Time-dependent Gutzwiller theory for multiband Hubbard models

Based on the variational Gutzwiller theory, we present a method for the computation of response functions for multiband Hubbard models with general local Coulomb interactions. The improvement over the conventional random-phase approximation is exemplified for an infinite-dimensional two-band Hubbard model where the incorporation of the local multiplet structure leads to a much larger sensitivity of ferromagnetism on the Hund coupling. Our method can be implemented into local-density approximation and Gutzwiller schemes and will therefore be an important tool for the computation of response functions for strongly correlated materials.     

Generalized classical theory of magnetism

We consider an Ising model with Kac potential ^dK(¦x¦) which may have arbitrary sign, and show, following Gates and Penrose, that the free energy in the classical limit0+ can be obtained from a variational principle. When the Fourier transform of the potential has its maximum atp=0 one recovers the usual mean-field theory of magnetism. When the maximum occurs forp ₀0, however, one obtains an oscillatory or helicoidal phase in which the magnetization near the critical point oscillates with period 2/¦p ₀¦. An example with a potential possessing parameter-dependent oscillations is shown to exhibit crossover phenomena and a multicritical Lifshitz point in the classical limit.     

Phase diagram of NaxCoO2 studied by Gutzwiller density-functional theory

The ground state of NaxCoO2 (0.00.6 due to a_{1g} van Hove singularity near the band top, (2) correlated nonmagnetic metal without e_{g};{'} pockets for 0.3相似文献   

Peculiarities of the band magnetism in the higher manganese silicide

Magnetism of MnSi_x, x=1.746 (Mn₂₇Si₄₇) was investigated by SANS, neutron diffraction, magnetization and magnetic susceptibility measurements. MnSi_x single crystalline specimens were characterized by X-ray and neutron diffraction. A spiral spin structure with periodicity ?=(163±4) Å along the c axis and a spiral component of the magnetic moment per Mn of p_o=0.056 was determined. From the field dependence of ? it is indicated that the magnetic order below T_N=42 K is an incommensurate state. From the large difference of the magnetic moments in the paramagnetic state and ordered state MnSi_x is classified as an itinerant magnet. Below T_N the difference of the magnetic moments per Mn between p_o=0.056 at H = 0 from SANS and p_s = 0.014 at saturation field from magnetization measurements is explained by longitudinal spin fluctuations.     

The physical theory of rock magnetism

Magnetite and hematite are representative of the ferrimagnetic and weakly ferromagnetic minerals which are responsible for the magnetic properties of rocks reviewed in this paper. Magnetite grains are multi-domains in the size range of interest (0.1 μ–1000 μ), whereas hematite grains in the same size range are almost certainly single domains. Properties discussed are coercivity, susceptibility, magnetic viscosity, thermo- and isothermal remanence, alternating field demagnetization, anhysteretic, chemical and detrital magnetization, anisotropy, piezomagnetic effects and self-reversals. Problems requiring more experimental data are emphasized, especially the basal plane anisotropy of hematite, the Barkhausen discreteness of domains in magnetite and the possibility that grains of cubic minerals may have some uniaxial character arising from grain shape or internal strains.     

Paraconductivity of doped LaOFeAs superconductors

The paraconductivity of both electron- and hole-doped LaOFeAs superconductors is studied within the existed fluctuation mechanisms. It is found that the FC data at the temperature close to T_c can be explained with the three-dimensional (3D) Aslamazov-Larkin (AL) theory. The Gaussian-Ginzburg-Landau (GGL) approach under various cutoff conditions can only account for the fluctuation conductivity (FC) data in the 3D AL regime. While the approach taking into account the pseudogap effect can describe the paraconductivity very well in the whole temperature region. This result suggests that the pseudogap state of doped LaOFeAs system is probably due to the formation of paired fermions in the form of strongly bound bosons (local pairs) at T_c<T<T^∗.     

Recent progress in the theory of itinerant electron magnetism

A review is given of recent theoretical investigations toward unified understanding of magnetism in narrow-band electron systems. It is emphasized that the classical controversy between the itinerant and localized models have been resolved into a more general and well-defined problem of spin density fluctuations in a general sense. The local moment picture is a limiting form of general spin fluctuations; and in its opposite limit we have weakly ferro- and antiferromagnetic metals. As an approach from the latter limit, the self-consistent renormalization theory of spin fluctuations is shown to have been quite successful in explaining and predicting a number of qualitatively new physical properties of this class of materials. More recent theoretical studies of spin fluctuations from a general point of view interpolating between the above-mentioned two limits seem to lead to a unified picture of magnetism in narrow-band electron systems including 3d transition metals and magnetic compounds.     

Orthogonal- and nonorthogonal-orbital methods in the theory of magnetism

The effective spin Hamiltonian is obtained in the Dirac-van Vleck form by the method of nonorthogonal atomic orbitals with an estimate of virtual interatomic transitions. The equivalence of the orthogonal- and nonorthogonal-orbital methods is demonstrated for a bounded configurational interaction, with an accuracy to quadratic terms in the overlap integral.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 12, No. 3, pp. 59–67, March, 1969.In conclusion the author thanks B. V. Karpenko for formulating the problem, for consultation, and for discussions.     

A theory of magnetism for semi-metallic uranium compounds

A theoretical study is carried out to explain the magnetic properties of uranium compounds of NaCl, anti-Cu₂Sb and Th₃P₄ type. The electronic model used is a modified free electron model along the lines of the band structure for NaCl type uranium compounds previously reported. An exchange interaction via the conduction electrons is assumed for the mechanism of magnetic ordering. The molecular field approximation is used to calculate θ, T_N and the exchange interaction constants. The stability range of various magnetic ordering states and the variation of θ, T_N and exchange interaction constants are obtained as functions of lattice parameter. These results well explain the experimental data.     

The impact of renormalization group theory on magnetism

The basic issues of renormalization group (RG) theory, i.e. universality, crossover phenomena, relevant interactions etc. are verified experimentally on magnetic materials. Universality is demonstrated on account of the saturation of the magnetic order parameter for T ↦ 0. Universal means that the deviations with respect to saturation at T = 0 can perfectly be described by a power function of absolute temperature with an exponent ε that is independent of spin structure and lattice symmetry. Normally the T^ε function holds up to ~0.85T_c where crossover to the critical power function occurs. Universality for T ↦ 0 cannot be explained on the basis of the material specific magnon dispersions that are due to atomistic symmetry. Instead, continuous dynamic symmetry has to be assumed. The quasi particles of the continuous symmetry can be described by plane waves and have linear dispersion in all solids. This then explains universality. However, those quasi particles cannot be observed using inelastic neutron scattering. The principle of relevance is demonstrated using the competition between crystal field interaction and exchange interaction as an example. If the ratio of crystal field interaction to exchange interaction is below some threshold value the local crystal field is not relevant under the continuous symmetry of the ordered state and the saturation moment of the free ion is observed for T ↦ 0. Crossover phenomena either between different exponents or between discrete changes of the pre-factor of the T^ε function are demonstrated for the spontaneous magnetization and for the heat capacity.     

Ultrafast quasiparticle dynamics in spin-density-wave LaOFeAs single crystal

Ultrafast quasiparticle dynamics of single crystalline LaOFeAs were investigated by pump-probe measurement.The compound experiences structural and spin-density-wave(SDW)phase transitions at 150 K(TS1)and 130 K(TS2),respectively.The relaxation time of quasiparticles was somewhat temperature independent at high temperature but exhibited a sharp upturn at TS1and reached the maximum at approximately TS2.The remarkable slowing down of quasiparticle relaxation time is caused by the formation of energy gap.By employing the Rothwarf-Taylor model analysis,we found that there should be already energy gaps opening just below the structural transition.The magnitude of SDW gap was identified to be 72 meV.     

Structure and magnetism in Mn-doped zirconia: Density-functional theory studies

Using the first-principles density-functional theory plane-wave pseudopotential method, we investigate the structure and magnetism in 25% Mn substitutive- and interstitial-doped monoclinic, tetragonal, and cubic ZrO₂ systematically. Our studies show that the introduction of Mn impurities into ZrO₂ not only stabilizes the high-temperature phase, but also endows ZrO₂ with magnetism. Based on a simple crystal field model, we discuss the origination of magnetism in Mn-doped ZrO₂. Finally, we discuss the effect of electron donor on the magnetism.     

The interaction of electricity and magnetism in Einstein–Maxwell theory

I consider stationary axially symmetric solutions of the Einstein–Maxwell equations. These show that, in general, time-independent electric and magnetic fields acting together cause rotational effects in the spacetime. An electric charge placed on the axis of a magnetic dipole induces a region of closed timelike curves.     

Introduction in the band theory of molten salts

The electrochemical properties of molten salts are considered in the frame of band theory for ideal crystalline dielectrics. It turned out that metal impurities at low concentration in them generate the local electron levels in the wide band gap (>4 eV) of molten salt divided by 50 meV and more. They are occupied by electrons when Fermi level is approached to them from below. Then, the minor metal impurities fall out the molten salt when the electron occupation of their energy levels in the band gap achieves their solubility (<0.1 ppm) in the molten salt. At the same time, the given RedOx-potential of molten salt can be maintained without chemical additives but forced shifting of Fermi level by the electrochemical cell with one electrode strongly polarized and the other in equilibrium with this molten salt.     

Prediction of band gap reduction and magnetism in (Cu,S)-codoped ZnO

By employing a density functional theory plane-wave pseudopotential method, we investigated band gap reduction and magnetism as well as electronic structures of (Cu, S)-codoped ZnO. Our calculations indicated that Cu and/or S-doped ZnO can reduce the band gap of ZnO. The (Cu, S)-codoped ZnO has a large band gap reduction of 0.37 eV, two times larger than that in Cu-doped ZnO. S atom has no contribution for the total magnetic moment of (Cu, S)-codoped ZnO, whereas it plays a central role in spin-polarizing of both Cu and S dopants due to strong coupling between Cu 3d and S 3p states. This would offer a new strategy for designing narrow band gap devices with magnetism.     

The origin of magnetism in UNi2 — a simple band approach

The magnetization and susceptibility investigation of pseudo-binary U(Ni_1?xFe_x)₂ and U(Ni_1?xCu_x)₂ for chosen concentrations of x ? 0.1, x ? 0.8, and x ? 0.06, respectively, are presented. The most significant result is that the substitution of Ni (in UNi₂) by Fe reduces both the magnetic moment and the ordering temperature rapidly although it appears to be established that the magnetic moment of UFe₂ is predominantly residing on the Fe sites. The small concentration of Cu substituted into UNi₂, on the other hand, increases the magnetic moment. The obtained results are discussed together with those of U(Ni_1?xCo_x)₂ and seem to support the recently proposed explanation on the origin of magnetism in UNi₂.     

Electronic structure theory of surface, interface and thin-film magnetism

Low dimensional magnetic systems including surfaces, interfaces and thin-films, have attracted a great amount of attention in the past decade because, as expected, the lowered symmetry and coordination number offer a variety of opportunities for inducing new and exotic phenomena and so hold out the promise of new device applications. Local spin density functional (LSDF) ab initio electronic structure calculations played a key role in the development of this exciting field by not only providing a clearer understanding of the experimental observations but also predicting new systems with desired properties. Extensive calculated results reviewed here demonstrate that (1) weakened interatomic hybridization at clean surfaces or interfaces with inert substrates give rise to strong magnetic enhancement and (2) the strong interaction with nonmagnetic transition metals diminishes (entirely in some cases) the ferromagnetism and usually stabilizes the antiferromagnetic configuration. Surprisingly, experimentally observed surface (interface) magnetic anisotropy can be reproduced correctly in the theoretical calculations, although the anisotropy energy is only 10⁻⁴−10⁻⁵ eV.