Many-electron model of band structure and metal-insulator transition under pressure in FeBO<Subscript>3</Subscript>

A many-electron model is proposed for the band structure of FeBO₃ with regard to strong electron correlations in the d⁴, d⁵, and d⁶ configurations. Under normal conditions, FeBO₃ is characterized by a dielectric charge-transfer gap in the strong correlation regime U?W. With increasing pressure, not only does the d-band W width grow but simultaneously the effective Hubbard parameter U_eff sharply drops, which is due to the crossover of high-spin and low-spin ground state terms of the Fe²⁺, Fe³⁺, and Fe⁴⁺ ions. It is predicted that a transition from the semiconducting antiferromagnetic state to the metallic paramagnetic state will occur in the high-pressure phase with increasing temperature.     

Electronic and magnetic properties of Fe<Subscript>2</Subscript>SiC

The electronic structure and magnetic properties of Fe₂SiC compound have been studiedusing the framework of an all-electron full-potential linearized augmented-plane wave(FP-LAPW) method within the local density (LSDA) and + U corrected(LSDA + U)approximations. An antiferromagnetic spin ordering of Fe atoms is shown to be the groundstate for this compound. From the electronic band structures and density of states (DOS),Fe₂SiC has ametallic character and from the analysis of the site and momentum projected densities, itis deduced that the bonding is achieved through hybridization of Fe-3d with C-2p states andFe-3d withSi-3pstates. It is also pointed out that the Fe-C bonding is more covalent than Fe-Si. In theFM phase, the spin polarized calculations indicate that the total magnetic moment ofFe₂SiC increasesfrom 0.41 to 4.33μ _B when the Hubbard U parameter for iron isconsidered.     

Similarity and dimensional theory for galaxies: Explanation of long-known results of observations

A practical extension of the similarity and dimensional theory to the case of several similarity parameters is proposed. On this basis, for galaxies an explanation is given for the empirical correlations noticed in the last quarter of the 20th century: the Tully–Fisher relation, the concept of a fundamental plane, etc. For galaxies, apart from the virial, there is another similarity parameter whose choice is arbitrary. Here, it is introduced in the simplest form for an empirical determination:Π₁ = U ₀/U _d, U ₀ is the observed velocity, the scale U _d = (GL)^1/5, where L is the object luminosity, G is the gravitational constant.     

On the theory of superconductivity in the extended Hubbard model

A microscopic theory of superconductivity in the extended Hubbard model which takes into account the intersite Coulomb repulsion and electron-phonon interaction is developed in the limit of strong correlations. The Dyson equation for normal and pair Green functions expressed in terms of the Hubbard operators is derived. The self-energy is obtained in the noncrossing approximation. In the normal state, antiferromagnetic short-range correlations result in the electronic spectrum with a narrow bandwidth. We calculate superconducting T _c by taking into account the pairing mediated by charge and spin fluctuations and phonons. We found the d-wave pairing with high-T _c mediated by spin fluctuations induced by the strong kinematic interaction for the Hubbard operators. Contributions to the d-wave pairing coming from the intersite Coulomb repulsion and phonons turned out to be small.     

First principles study on magnetic properties in ZnS doped with palladium

The electronic structures and magnetic properties in zinc-blende structure ZnS doped with nonmagnetic noble metal palladium have been investigated by means of density functional theory (DFT) calculations employing the generalised gradient approximation (GGA) and the GGA plus Hubbard U (GGA + U). Both the GGA and GGA + U methods demonstrate half-metallicity in Pd-doped ZnS with total magnetic moments of about 2.0μ _B per supercell. The half-metallic ferromagnetism stems from the hybridisation between Pd-4d and S-3p states and could be attributed to a double-exchange mechanism. These results suggest a recipe for obtaining a promising dilute magnetic semiconductor by doping nonmagnetic 4d elements in ZnS matrix.     

Effect of spin structures and nesting on the shape of the fermi surface and pseudogap anisotropy in the <Emphasis Type="Italic">t</Emphasis>-<Emphasis Type="Italic">t</Emphasis>′-<Emphasis Type="Italic">U</Emphasis> Hubbard models

The effect of two types of spin structures on the shape of the Fermi surface and on the map of photoemission intensities for the t-t′-U Hubbard model is investigated. The stripe phase with a period of 8α and the spiral spin structure are calculated in the mean field approximation. It is shown that, in contrast to electron-type doping, hole-doped models are unstable to the formation of such structures. Pseudogap anisotropy is different for h-and e-doping and is determined by the spin structure. In accordance with ARPES data for La_2?xSr_xCuO₄, the stripe phase is characterized by quasi-one-dimensional FS segments in the vicinity of points M(±π, 0) and by suppression of the spectral density for k _x =k _y . It is shown that spiral structures exhibit polarization anisotropy: different segments of the FS correspond to electrons with different spin polarizations.     

Energy levels of ions of silver and rhodium isoelectronic sequences with <Emphasis Type="Italic">Z</Emphasis> < 86

The method of extrapolation of the parameter of a model potential for states of one electron (4f, 5s, 5p, 5d, 5f) above the core 1s ²2s ²2p ⁶3s ²3p ⁶3d ¹⁰4s ²4p ⁶4d ¹⁰ and one vacancy (4d ⁹) in the same core is applied to calculate the energy levels in the silver and rhodium isoelectronic sequences with the maximum nuclear charge Z = 86. The energy levels of Ag-and Rh-like ions were used for the calculation of the energies of resonance transitions to the ground state ¹ S ₀ in Pd-like ions. Good agreement between the theoretical and calculated energies of the resonance transitions in Pd-like ions indicates the reliability of the results obtained.     

Ground State Energy of the Low Density Hubbard Model

We derive a lower bound on the ground state energy of the Hubbard model for given value of the total spin. In combination with the upper bound derived previously by Giuliani (J. Math. Phys. 48:023302, [2007]), our result proves that in the low density limit the leading order correction compared to the ground state energy of a non-interacting lattice Fermi gas is given by 8π a ? _u ? _d , where ? _u(d) denotes the density of the spin-up (down) particles, and a is the scattering length of the contact interaction potential. This result extends previous work on the corresponding continuum model to the lattice case.     

Calculation of the binding energy of a highly excited electron in heavy Be-like ions

The binding energy of a highly excited electron (n=20–50, where n is the principal quantum number) in the Be-like ions Au⁷⁵⁺, Pb⁷⁸⁺, Bi⁷⁹⁺, and U⁸⁸⁺ is calculated in order to determine the energy of the 2p _1/2-2s transition from experiments on dielectronic recombination in Li-like ions. A formula approximating the calculated data with an error not exceeding 0.04% is proposed.     

Coexistence of superconducting and spiral spin orders: Models of ruthenate

The effect of a spiral spin structure on superconducting (SC) pairing in a three-band Hubbard model related to Sr₂RuO₄ is analyzed in the mean-field approximation. Such a structure with incommensurate vector Q=2π (1/3, 1/3) is the simplest one that removes the nesting instability of α and β bands. It is assumed that there is an intralayer pairing interaction between two types of neighbor sites, those with attraction in a singlet channel and with attraction in both two-singlet and triplet channels. In both cases, a mixed singlet-triplet SC order is observed in the γ band: a d-wave singlet order is accompanied by the formation of p-wave triplet pairs (k,-k-Q)? and (k,?k+Q)? with large total momenta ?Q and the spin projections ±1 onto an axis perpendicular to the spin rotation plane of the spiral spin structure. Both the SC and normal states are states with broken time-reversal symmetry. In contradiction to the experiment, the models give different scales of T _c for the γ band and for α and β bands. This fact shows that the models with intralayer interactions or with the spin structure assumed are insufficient.     

Self-similar magnetic structures and magnetic flux “Giant” creep

The penetration of a magnetic flux into a type-II high-T _c superconductor occupying the half-space x > 0 is considered. At the superconductor surface, the magnetic field amplitude increases in accordance with the law b(0, t) = b ₀(1 + t)^m (in dimensionless coordinates), where m > 0. The velocity of penetration of vortices is determined in the regime of thermally activated magnetic flux flow: v = v ₀exp?ub;?(U _0/T )(1-b?b/?x)?ub;, where U ₀ is the effective pinning energy and T is the thermal energy of excited vortex filaments (or their bundles). magnetic flux “Giant” creep (for which U ₀/T? 1) is considered. The model Navier-Stokes equation is derived with nonlinear “viscosity” v ∝ U ₀/T and convection velocity v _f ∝ (1 ? U ₀/T). It is shown that motion of vortices is of the diffusion type for j → 0 (j is the current density). For finite current densities 0 < j < j _c, magnetic flux convection takes place, leading to an increase in the amplitude and depth of penetration of the magnetic field into the superconductor. It is shown that the solution to the model equation is finite at each instant (i.e., the magnetic flux penetrates to a finite depth). The penetration depth x _eff ^A (t) ∝ (1 + t)^{(1 + m/2)/2} of the magnetic field in the superconductor and the velocity of the wavefront, which increases linearly in exponent m, exponentially in temperature T, and decreases upon an increase in the effective pinning barrier, are determined. A distinguishing feature of the solutions is their self-similarity; i.e., dissipative magnetic structures emerging in the case of giant creep are invariant to transformations b(x, t) = β^m b(t/β, x/β^{(1 + m/2)/2}), where β > 0.     

The mystery of abnormally large volume of PbCrO<Subscript>3</Subscript> with a structurally consistent Hubbard U from first-principles

PbCrO₃ is found experimentally to be a cubic perovskite with an abnormally large lattice constant of about 4.00 Å. To date, no successful first principle calculations have been carried out for cubic PbCrO₃ perovskite. Combining the generalized-gradient approximations and a structurally consistent Hubbard U approach, the crystal, electronic and magnetic structures of PbCrO₃ are calculated to explore the mystery of abnormally large volume of PbCrO₃ in cubic structure. The crystal structure, magnetic order and half-metallic properties of PbCrO₃ are determined. For the first time, a structurally consistent Hubbard U _eff of about 8.28 eV for PbCrO₃ is calculated based on linear-response approach. With the structurally consistent U obtained by first principles, a \(Pm\bar 3m\) cubic structure with a lattice constant of about 3.98 Å for PbCrO₃ is obtained successfully. Moreover PbCrO₃ is found to be half-metallic ferromagnet with an integral magnet moment of 2.00μ _B per unit cell. The anomalously large volume of PbCrO₃ in cubic structure results from the strongly correlated electron interaction. The results obtained agree well with experimental data.     

Electronic structure of the Np<Emphasis Type="Italic">MT</Emphasis> <Subscript>5</Subscript> (<Emphasis Type="Italic">M</Emphasis> = Fe,Co, Ni; <Emphasis Type="Italic">T</Emphasis> = Ga,In) series of neptunium compounds

Evolution of the electronic structure of the NpMGa₅ (M = Fe, Co, Ni) series of neptunium compounds, whose crystal structure is similar to that of the known family of Pu115 superconductors, was studied by the LDA + U + SO method. The calculations took into account both the strong electron correlations and the spin?orbit coupling in the 5f shell of neptunium. For the first time, the electronic structure was calculated for a hypothetical series of compounds in which gallium is replaced with indium. Parameters of the crystal structure of the given series were obtained using the relationship between the parameters of the crystal structure of the earlier-studied compounds PuCoGa₅ and PuCoIn₅. The analysis of the electronic structure and characteristics of neptunium ions calculated in the framework of the LDA + U + SO method showed that the neptunium ions in NpMIn₅ with M = Fe, Co, and Ni should have an electron configuration closer to f⁴, but a spin and magnetic characteristics close to those in NpMGa₅.     

Thermal quenching of luminescence of BaY<Subscript>2</Subscript>F<Subscript>8</Subscript> crystals activated with Er<Superscript>3+</Superscript> and Tm<Superscript>3+</Superscript> ions

Thermal quenching of interconfigurational 5d-4f luminescence of Er³⁺ and Tm³⁺ ions in BaY₂F₈ crystals is studied in the temperature range of 330–790 K. The quenching temperatures are ~575 and ~550 K for Er³⁺ and Tm³⁺, respectively. It is shown that quenching of 5d-4f luminescence of Tm³⁺ ions is caused by thermally stimulated ionization of 5d electrons to the conduction band.     

X-ray photoelectron spectroscopy of FeP phosphide

The structure of the outer and inner electron spectra of iron (2p, 3p, 3s, and 3d) and phosphorus (3s and 3p) atoms in FeP monophosphide is studied in detail by the X-ray photoelectron spectroscopy (XPS) method. On the basis of the analysis of the binding energy of electrons, as well as the parameters characterizing the structure of experimental spectra, a conclusion is made that Fe³⁺ (d ⁵) cations in FeP are stabilized in a state with intermediate value of the total spin (IS, S = 3/2). The range of values of intra-atomic parameters (10Dq, J _H ) is established in which the consideration of the high degree of covalence of Fe–P bonds may lead to the stabilization of (FeP₆)^15– clusters in the IS state.     

Investigation of a Spin Transition in a LaCoO<Subscript>3</Subscript> Single Crystal by the Method of X-Ray Magnetic Circular Dichroism at the Cobalt <Emphasis Type="Italic">K</Emphasis>- and <Emphasis Type="Italic">L</Emphasis><Subscript>2,3</Subscript>-Edges

Spin transitions of cobalt ions in LaCoO₃ single crystals have been studied by the method of X-ray magnetic circular dichroism (XMCD) at the K- and L_2,3-edges of Co³⁺ ions. The orbital momentum of cobalt ions obtained for the K-edge at the 3d level in the region of the spin transition in the temperature range from 25 to 120 K increases by a factor of approximately 1.6, whereas the slope of the magnetization curve value in the same temperature range and magnetic field increases by a factor of more than 10. XMCD experiments at the cobalt L_2,3-edges demonstrate gradual growth of the ratio of the orbital momentum to the spin one L/S from 0.48 to 0.53 in the temperature range from 60 K to 120 K.     

Determination of the effective nuclear charge for free ions of transition metals from experimental spectra

The refined set of values of the effective nuclear charge (\(\bar Z_{ef} \)) and the set of one-electron spin-orbital constants (ξ _3d ^teor ) are obtained for ions of transition metals with the help of a semiempirical method of calculation of the spectrum of free ions with allowance for the spin-orbital interaction. The effective nuclear charge, which is a variable theoretical parameter, was determined from a comparison of theoretically calculated ion absorption spectra with experimental ones. The form of the potential for the calculation of the spin-orbital constant was chosen such that the calculated value ξ _3d ^teor would coincide with the experimental one ξ _3d ^exp . The calculated set of values \(\bar Z_{ef} \), ξ _3d ^teor } can be used in semiempirical calculations of energy levels of 3d ions (ions with an incomplete 3d shell).     

The electron energy spectrum and superconducting transition temperature of strongly correlated fermions with three-center interactions

The renormalizations of the fermionic spectrum are considered within the framework of the t-J* model taking into account three-center interactions (H₍₃₎) and magnetic fluctuations. Self-consistent spin dynamics equations for strongly correlated fermions with three-center interactions were obtained to calculate quasi-spin correlators. A numerical self-consistent solution to a system of ten equations was obtained to show that, in the nearest-neighbor approximation, simultaneously including H₍₃₎ and magnetic fluctuations at n>n₁ (n₁ ≈ 0.72 for 2t/U = 0.25) caused qualitative changes in the structure of the energy spectrum. A new Van Hove singularity is then induced in the density of states, and an additional maximum appears in the T_c(n) concentration dependence of the temperature of the transition to the superconducting phase with order parameter symmetry of the d _x ²?y² type.     

The effect of high pressure on the structure and on the magnetic and electronic properties of nickel monoxide

A diamond anvil cell is used to investigate the effect of high pressure (up to 37.5 GPa) on the optical absorption spectra of a single crystal of nickel oxide (NiO). In addition, strain-gage measurements are used to experimentally investigate the V(P) equation of state at a hydrostatic pressure of up to 8.5 GPa in a high-pressure chamber of the “toroid” type. Measurements are performed at room temperature. Absorption bands are observed, which correspond to optical d-d transitions of Ni²⁺ ion in the crystal field of ligands ³A_2g → ³T_2g, ³A_2g → {au1}E_1g, ³A_2g → ³T_1g(F), and ³A_2g → ¹T_2g. The values of energy of these transitions increase linearly with pressure, and their pressure coefficients are 7.3 ± 0.2, 2.87 ± 0.9, 9.7 ± 0.5, and 8.9 ± 0.3 meV/GPa, respectively. The pressure derivative of the crystal field parameter 10Dq corresponding to the ³A_2g → ³T_2g transition gives the pressure dependence of the magnitude of exchange integral J in the Anderson hybridization model. It is found that, in the pressure range from zero to 37.5 GPa, the behavior of the exchange integral J is largely defined by the hybridization parameter b = (10Dq/3). At the same time, the Coulomb interaction parameter U_eff is independent of pressure and, therefore, has no effect on the variation of J. The Coulomb interaction U_eff ≈ 7.47 ± 0.005 eV is determined. The experimental data on the equation of state are used to derive the \(J \propto V^\varepsilon \) correlation, where ε = ?2.99 ± 0.15, which is in good agreement with the predictions of Bloch’s theory (ε = ?10/3).     

Hyperfeinstruktur-Untersuchungen im Rhenium-I-Spektrum

By optical means (i.e. hollow cathode, Fabry-Perot-interferometer and spectrograph) the hyperfine structure of 15 transitions in the Re-I spectrum was investigated. This permits to state the hfs coupling constantsA, B for 15 levels belonging to the configurations (5d ⁵) 6s ², (5d ⁵) 6s 6p, (5d ⁵) 6s 7s, (5d ⁶) 6s, (5d ⁶) 6p and (5d ⁴) 6s ² 6p. It was found that the (5d ⁵) 6s ^{2 6} S _5/2 ground state, too, shows a small hfs. Of its magnetic splitting both size and sign could be determined (A _0,185,187=?2,2±0,4 mK); for a possible quadrupole splitting upper limits were obtained. Estimates of the quadrupole moments resulted inQ _185,187?2,9b. — In the case of two levels (x ⁶ P _7/2 andy ⁶ F _11/2) the hfs analysis raises doubts as to their classification.