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.     

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   

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.     

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.     

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^∗.     

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.     

LaOFeAs的反铁磁能带结构

基于密度泛函理论（DFT）,采用线性缀加平面波展式结合改进的局域轨道方法（APW+lo）,对新型超导材料LaOFeAs的结构进行了计算和分析。反铁磁计算的结果表明,由于巡游电子的贡献,系统具有负的磁矩。费米面附近两种自旋电子的能带在Г点附近具有显著差异,自旋向下的一条空带在此形成了与自旋向上未满填充导带局域相似的结构。而在其他K点,这两种自旋能带的结构基本相同。     

LaOFeAs的反铁磁能带结构

基于密度泛函理论(DFT),采用线性缀加平面波展式结合改进的局域轨道方法(APW+lo),对新型超导材料LaOFeAs的结构进行了计算和分析.反铁磁计算的结果表明,由于巡游电子的贡献,系统具有负的磁矩.费米面附近两种自旋电子的能带在Г点附近具有显著差异,自旋向下的一条空带在此形成了与自旋向上未满填充导带局域相似的结构.而在其他K点,这两种自旋能带的结构基本相同.     

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.     

Enhanced Orbital Degeneracy in Momentum Space for LaOFeAs

The Fermi surfaces （FS） of LaOFeAs （in kz = 0 plane） consist of two hole-type circles around F point, which do not touch each other, and two electron-type co-centred ellipses around the M point, which are degenerate along the M - X line. By first-principles calculations, here we show that additional degeneracy exists for the two electron-type FS, and the crucial role ofF-doping and pressure is to enhance this orbital degeneracy. It is therefore suggested that the inter-orbital fluctuation is important to understand the unconventional superconductivity in these materials.     

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.