Zhang Yu-Mei

Projection operator formulation for second order response

Combined first and second order dynamic susceptibilities for a set of observables are expressed by a matrix resolvent operator in Liouville-space. A well-defined triangular projector is introduced to rewrite second order susceptibilities in terms of frequencies and memory functions. For these a Markovian type of approximation is established and connected with quantities of first order response.

Ground-State Properties of a Local Sine-Gordon Model with Two Frequencies

The local sine-Gordon model with two frequencies is studied by a variational method. The renormalization mass and classical field path are calculated. We find an Ising-like phase transition, which is analogous in the global sine-Gordon model with two frequencies. A full phase diagram is given.

CRITICAL EXPONENTS OF THE ONE-DIMENSIONAL CONTINUUM HEISENBERG-ISING MODEL

The continuum generalization of the one-dimensional Heisenberg-Ising model is studied by the bosonization method. With reference to the results of the renormalization group theory, we are able to obtain the critical exponents by considering the influence of the large momentum transfer and improve Luther and Peschels results to the next order. A comparison of the exact result of the lattice model with that of the continuum model by various treatments are also given.

Ground State Properties of a Local Sine-Gordon Model with Two Scatters

We investigate the local sine-Gordon model with two scatters by a variational method. The renormalization mass is calculated. It is found that there is an Ising-like phase transition, which is analogous in the global sine-Gordon model with two frequencies. The renormalization mass can be modulated by the distance between the two scatters.

Soliton Lattice State of Spin- Antiferromagnetic Chain in an External Magnetic Field*

One-dimensional spin- anisotropic Heisenberg antiferromagnet in a longitudinal external magnetic field is studied using bosonization method and Gaussian wave functional techniques which take account of the spatial structure. The magnetization and the energy of the ground state which depend on the external magnetic field are calculated. For the case of anisotropic parameter , increasing of the external magnetic field above the threshold value leads to the appearance of the soliton lattice state in the ground state, until to an another critical field where the ground state changes to the canted state phase. Therefore, with increasing external magnetic field, the ground state experiences four different phases successively, namely, antiferromagnetic Ising, soliton lattice state canted state, and magnetization saturated phases. When , the soliton lattice state phase does not appear, with increasing external field, the paramagnetic phase smoothly evolves into the canted state phase, finally reaches magnetization saturated phase.

The Critical Properties of One-Dimensional Extended Hubbard Model

In the framework of nonperturbative quantum field theory, the critical phenomena of one-dimensional extended Hubbard model (EHM) at half-filling are discussed from weak to intermediate interactions. After the EHM being mapped into two decoupled sine-Gordon models, the ground state phase diagram of the system is derived in an explicit way. It is confirmed that the coexisting phases appear in different interaction regimes which cannot be found by conventional theoretical methods. The diagram shows that there are seven different phase regions in the ground state, which seems not to be the same as previous discussions, especially the boundary between the phase separation and condensed phase regions. The phase transition properties of the model between various phase regions are studied in detail.

The Critical Properties of a Modulated Quantum sine-Gordon Model

A new procedure of trial variational wave functional is proposed for investigating the mass renormailzation and the local structure of the ground state of a one-dimensional quantum sine-Gordon model with linear spatial modulation, whose ground state differs from that without modulation. The phase diagram obtained in parameters

Instability of non-fermi liquid behavior in the two-channel Kondo model.

(\alpha \Lambda ^{-2},\beta ^{2})

Phase Diagram of One-Dimensional Bosons in an Array of Local Nonlinear Potentials at Zero Temperature

plane shows that the vertical part of the boundary between soliton lattice phase and incommensurate (IC) phase with vanishing gap sticks at

Variational study on missing bond-charge repulsion on the extended Hubbard model: effects on polyacetylene

\beta ^{2}