First-Principles Calculations of the Instabilities in Fe-(Ni,Co, Pt) Alloys

First-principles calculations using the Korringa-Kohn-Rostocker method and the Coherent-Potential Approximation for Fe-Ni, Fe-Ni-Co and Fe-Pt alloys show that several features are responsible for the Invar anomalies. Atomic short range ordering in the alloys is responsible for the appearance of antiferromagnetic and non-colinear magnetic moments. The antiferromagnetic contributions are responsible for two effects, the negative anharmonicity due to the tendency of the alloys to have a smaller lattice constant, as well as the tendency to have a larger lattice constant because of additional density of states of antibonding majority-spin orbitals at the Fermi level, which simultaneously stabilizes the antiferromagnetic moments.     

Orbital ordering transition in Ca2RuO4 observed with resonant X-ray diffraction

Resonant x-ray diffraction performed at the L(II) and L(III) absorption edges of Ru has been used to investigate the magnetic and orbital ordering in Ca2RuO4 single crystals. A large resonant enhancement due to electric dipole 2p-->4d transitions is observed at the wave-vector characteristic of antiferromagnetic ordering. Besides the previously known antiferromagnetic phase transition at T(N)=110 K, an additional phase transition, between two paramagnetic phases, is observed around 260 K. Based on the polarization and azimuthal angle dependence of the diffraction signal, this transition can be attributed to orbital ordering of the Ru t(2g) electrons. The propagation vector of the orbital order is inconsistent with some theoretical predictions for the orbital state of Ca2RuO4.     

The first principle study on the electronic structure and spin ordering of the rare earth palladium sulfide, EuPd3S4

We have performed relativistic first-principles full-potential linearized augmented plane wave (FLAPW) calculation for rare earth palladium sulfide EuPd₃S₄ in the ferromagnetic and antiferromagnetic states. The density of 4f electrons of Eu is taken from a local-spin-density approximation self-interaction correction (LSDA-SIC) atomic calculation. EuPd₃S₄ is found to exhibit antiferromagnetic ordering in its ground state. The charge, orbital, magnetic moment and spin ordering are explained with the electronic structure, the orbital-projected density of states and the total energy study. EuPd₃S₄ is found to be stable in the body-centered Type-I antiferromagnetic state, in agreement with experimental results. Different Eu states are found in antiferromagnetic ordering. The magnetic moments of different states obtained through spin-polarized calculation are also in good agreement with experimental results. The phenomena observed are explained by the orbital hybridization of Eu and Pd ions as compared with the free ions.     

Spin order due to orbital fluctuations: cubic vanadates

We investigate the highly frustrated spin and orbital superexchange interactions in cubic vanadates. The fluctuations of t(2g) orbitals trigger a novel mechanism of ferromagnetic interactions between spins S = 1 of V3+ ions along one of the cubic directions which operates already in the absence of Hund's rule exchange J(H), and leads to the C-type antiferromagnetic phase in LaVO3. The Jahn-Teller effect can stabilize the orbital ordering and the G-type antiferromagnetic phase at low temperatures, but large entropy due to orbital fluctuations favors again the C phase at higher temperatures, as observed in YVO (3).     

Magnetic order driven by orbital ordering in the semiconducting KFe1.5Se2

The two-orbital Hubbard model is studied numerically by using the Hartree-Fock approximation in both real space and momentum space, and the ground-state properties of the alkali metal iron selenide semiconducting KFe_1.5Se₂ are investigated. A rhombus-type Fe vacancy order with stripetype antiferromagnetic (AFM) order is found, as was observed in neutron scattering experiments [J. Zhao, et al., Phys. Rev. Lett. 109, 267003 (2012)]. Hopping parameters are obtained by fitting the experimentally observed stripe AFM phase in real space. These hopping parameters are then used to study the ground-state properties of the semiconductor in momentum space. It is found to be a strongly correlated system with a large on-site Coulomb repulsion U, similar to the AFM Mott insulator — the parent compound of copper oxide superconductors. We also find that the electronic occupation numbers and magnetizations in the d_xz and d_yz orbitals become different simultaneously when U>U_c (～3.4 eV), indicating orbital ordering. These results imply that the rotational symmetry between the two orbitals is broken by orbital ordering and thus drives the strong anisotropy of the magnetic coupling that has been observed by experiments and that the stripe-type AFM order in this compound may be caused by orbital ordering together with the observed large anisotropy.     

Unusual symmetries in the Kugel-Khomskii Hamiltonian

The Kugel-Khomskii Hamiltonian for cubic titanates describes spin and orbital superexchange interactions between d(1) ions having threefold degenerate t(2g) orbitals. Since orbitals do not couple along "inactive" axes, perpendicular to the orbital planes, the total number of electrons in |alpha> orbitals in any such plane and the corresponding total spin are both conserved. A Mermin-Wagner construction shows that there is no long-range spin ordering at nonzero temperatures. Inclusion of spin-orbit coupling allows such ordering, but even then the excitation spectrum is gapless due to a continuous symmetry. Thus, the observed order and gap require more symmetry breaking terms.     

Magnetic and orbital ordering in the spinel MnV2O4

Neutron inelastic scattering and diffraction techniques have been used to study the MnV2O4 spinel system. Our measurements show the existence of two transitions to long-range ordered ferrimagnetic states, the first collinear and the second noncollinear. The lower temperature transition, characterized by development of antiferromagnetic components in the basal plane, is accompanied by a tetragonal distortion and the appearance of a gap in the magnetic excitation spectrum. The low-temperature noncollinear magnetic structure has been definitively resolved. Taken together, the crystal and magnetic structures indicate a staggered ordering of the V d orbitals. The anisotropy gap is a consequence of unquenched V orbital angular momentum.     

Instability of one-dimensional dangling-bond wires on H-passivated C(001), Si(001), and Ge(001) surfaces

We investigate the instability of one-dimensional dangling-bond (DB) wires fabricated on the H-terminated C(001), Si(001), and Ge(001) surfaces by using density-functional theory calculations. The three DB wires are found to show drastically different couplings between charge, spin, and lattice degrees of freedom, resulting in an insulating ground state. The C DB wire has an antiferromagnetic spin coupling between unpaired DB electrons, caused by strong electron–electron interactions, whereas the Ge DB wire has a strong charge-lattice coupling, yielding a Peierls-like lattice distortion. For the Si DB wire, the antiferromagnetic spin ordering and the Peierls instability are highly competing with each other. The physical origin of such disparate features in the three DB wires can be traced to the different degree of localization of 2p, 3p, and 4p DB orbitals.     

Hubbard model calculation of (2 × 1) reconstructed (111) surfaces of diamond-structure solids

From a Hubbard Hamiltonian for the electrons in the dangling bond orbitals of a (111) surface of diamond-type solids, different superstructures of periodicity (2 × 1) are calculated. The most stable one is an antiferromagnetic ordering of the electron spins, where alternating rows of orbitals show alternating magnetization. This superstructure vanishes in a second order phase transition at a critical temperature, above which the surface shows the unreconstructed (1 × 1) periodicity.     

Ferroelectricity due to orbital ordering in E-type undoped rare-earth manganites

Aiming at understanding the origin of the electronic contribution to ferroelectric polarization in undoped manganites, we evaluate the Berry phase of orbital-polarizable Bloch electrons as an orbital ordering (OO) establishes in the background of an antiferromagnetic E-type configuration. The onset of OO is tuned by the Jahn-Teller (JT) interaction in a tight-binding model for interacting electrons moving along zigzag chains. A finite polarization is found as soon as the JT coupling is strong enough to induce OO, supporting the large electronic contribution predicted from first principles.     

Photoelectron and electron impact spectrum of HCN

The electronic structures of HCN and DCN have been determined by examining high resolution He(I) photelectron spectra of HCN and DCN, He(II) photoelectron spectrum of HCN, and the electron impact energy loss spectra of HCN and DCN. The present investigation supports an earlier assignment of the orbital sequence in HCN. New vibrational data are presented and the Rydberg series and valence transitions are reinvestigated. The adiabatic ionization energies for the 1π and 5σ orbitals in HCN are found to be 13.607 ± 0.002 eV and 14.011 ± 0.003 eV respectively.As mentioned above the investigation of the Rydberg series indicated that the first IP at 13.607 eV is the 1π ionization and the second IP at 14.011 eV is the 5σ ionization. A comparison of the experimental and theoretical intensity ratio between the two first PES progressions also supports this assignment. It is further supported by the fact that in the second IP the ν₃ vibration frequency is not changed as much as it is in the first IP, which is in agreement with the PES of N₂ and CO. The analysis of the bending vibrations also supports this ordering of the orbitals.The same orbital assignment has recently been proposed by Frost et al.⁵, using a comparison with the HCP photoelectron spectrum. The present paper supports their assignment of orbitals and (00⁰0)-(00⁰0) transitions. There are, however, some disagreements concerning the vibrational analysis. This is probably due to the fact that the HCN spectrum of Frost et al.⁵ revealed less structure than ours. As indicated by Figure 5 there is possibly still more structure to be revealed.     

Specific features of magnetic structure formation in orbitally degenerate BiMnO3 manganite

The orbital structure and magnetic ordering of the Jahn-Teller multiferroic BiMnO₃ manganite have been theoretically studied. It is shown that the orbital structure depends not only on the nearest-neighbor oxygen environment of manganese ions, but also on their next-to-nearest neighbors. The orbital structure significantly influences the magnetic order that forms as a result of competition between ferromagnetic and antiferromagnetic exchange interactions.     

Long-range coulomb interaction in ionic crystals

Expressions have been proposed for calculating the matrix elements of the Coulomb interaction of p and d electrons in a chosen ion of a crystal with an infinite crystal lattice. The matrix elements have been calculated at Gaussian-type orbitals. The Coulomb interaction energy per molecular unit of the ????-NaV₂O₅ crystal has been calculated in the ionic approximation for homogeneous and chain orderings. It has been shown that the more correct determination of the energetic favorability of one or other ordering requires calculation of the Coulomb interaction energy with an infinite crystal lattice of electrons that are at different orbitals of the ion under consideration.     

Predicted spin-orbit coupling effect on the magnetic ordering of crystalline uranium dioxide

The magnetic ordering of fluorite structure uranium dioxide has been investigated using fully-relativistic linear combinations of Gaussian type orbitals - fitting function (LCGTO-FF) calculations, within the generalized gradient approximation (GGA) to density functional theory. Three types of collinear spin-orderings were considered; ferromagnetic with spins aligned in the (001) direction and two antiferromagnetic (001) layer structures with spins aligned either perpendicular to each plane (001) or parallel to each plane (100). For each ordering, the total energy and spin-moment were calculated both with and without spin-orbit coupling. The ferromagnetic ordering is found to be energetically preferred to the antiferromagnetic orderings, contrary to experiment, whether or not spin-orbit coupling is included. Spin-orbit coupling is shown to have a significant quenching effect on the spin-moment and also introduces a strong magnetic anisotropy in the antiferromagnetic state that favors the (001) alignment over the (100) alignment.Received: 9 August 2003, Published online: 19 November 2003PACS: 71.15.Rf Relativistic effects - 75.25. + z Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source X-ray scattering, etc.) - 75.50.Ee Antiferromagnetics - 75.30.Gw Magnetic anisotropy - 64.30. + t Equations of state of specific substances     

Symmetry properties of the phase of coexistence of superconductivity and antiferromagnetism in 2D systems with strong electron correlations

Using the diagram technique for Hubbard operators, the effect of quasi-two-dimensionality and hybridization of the 4f electrons of cerium ions and p electrons of indium ions on the properties of the antiferromagnetic, superconducting, and mixed phases in heavy-fermion intermetallic compounds of cerium is studied. It is shown that taking into account quasi-two-dimensionality, low-energy hybridization processes renormalize the antiferromagnetic and superconducting order parameters in the broken time-reversal symmetry phase. Estimates of the critical temperatures of antiferromagnetic ordering and Cooper instability, obtained by the developed approach, are in good agreement with experimental data for cerium-based intermetallic compounds.     

Photoelectron spectroscopy of 9,10-dihaloanthracenes

The photoelectron spectra of anthracene and its 9,10-dichloro- and dibromo-derivatives have been obtained. Analysis of the spectra is based mainly on the shifts in the anthracene molecular orbital ionization energies observed upon substitution. These shifts are interpreted on the basis of a perturbation model comprising short-range and long-range inductive terms and a mesomeric term. The electronic perturbation caused by the halogen atom is observed to depend largely on the unperturbed energy of the halogen np electrons and the charge distribution in the anthracene molecular orbitals. It has been possible to separate out Coulombic interactions and mesomeric interactions which depend on the energy gap between hydrocarbon and substituent orbitals. Although the observations are not conclusive, owing to the fact that only two orbitals have measurable mesomeric shifts, results indicate that α_Cl′, the Coulombic perturbation parameter, may well be negative, in contrast to previous work.     

Photoelectron spectra of bis-cyclopentadienyl metal dihalides

He(I) and HE(II) photoelectron spectra are reported for (η-C₅H₅)₂MX₂ (M = Ti; X = F, Cl, Br, I: M = Zr, Hf; X = Cl, Br: M = Ta; X = Cl, Br) and (η-MeC₅H₄)₂MX₂ (M = Nb, X = Cl: M = Mo; X = Cl, Br, I). A substantial variation is found in the ordering of the halogen and cyclopentadienyl ionizations, the order being dependent on the metal as well as on the halogen. The compounds may be divided into three classes, namely, those in which the electrons in cyclopentadienyl e₁ orbitals ionize at a lower energy than those occupying halogen pπ orbitals, those in which halogen pπ electrons have lower ionization energy than cyclopentadienyl e₁ electrons and those in which the corresponding electrons arise from extensively delocalized molecular orbitals with significant contributions from both these categories of fragment orbital.     

Microstrain sensitivity of orbital and electronic phase separation in SrCrO3

An orbital ordering transition and electronic phase coexistence have been discovered in SrCrO3. This cubic, orbitally-degenerate perovskite transforms to a tetragonal phase with partial orbital order. The tetragonal phase is antiferromagnetic below 35-40 K, whereas the cubic phase remains paramagnetic at low temperatures. The orbital ordering temperature (35-70 K) and coexistence of the two electronic phases are very sensitive to lattice strain. X-ray measurements show a preferential conversion of the most strained regions in the cubic phase. This reveals that small fluctuations in microstrain are sufficient to drive long range separation of competing electronic phases even in undoped cubic oxides.     

典型磁性材料价电子结构研究面临的机遇与挑战

目前在磁性材料磁有序现象研究中广泛使用的交换作用、超交换作用和双交换作用模型形成于1950年代及其以前,这些模型都涉及材料中的价电子状态,但那时还没有充分的价电子状态实验依据.1970年代以来,有关价电子结构实验结果的报道越来越多,这些实验结果表明传统的磁有序模型需要改进.首先,大量电子谱实验表明,在氧化物中除存在负二价氧离子之外,还存在负一价氧离子,并且负一价氧离子的含量可达30%或更多.这说明以所有氧离子都是负二价离子为基本假设的超交换和双交换作用模型需要改进.其次,一些实验证明,铁、钴、镍自由原子的一部分4s电子在形成铁磁性金属的过程中变成了3d电子,这为探讨金属磁性与电输运性质的关系提供了依据.此外,即使在现代的密度泛函计算中,仍不能给出磁性交换作用能的函数表达式,只能采取各种不同模型进行模拟计算,从而使磁性材料的模拟计算遇到严重困难.寻求一个磁有序能的函数表达式可能是解决这个困难的途径.这些研究表明磁性材料价电子结构研究面临着重大的机遇与挑战.本文首先介绍一些典型的实验例证,然后介绍了基于这些实验结果的一套典型磁性材料的磁有序新模型,随后介绍了基于新模型的磁性材料价电子结构与旧模型的主要区别,最后指出了未来研究工作面临的挑战.     

The role of orbital ordering in the formation of electron structure in undoped LaMnO3 manganites in the regime of strong electron correlations

The electron structure of undoped LaMnO₃ and slightly doped La_1?x Sr_xMnO₃ manganites has been calculated within the framework of a generalized tight binding method with explicit allowance for strong intraatomic electron correlations. According to the results of these calculations, the ground state in orbitally disordered undoped LaMnO₃ ferromagnets would be metallic despite the Mott-Hubbard correlation gap in the spectrum of quasiparticles. Owing to the orbital ordering, the insulating state is stabilized in both antiferromagnetic and paramagnetic phases. In-gap states of a polaron nature with a spectral weight proportional to the dopant concentration have been found near the top of the valence band in La_1?x Sr_xMnO₃. As the doping level increases, a metal state appears in the ferromagnetic phase, which has a metallic character for one spin subband and an insulating character for the other subband (representing the so-called half-metallic state).