We study the squeezing and the amplitude-squared squeezing of radiation field in the interaction of squeezed vacuum state with two two-level atoms, and discuss the effects of the relative value of two atom-field coupling constants on them. The numerical results show that the presence of one additional atom introduces noatrivival cooperative effects into the squeezing and the amplitude-squared squeezing of radiation field.

CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

We consider the soliton excitations in a diatomic chain with nonlinear on-site potential based on a quell-discreteness approximation, Different from the theoretical approach devel-oped before, our method is valid in the whole Brillouin zone of phonons. The gap solitons observed recently in a nonlinear diatomic pendulum lattice can be well explained qualita-tively. It is also shown that if the wave number of the carrier wave is near the edge of the Brillouin zone, the results coinside with that of Kivshar and Flytzanis about the gap solitons in diatomic lattices when the difference of mass between two kinds of atoms becomes small.

We have calculated the vibrational modes around a polaron to test the effects of the next-nearest-neighbor hopping interactions on the localized medes within a framework of the two-dimensional mode. The results show that: (1) For a negative polaron, eleven localized modes have been found, compared with ten modes obtained on a previous work without non-nearest neighbor hopping. (2) For a positive polaron, two additional localized modes occur. (3) The localization of localized modes are strengthened and their frequencies move after turning on the next-nearest-neighbor hopping interactions.

CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES

A new quasi-one-dimensional organic ferromagnet model is proposed. Magnon excitations for finite and infinite on-site repulsion of electrons on main carbon chain are investigated. It is found that the lowest branch of spectrum preserves the characteristics of the ferromagnetic magnon. It is also shown that with increasing of the on-site repulsion energy, the itinerant π-electrons become tightly localized.

After establishing the method of constructing a class of two-dimensional Fibonacci quasi-lattices, by means of a decomposition-decimation method based on the renormalization-group technique, we have studied the rule. of energy spectrum splitting for the two-dimensional Fibonacci quasilattices in the framework of single-electron tight-binding on-site model. Analytic results show that there are only six kinds of clusters and the electronic energy band consists of nine subbands.

Photoluminescence (PL) ia used to study the interface properties of GaAs/AlGaAs quan-tum well (QW) heterostructures prepared by molecular beam epitaxy with growth interrup-tion (GI). The discrete luminescence lines observed for the sample with GI are assigned to the splitting of the heavy-hole exciton associated with heterointerface islands with the lateral size greater than excjton diameter and mean height less than one monolayer, and the spectra have the Gaussian lineshapes. The results strongly support the microroughness model. We also study the temperature dependence of the exciton energies and find that excitons are localized at the interface roughness at low temperature even in the sample with GI, The lateral size of the microroughness of the GI sample is estimated to be less than 5nm from the exciton localization energy.

Heavily C-doped GaAs films, grown by metalorganic chemical vapor deposition with CCl_{4} as external carbon source, have been studied by Hall-effect measurements, high-resolution double-crystal X-ray diffraction, and secondary-ion-mass spectroscopy (SIMS). Comparison among X-ray diffraction and Hall-effect measurements and SIMS results indicates that carbon is preferentially incorporated as acceptor on As lattice sites and electrical activation rate achieves 100%. There is no evidence of carbon incorporated on Ga sites as donors and incorporated on interstitial lattice sites.

A formula of thermoelectric power (TEP) for the local pair model has been derived. It has been found that from T_{c} to about 125K the TEP data of YBa_{2}Cu_{3}O_{7} (Y-123) can be described by this formula. We have also found that the formula of the diffusion thermoelec-tricity in the free electron approximation can be used to express the TEP data of Y-123 in the region of 220 to about 300 K. It seems that a general transition occurs in the intermediate temperature region, that is from the regime in which the local pair subsystem is dominating to the regime in which the itinerant carrier subsystem is dominating.

Based upon the spin wave theory, the influence of the size of a three-dimensional Heisenberg system on its thermodynamic properties was studied. It is found that the specific heat increases due to the finite size and free surface of the system. For a magnetic film with finite thickness, the interaction of spin waves was also discussed. There exist three additional scattering processes, namely, the scattering between spin waves with wave-vectors parallel to the surface of the film (two-dimensional spin wave), the scattering between two and three-dimensional waves, and the scattering between those waves with the same component in the direction along the thickness of the film. As a result, the T^{4} term, arising from the coupling of spin waves, in the expression of the specific heat of the system, splite into three parts proportional to T^{5/2},T^{7/2} and T^{4}, respectively. Here T is the temperature.

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