Intra-atomic Coulomb Integrals Research Articles

Itinerant-electron ferrimagnetism of transition-metal compounds

The onset of ferrimagnetic spin moment is investigated on the itinerant-electron model for compounds composed of magnetic and non-magnetic transition-metal atoms. The d-states of non-magnetic atoms mix with those of the magnetic atoms. In the magnetic state the strength of the d-d mixing is different in the majority and minority spin bands and then the magnetic moment on non-magnetic atom is induced. By using a two-band model, the induced moment on the non-magnetic atoms is shown explicitly to align in either the parallel or antiparallel direction to that of the magnetic atoms, depending on the value of intra-atomic Coulomb integral on the magnetic atom.

Theory of magnetic and thermal properties of the weak itinerant ferromagnet Ni 3Al

Results of band structure calculations are used to calculate the temperature dependence of magnetization, fluctuation of the local moment, paramagnetic susceptibility and the specific heat of the weak itinerant ferromagnet Ni 3Al. Calculations of these quantities are performed over a broad temperature range from 0 K to about 5 T c. Arrott plots are presented. The functional integral method in the static approximation is used. The model with the intra-atomic Coulomb integral as the only interaction parameter is applied, its value is determined from the ground state magnetic moment. Results compare favourably with experimental data.

Correlated one-hole spectra of YBa2Cu3O7 in the bulk and at the surface

The one-hole valence spectrum of YBa2Cu3O7 is calculated using an accurate tight-binding hamiltonian and a Hubbard model to treat correlation effects. Intra-atomic Coulomb integrals for holes sitting on Cu or O sites are considered and the one-hole self-energy is computed in the low density approximation. The calculated spectrum is compared with photoemission and X-ray emission experiments. This allows to point out a number of problems that arise in assigning the observed structures to a specific atom and to understand some aspects of the data. To evaluate the role of the surface in determining the shape of the photoemission energy distribution curves, the one-hole spectrum is calculated for the Ba-O4 surface. Oxygen induced surface states are shown to be important in determining the intensity of the observed structures.

Tight-binding calculation of the local moments in ordered FeV alloy

A tight-binding type self-consistent band calculation is used to study the local magnetic moments of the ordered FeV alloy with CsCl-type crystal structure within the Hartree-Fock approximation of the Hubbard model for the spin polarisation. It is shown that the reasonable choice of the parameters for the numbers of d-electrons and intra-atomic Coulomb integrals can account for the experimental values of the local moments if the hopping integrals between Fe and V are chosen to be about 6% smaller than the usual geometrical average of the hopping integrals for pure metals.

Three-Body Problem in the One-Dimensional Hubbard Model

Electron correlation is studied in the special case of a single reversed spin in an otherwise fully aligned band in the one-dimensional Hubbard model for the purpose of examining our previous theory which exactly takes account of the three-body correlation. The importance of the three-body correlation is demonstrated through a comparison of our results with those of exact theories. The ground-state energy and the single particle spectra are in good agreement with McGuire-Lieb-Wu's exact solution and also with exact numerical solutions for finite size systems over a wide range of the intraatomic Coulomb integral and the electron number. The properties of the ground state are studied in detail through the calculation of pair correlation functions.

Magnetic Structure at Ground State of Hubbard Model

The magnetic structure at the ground state of the single band Hubbard model is investigated. The extended Gutzwiller method is adopted. It is found, if the intra-atomic Coulomb integral is sufficiently large, that the ferromagnetic or antiferromagnetic ground states are obtained at nearly half-filled band in the cases of the simple cubic and body centered cubic structures and that the ferromagnetic ground states are obtained at more than half-filled band in the cases of the face centered cubic and hexagonal closed packed structures.

Remarks on coupled spin and charge fields in the Hubbard hamiltonian

This paper deals with the combined effect of spin and charge fluctuations on the thermodynamic properties of an itinerant electron paramagnet described by the Hubbard Hamiltonian. Firstly, we discuss the functional integral formulation of the partition function. We argue that a correct formulation must preserve the spin rotational invariance of the Hamiltonian. As a consequence, the Landau-Ginzburg-Wilson free energy functional contains both spin and charge fluctuation fields, with coupling terms of the form ρM2, ρ2 M2, etc... where p is a charge (scalar) field, and M is a vector spin field with 3 components. The relevant coupling between spin and charge fields is [FORMULA] where n(e) is the density of states, eF is the Fermi level, and u is the intra-atomic Coulomb integral. It is pure imaginary. Next, we exploit the similarity between the free energy which we obtain and various other cases of coupled fields, such as metamagnets, or other phenomenological functionals. The role of constraints such as charge neutrality is discussed, with respect to critical properties, using the renormalization group method.

Spin Paramagnetic Susceptibilities of Pd-Ni, Pt-Ni and Pd-Pt Alloys

The generalized spin susceptibility of isoelectronic and paramagnetic alloys is calculated by assuming the Wolff-Moriya model in the random phase approximation and in the coherent potential approximation. It is just an extension of Lederer and Mills's result for one impurity system to concentrated alloys. A critical concentration at which ferromagnetism appears can be found from the divergence of the uniform static spin susceptibility of alloys. Uniform static spin susceptibilities of Pd -Ni, Pt -Ni and Pd-Pt alloys are numerically calculated by choosing appropriate values of intra-atomic Coulomb integrals on Pd, Pt and Ni atoms. The calculated results are compared with the experimental results. The estimated values of the intraatomic Coulomb integrals in these alloys are compared with each other.

Magnetic Properties of (Zr1-xTix)Zn2 and (Zr1-xHfx)Zn2 Alloys

Magnetic properties of ZrZn 2 alloys with Ti and Hf are investigated by making use of the Wolff-Clogston model extended to the case of many impurities in the self-consistent Hartree-Fock approximation. The self-consistent equations for Green's function in the magnetic case are derived, by using the result obtained by the graphic method for a disordered system. They are solved explicitly in lower concentrations up to the second-order terms with respect to the impurity potential and the difference between intra-atomic Coulomb integrals at an impurity and a host sites. By the solution corresponding to the Stoner model for a virtual crystal, the experimental results of the spontaneous magnetization, Curie temperature and spin susceptibilities of (Zr 1- x Ti x )Zn 2 and (Zr 1- x Hf x )Zn 2 alloys are explained satisfactorily. The correction to the solution for a virtual crystal is estimated to be small. It is concluded that the difference between magnetic properties of these alloys can be explained by the chan...

Magnetic field dependence of the metal-insulator 'Mott transition': a study of the compound VO2

A theory of the metal-insulator transition, taking into account the effects of an external magnetic field is presented. Comparison with experimental susceptibility and optical data for VO2 is good and yields for theoretical parameters the values: Intra-atomic Coulomb integral I approximately=0.7 eV; density of states N(Ef) approximately=1023 cm-3 eV-1 ratio IN(Ef)/N approximately=10; optical energy gap Eg approximately=0.7 eV. The insulating phase stability criterion is shown to be identical with Mott's. The existence of a critical field Hc such that higher magnetic fields destroy the insulating phase is derived.

Theory of Localized vs Band Magnetic Semiconductors

We have considered the question of whether the Hubbard Hamiltonian can lead to properties characteristic of two types of semiconductors, depending on the value of the ratio Δ/I of bandwidth to intra-atomic Coulomb integral. In one type there is a transformation, with increasing T, from a magnetic insulating state to a paramagnetic insulating state, and in the other the system goes from a magnetic insulating state to a paramagnetic metallic state. We have applied a new variational single-determinantal approximation which, in contrast to the standard thermal Hartree-Fock approximation, duplicates the exact behavior of the model both in the atomic limit, Δ/I=0, and the band limit Δ/I=∞. Limiting ourselves to well-known types of one-electron states, we have obtained stability boundaries (as determined by the free energy) between various phases. While the boundaries obtained to date have intrinsic interest, we find that further calculations are necessary to completely answer the question raised above.

Dynamical Spin Susceptibility and Spin Waves in Ferromagnetic Dilute Alloys

A general expression of the dynamical transverse spin susceptibility for ferromagnetic dilute alloys of transition metals is obtained in the ladder approximation by using the Wolff-Clogston model for dilute alloys and by taking into account exchange interactions among electrons in the host metal. The dispersion relation of spin waves is calculated from the pole of the dynamical transverse spin susceptibility by making use of the one-particle Green's function obtained in the Hartree-Fock approximation and by assuming that both impurity potential and the difference between intra-atomic Coulomb integrals in the impurity and host atoms are small. The values of exchange stiffness constants are estimated and compared with experimental data for dilute nickel alloys with iron, copper and palladium. The possibility of the appearance of a local spin wave mode is discussed.