Low-Density Ferromagnetism in the Hubbard Model

We study analytically the stability of the saturated ferromagnetic ground state(the Nagaoka state)of the Hubbard model in the low electron density limit with the mean-field approach.We find that the critical U_(c) is just 4πt,where t is the hopping integral between the nearest neighbours,i.e.,when U is larger than 4πt,the paramagnetic state is unstable with respect to the ferromagnetic state,and vice versa.

Competition Between Two Ordering Processes in Two-Dimensional Doped Antiferromagnets

We provide a Landau theory of the coupled charge density wave and spin density wave order parameters to investigate the possibility of coexistence or competition between the two ordering processes occurring in the two-dimensional doped antiferromagnet.We find that the two ordering processes can coexist only in a certain region of coupling parameters,andthe one with lower transition temperature can only coexist together with the other one with higher transition temperature and can not be stabilized separately.Phase diagrams in different situations are given.

Evidence of Phase Separation in the Hubbard Model

We examine theoretically the possibility of phase separation due to bole aggregation in the background of anti-ferromagneticallyr ordered CuO_(2) planes.The results which are calculated within a self-consistent mean-field-type approximation give obvious evidence of phase separation.Our conclusions agree with previous numerical results(from exact diagonalization).

Interaction Between Two Holes in the CuO_(2) Plane of Cuprate Superconductors

We have studied the behavior of two holes in the CuO_(2) plane of cuprate superconductors which is important for making clear the origin of the superconductivity.The interaction between two holes does not show a simple form but a damped oscillating function of the distance between them.It is favorable for them to get closer with a distance about five or ten lattice constant just by thermal motion.Around the equilibrium positions,the vibration motion is considered.

Light Transmission Through Symmetric Fibonacci-Class Multilayers

We have studied the optical transmission of one-dimensional Fibonacci-class quasiperiodic multilayers, which possess a mirror symmetry. We find that the transmission coefficient is unity for all studied sequences at the central wavelength λ=λ0, where λ0 = (1/4)nA(B)dA(B) with nA(B) and dA(B) being the index of refraction and thickness of twokinds of layers, respectively. Two-cycle or three-cycle has been found around λ = λ0, which is different from the features of the sequences without symmetric structure.

Pressure-Induced Charge Transfer and Pressure Dependence of the Superconducting Transition Temperature in HgBa_(2)CuO_(4+δ)

The pressureinduced changes of hole concentration and pressure effects on Tc in HgBa2CuO4+δ(δ=0.07-0.39)have been investigated on the basis of the pressureinduced charge transfer model.Detailed calculations show that large enhancements in Tc under high pressure can be obtained for materials which are underdoped,and not for compounds which have nearly optimal hole concentration.On the other hand,Tc enhancements in compounds which are heavily overdoped are found to be quite modest.The theoretical prediction is in agreement with experiments.The possible intrinsic factors which are responsible for the pressure dependence of the maximum Tc are discussed within the van Hove singularity scenario.

Instability of the Random Fixed Point of Renormalization Group for the Spin Systems with Quenched Disorder

The replica trick used in the renormalization group(RG)for the disordered spin systems is discussed.The recursion relations of RG are derived in a more general situation in which the coupling constants can have arbitrary symmetry to the replica indices.It is shown that the random fixed point found by Luther,Grinstein and Lubensky is unstable.This instability implies that the replica symmetry should be broken in the RG for the disordered systems.

ULTRASONIC PROPAGATIONS IN COMPOSITIONAL MODULATED STRUCTURES(CMS)

The dynamic equation of acoustic vibration in CMS can be obtained by means of the mean field approximation.Sound propagation features in CMS are discussed,while the corresponding dispersion law,forbidden bands of the sound waves also determined.