Showing papers by "Shoji Yamamoto published in 1992"

Broken symmetry solutions of the two-dimensional extended Hubbard model and their instability conditions

Abstract: Instabilities of the restricted Hartree–Fock (HF) state of the two-dimensional extended Hubbard model under doping are studied with the linear response theory employing the linearized Bethe–Salpeter equation. Boundaries of the instabilities for the ordering vectors q = Q ≡ (π, π) and q = Q0 ≡ (0, 0) are given in terms of the interaction strength parameters at various doping levels. With doping, dominant instabilities generally change from ones for q = Q to ones for q = Q0. Broken symmetry phases obtained from the instabilities are also studied and classified using the group-theoretical technique. For q = Q, two more new phases are obtained in addition to the four nonmagnetic and five magnetic phases that have been already derived by one of the authors. For q = Q0, three nonmagnetic and five magnetic phases are obtained. Deriving the explicit forms of the charge current and spin current operators in the HF approximation, charge current and spin current densities in the broken symmetry states are discussed in detail. © 1992 John Wiley & Sons, Inc.

Superpolaron Model for Metallic Polyacetylene

Abstract: We present a theory for the metallic phase of heavily doped polyacetylene. The Hartree-Fock (HF) ground state in this system is a charged soliton (S ± ) lattice that has a nearly periodic long range order in the charge density as well as the one in lattice distortion. We take account of electronic correlation effects by considering superpolarons (SP's) as local fluctuations in the electronic order parameter (OP). They are electrons and holes injected into a S ± lattice that make no lattice distortions but are selftrapped and make local defects in the electronic OP. They make intrachain quantum translation and interchain hopping through dopants. The formation energy of a SP pair is reduced by these quantum motions and become negative in a high doping regime. Therefore, the ground state in this regime is a S ± lattice with spontaneously produced SP's. The SP's may be regarded as Fermion quasi-particles. Since they partially occupy the translational bands of SP's, metallic properties appear in this phase.

Resonating Hartree-Fock Theory of the One Dimensional Hubbard Model. : II. Breather Fluctuations in the Strong Correlation Regime

Abstract: The electronic structure of the strongly correlated one dimensional Hubbard model is calculated by the resonating Hartree-Fock (Res HF) method. In the strong correlation regime with the Coulomb repulsion U >∼6, SDW solitons localize almost on the central two sites and only one kind of neutral soliton pair has a much smaller energy than the other pairs. As quantum fluctuations in a periodic chain with 12 sites, we consider up to three diradical breathers consisting of the low energy neutral soliton pair. The Res HF ground state can explain 77.8% ( U =6) and 80.1% ( U =8) of the exact correlation energy. This is much better than the Gutzwiller method to use the SDW as the reference state that explains about 50% of the correlation energy. The breather fluctuations explain the rapid decreases in the short distance of the spin and staggered spin correlation functions. Other correlation functions and the excited states due to the quantum fluctuations are studied.

Physics aspects of tke iter design

Abstract: The ITER physics group has been responsible for developing the physics guidelines and physics design requirements for ITER, the design of the diagnostic systems and the development and coordination of the ITER Physics R and D Program. These requirements have been based on tokamak experimental data and credible extrapolations of that data. Assessment of the energy confinement and MHD stability requirements led to the choice of the major plasma parameters of 22 MA for the plasma current, a toroidal field of - 5 T, aspect ratio of - 3 and an elongation of - 2. Among the major accomplishments of the physics group has been the development of a database and an empirical xaling for L-mode energy confinement and the facilitation of an H-mode database and scaling. The divertor heat loads have been estimated by using experimentally validated models. The thermal and mechanical loads due to offnormal events such as disruptions have been based on analysis of the data from tokamaks such as TFTR, JET, JT-60, DIII-D, and TORESupra. To achieve the required availability of IO%, the pulse length has been extended by the use of current drive using 75 Mw 1.3 MeV neutral beam and 45 MW Lower Hybrid systems. Plasma shaping and control of the high beta, elongated plasma is provided by seven pairs of poloidd field coils located exterior to the toroidal field coils. A relatively complete set of plasma diagnostics is planned for ITER. Finally, a Physics R and D program has been developed to ensure that the intemational fusion program will address the issues which either introduce uncertainties into the design or lead to demanding design requirements. Phvsics Basis of ITER The goals of the Intemational Thermonuclear Experimental Reactor (ITER) are to demonstrate and study controlled, long pulse ignited plasma operation and to cany out engineering tests of high heat flux and nuclear components in order to establish the enginering and physics database for the design of a demonstration power reactor[ 11. The testing mission will require integrated plasma operation of about one year ( 3 x 107 s) with an average neutron wall loading of 1 MW/m2. To achieve these goals, the ITER plasma must be able to achieve an adequate level of plasma performance. The plasma must have adequate energy and fast alpha particle confinement to ignite. It must have sufficient MHD stability to achieve a high level of fusion power without disruptions. The power and particle control system must exhaust the fusion and auxiliary power and thermalized alpha particles without contaminating the plasma or damaging the plasma facing components. The damage from plasma disruptions must be acceptable. The current drive and auxiliary heating systems must be able to heat the plasma to ignition and to provide non-inductive current drive sufficient to increase the pulse length to at least 1000 s with an ultimate goal of steady state operation. The poloidal field system must be able to shape and control the plasma. The necessary plasma diagnostics systems must be developed and integrated into the machine design. The development of the physics to support the accomplishment of these tasks must be closely coordinated with the engineers who design the tokamak and tokamak systems. In many areas, the physics tasks lead to the definition of design requirement such as the plasma current required for ignition. In other areas, the tasks involve joint development by physicists and engineers of design concepts for the relevant systems, such as the divertor or current drive systems. The physics basis of ITER has been developed from an assessment of present tokamak physics and credible extrapolations of that physics. This assessment has been carried out with the assistance of the intemational fusion community, including participation by scientists from all of the major toroidal experiments in the world and a large portion of the theoretical plasma physics community. On this basis, a set of guidelines for energy confinement, operational limits, power and particle control, disruptions, current drive and heating, alpha particle physics, and plasma control has been developed. The formation and implementation of the guidelines has been an integrated physics-engineering activity.

Resonating Hartree-Fock theory of a one dimensional electronic system

Abstract: The electronic structure of the one dimensional (1D) Hubbard model is calculated by the resonating Hartree-Fock (Res HF) method. As local fluctuations in the spin density wave (SDW) in the half filled case, we consider a solito,-antisoliton pair making quantum trnaslational and breathing motions, i.e., breathers. The breather explains 76.5% (U=3) and 70.1% (U=4) of the ground state correlation energy, which is better than the result of the Gutzwiller method, and gives low energy excited states with the symmetries and energetic ordering characteristic in polyenes