchange interactions. parameters. Orbital-dependent LDA+U potential gives correct orbital polarization and corresponding Jahn-Teller distortion. To calculate the spectra of the strongly correlated systems the impurity Anderson model should be solved with a many-electron trial wave function. All parameters of the many-electron hamiltonian are taken from LDA+U calculations. The method was applied to NiO and has shown good agreement with experimental photoemission spectra and with the oxygen Kα X-ray emission spectrum.

https://iopscience.iop.org/article/10.1088/0953-8984/9/4/002/pdf

First-principles calculations of the electronic structure and spectra of strongly correlated systems: the LDA+ U method

Since the discovery of superconductivity above 30 K by Bednorz and M\"uller in the La copper oxide system, the critical temperature has been raised to 90 K in Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ and to 110 and 125 K in Bi-based and Tl-based copper oxides, respectively. In the two years since this Nobel-prize-winning discovery, a large number of electronic structure calculations have been carried out as a first step in understanding the electronic properties of these materials. In this paper these calculations (mostly of the density-functional type) are gathered and reviewed, and their results are compared with the relevant experimental data. The picture that emerges is one in which the important electronic states are dominated by the copper $d$ and oxygen $p$ orbitals, with strong hybridization between them. Photon, electron, and positron spectroscopies provide important information about the electronic states, and comparison with electronic structure calculations indicates that, while many features can be interpreted in terms of existing calculations, self-energy corrections ("correlations") are important for a more detailed understanding. The antiferromagnetism that occurs in some regions of the phase diagram poses a particularly challenging problem for any detailed theory. The study of structural stability, lattice dynamics, and electron-phonon coupling in the copper oxides is also discussed. Finally, a brief review is given of the attempts so far to identify interaction constants appropriate for a model Hamiltonian treatment of many-body interactions in these materials.

Electronic structure of the high-temperature oxide superconductors

A review of new developments in theoretical and experimental electronic-structure investigations of half-metallic ferromagnets (HMFs) is presented. Being semiconductors for one spin projection and metals for another, these substances are promising magnetic materials for applications in spintronics (i.e., spin-dependent electronics). Classification of HMFs by the peculiarities of their electronic structure and chemical bonding is discussed. The effects of electron-magnon interaction in HMFs and their manifestations in magnetic, spectral, thermodynamic, and transport properties are considered. Special attention is paid to the appearance of nonquasiparticle states in the energy gap, which provide an instructive example of essentially many-body features in the electronic structure. State-of-the-art electronic calculations for correlated d-systems are discussed, and results for specific HMFs (Heusler alloys, zinc-blende structure compounds, CrO2, and Fe3O4) are reviewed.

/pdf/half-metallic-ferromagnets-from-band-structure-to-many-body-1hrl8mp5lm.pdf

Half-metallic ferromagnets: From band structure to many-body effects

This review encompasses the story of the Verwey transition in magnetite over a period of about 90 years, from its discovery up to the present. Despite this long period of thorough investigation, the intricate multi-particle system Fe3O4 with its various magneto-electronic interactions is not completely understood, as yet - although considerable progress has been achieved, especially during the last two decades. It therefore appeared appropriate to subdivide this retrospect into three eras: (I) from the detection of the effect to the Verwey model (1913-1947), being followed by a period of: (II) checking, questioning and modification of Verwey's original concepts (1947-1979). Owing to prevailing under-estimation of the role of crystal preparation and qualitiy control, this period is also characterized by a series of uncertainties and erroneous statements concerning the reaction order (one or two) and type of the transition (multi-stage or single stage). These latter problems, beyond others, could definitely be solved within era (III) (1979 to the present) - in favour of a first-order, single-stage transition near 125 K - on the basis of experimental and theoretical standards established in the course of a most inspiring conference organized in 1979 by Sir Nevill Mott in Cambridge and solely devoted to the present topic. Regarding the experimental field of further research, the remarkable efficiency of magnetic after-effect (MAE) spectroscopy as a sensitive probe for quality control and investigation of low-temperature (4 K Tv) into a Wigner crystal (T<Tv), describes most adequately the various low-temperature mechanisms in Fe3O4 in terms of tunnelling and variable range hopping of small polarons. On the other hand, the well-elaborated Ihle-Lorenz model, assuming a superposition of polaron-band and -hopping conductivity, is in better agreement with the high-temperature data (Tv<T<600 K).

https://iopscience.iop.org/article/10.1088/0953-8984/14/12/203/pdf

The Verwey transition - a topical review

Review on spintronics: Principles and device applications

Band structure of the ferrimagnetic Fe 3 O 4 was calculated in the high temperature cubic phase by means of self-consistent APW method. Though the obtained band structure corresponds closely with the ionic Neel model, it was shown that the itinerant electron model is more adequate than the localized electron model. The minority spin d e bands of B site iron have an electron Fermi surface at the $ \varGamma $ point and hole surfaces around the W point. Experimental data including photoelectron emission, optical reflectivity, neutron scattering and transport phenomena were discussed qualitatively in relation to the calculated band structure.

Band Structure in the High Temperature Phase of Fe 3 O 4

Many rare earth and actinide compounds are well known as heavy fermion systems with a large specific heat coefficient or as valence fluctuation systems with an energy-gap of activation type. Band calculation is a powerful method for investigat­ ing the electronic structures of these materials, although many-body effects are not sufficiently contained. If hybridization between /-electrons and conduction electrons is large, the Fermi surfaces are .explained as a result of a conventional band calcula­ tion using the local density approximation (LDA). If /-electrons are well localized and have a large magnetic moment, the Fermi surfaces are expected to be similar to those of non-I reference materials. Therefore investigations for the electronic struc­ tures of /-electron systems, provided the heavy masses cannot be expected, are important to know the role of /-electrons in their ground states. For the compounds with an energy-gap, results of band calculations using LDA are needed to answer the question whether hybridization causes a gap or not. Both of SmB6 1 > and YbB1z 2 >' 3 > are known as valence fluctuation compounds with clear gap. CeNiSn is known as a dense Kondo compound with a formation of an energy-gap. 4 >' 5 > The existence of the gap is suggested in the experiments. The resistivity of YbB12 and SmB6 rise in low temperatures in order of magnitude, while the resistivity in the c-direction of CeNiSn becomes about only 4 times larger below 5 K at most in three directiQns of the orthorhombic axes. Therefore, the energy gap in CeNiSn may have different charac­ ter. The compound SmB6 has a CaB6 type crystal structure with the space 'group of Pm3m. Trivalent compounds with this structure such as LaB6, PrB6, NdB6 and YB6 are good metals with one conduction electron per metallic ion, while divalent com­ pounds such as CaB6 and SrB6 are insulator. The band calculation for CaB6 calcu­ lated with LDA gives a small direct gap between the valence and conduction bands at the X points in the simple cubic Brillouin zone. 6 > The magnitude of the energy gap is 0.3 eV, in reasonable agreement with the experimental value, 0.4 eV suggested by

Band Calculations on YbB12, SmB6 and CeNiSn

The self-consistent APW band calculation for the body centered tetragonal structure La2CuO4, which is a proper reference material for the study of the origin of high-temperature superconductivity in (La1-xMx)2CuO4 compounds, has been done using the local density approximation. The bonding-antibonding splitting of the Cu 3d(x2-y2) and O 2p states results in a pair of subbands of about 0.58 Ry width. The half-filled antibonding band has the two dimensional property and nearly square Fermi surface.

https://iopscience.iop.org/article/10.1143/JJAP.26.L352/pdf

Electronic Structure of La2CuO4 by APW Method

We have performed band calculatons for the high- T c related oxides, La 2 CuO 4 , YBa 2 Cu 3 O y ( y =6 or 7), La 2 CaCu 2 O 6 , CuO, and also NiO with the full-potential linear augmented plane wave method in order to derive information about the characteristic aspects of these materials. We pay particular attention to the energy separation between the oxygen p and the copper d levels, the assignment of a p orbital for the extra hole accommodation, the significance of the p - p hopping, the occupancy in the two-dimensional Cu-O bands, and the Cu valency.

Implications of Band-Structure Calculations for High- T c Related Oxides

The phase diagram in the superconducting Bi1.68PbvSrxCavCuzOw(v=0.28, 0.32) system was investigated by means of X-ray diffraction and electrical resistivity for samples calcined in air. The results revealed that the phase was classified into four regions, a single 110 K-phase region, two types of two-phase regions (110 K phase+Ca2CuO3 and 110 K phase+80 K phase) and a three-phase region (110 K phase+Ca2CuO3+80 K phase). The single 110 K phase with a Tc of about 106–108 K exists in a fairly wide composition range, especially in the Bi1.68Pb0.32Sr1.75CayCuzOw system, 1.75y1.85 and 2.65z2.85. This means that the single 110 K phase can be easily obtained, even by the usual calcination in air in the Bi-Pb-Sr-Ca-Cu-O system.

Akira Yanase

Papers

Band Structure in the High Temperature Phase of Fe 3 O 4

Band Calculations on YbB12, SmB6 and CeNiSn

Electronic Structure of La2CuO4 by APW Method

Implications of Band-Structure Calculations for High- T c Related Oxides

Single high-Tc phase region of the Bi-Pb-Sr-Ca-Cu-O system