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Condensed matters exhibit tremendous number of orders and quantum many-body phenomena due to correlated motion of electrons with spin 1/2. We are interested in understanding these phenomena and their diversity on the basis of the quantum mechanics developing new theoretical framework of electron correlations. Current topics we are dealing with are as follows.
1.LDA + Dynamical CPA and Quantitative Understanding of Magnetism
The magnetic properties such as the Curie temperature and susceptibility are well-known to be caused by electron correlations at finite temperatures. Quantitative description of these properties in the intermediate regime between the weak and strong Coulomb interaction limits has been a long-standing problem in magnetism, because there are no simple parameters controlling the magnetism in this regime and because simple perturbation methods are not applicable to this regime. It is indispensable for understanding of the diversity of magnetic materials at finite temperatures. In the past decade, we have developed a nonperturbative theory called the dynamical CPA (coherent potential approximation) on the basis of the functional integral method. The theory describes the dynamical spin fluctuations from the weak to the strong Coulomb interaction regime within the single-site approximation. Towards the quantitative description of the finite temperature magnetism, we are developing a new theory which combines the dynamical CPA with the first-principles tight-binding LMTO (Linear Muffin-Tin Orbital) + LDA (Local Density Approximation) scheme.
2.Nonlocal Excitations in Correlated Electron System
The single-site theory of electron correlations using an effective medium, which are well-known as the many-body CPA in the disordered system, the dynamical CPA in the itinerant magnetism, the dynamical mean field theory in the metal-insulator transition, and the projection operator method CPA (PM-CPA) in the excitation problem, have been much developed in the past decade. The single-site approximation (SSA) however neglects the k-dependence of the self-energy, therefore does not describe properly the momentum dependent properties such as the quasiparticle state, the magnetic short range order, and the spin frustrations in magnetic materials. We are developing the self-consistent projection operator method (SCPM) which guarantees a high accuracy of the spectra with high energy and high momentum resolutions and takes into account the long-range intersite correlations as well as the strong on-site correlations. The SCPM allows us to clarify the nonlocal effects on the quasiparticle excitations, especially, the nonfermi liquid behaviors and the kink structure found in the angle resolved photoemission spectroscopy (ARPES) experiments in the high-Tc cuprates. The SCPM is also applicable to the metal-insulator transition, the heavy fermion phenomena, as well as the nonlocal excitations in frustrated systems.
3.First-Principles Molecular Dynamics Approach to the Complex Magnetic Structure
The metals and alloys with competing magnetic interactions show the complex magnetic structure and magnetism such as the noncollinear magnetism, spin glass, multi-stage magnetic phase transition, and the partially disordered state. It is not easy problem to determine the magnetic structure of these systems from the theoretical point of view because there exist many local minima in the free energy due to competition between the long-range ferro- and antiferro-magnetic interactions. The difficulty has prevented us for a long time from theoretical understanding of their magnetic structure and magnetism based on the microscopic Hamiltonian. In order to solve the problem, we recently proposed a molecular dynamics (MD) approach which automatically determines at finite temperatures the complex magnetic structure of the system with a large unit cell consisting of several hundreds atoms. We are developing the first-principles theory of the magnetic structure combining the MD approach with the theory of electronic structure calculations. By making use of the first-principles MD approach, we can clarify the magnetic structure and magnetism of the bulk metals and alloys in the high-pressure region, surface and thin film, and amorphous alloys, for which the experimental neutron diffraction technique is not available. Furthermore, using the quantitative MD approach one can control and design the magnetic structure and magnetism for the developments of magnetic materials.
Takafumi SHIMABUKURO
Research Theme ‘Gutzwiller-Type Wavefunction and Electron Correlations’.
George TSUTSUI
Research Theme ‘Realistic Calculations of Magnetism of Iron’.
Ming YU
Department of Physics, University of Louisville, USA
‘Finite-Temperature Theory of Metallic Magnetism for Amorphous Alloys’(1995).
Mohammad Shawkat AKBAR
Bangladesh Atomic Energy Commission, BANGLADESH
‘Molecular-Dynamics Theory of Itinerant Electron System to Determine Complex Magnetic Structures’(1998).
H.M. AL-ATTAR
Department of Physics, Assiut University, EGYPT
‘Interpolation Theory of Metallic Magnetism from Crystals to Amorphous Structures and its Development to the Noncollinear Magnetism’(2000).
