International Journal of Modern Physics B

Squeezing and amplitude-squared squeezing for two two-level nonidentical atoms in lossless cavity has been investigated assuming the field to be initially in the coherent state. The time-dependent squeezing parameters have been calculated. The influence of the relative difference of two coupling constants on the squeezing parameters has been analyzed.

A new coupled-mode formulation based on scalar modes is developed for the anisotropic optical waveguide. In the new formulation, the birefringence property of the material is represented as additional coupling to the coupling due to the refractive-index perturbations. The theory is applied to the direction coupler made of parallel slab waveguides. The numerical results show that the numerical value of the birefringence coupling correction is around 10% as much as that of the refractive-index perturbation coupling for the special case.

Metal-insulator transitions (MITs) in doped uncompensated systems are investigated in the Mott–Hubbard and Anderson impurity models by considering the intercarrier correlation and screening effect of carriers in the same hydrogenic impurity center, the formation of the superlattices with different coordination numbers (z=6, 8 and 12) and by studying the effect of randomness in impurity distribution. We have obtained simple and quite general criteria for the Mott and Anderson transitions and used these criteria to describe quantitatively the correlation and disorder-induced MITs in doped semiconductors and high-T_c cuprates. We examine the validity of the obtained criteria for the Mott and Anderson MITs in these doped systems. It is found that the newly derived criteria for the Mott MIT are well satisfied in doped semiconductors, but they cannot be used to describe the observed MITs in the hole-doped high-T_c cuprates, whereas the newly derived criteria for the Anderson MIT are applicable equally to describe the MITs observed both in doped semiconductors (at weak and intermediate disorders) and in doped cuprates (at intermediate and strong disorders). The new criteria for the Anderson MIT are extended to the polaronic systems in p-type cuprates. Our results are in quantitative agreement with the existing well-established experimental data and shed more light on the different types of MITs that occur in doped uncompensated semiconductors and cuprates.

Persistent current and low-field magnetic susceptibility in single-channel normal metal rings threaded by a magnetic flux ϕ are studied in the tight-binding model considering long-range hopping of the electrons in shortest path. The higher order hopping integrals try to reduce the effect of disorder by delocalizing the energy eigenstates and accordingly, current amplitude in disordered rings is comparable to that of an ordered ring. The calculations of low-field magnetic susceptibility predict that the sign of the currents can be mentioned precisely for the rings with fixed number of electrons even in the presence of impurity in the rings. At low-fields current shows only diamagnetic sign in perfect rings irrespective of the total number of electrons, N_e. On the other hand, in disordered rings it exhibits diamagnetic and paramagnetic sign, respectively, for the rings with odd and even N_e. In the rings described by fixed chemical potentials μ, the sign of the low-field currents cannot be predicted precisely since then it strongly depends on the values of μ and the specific realizations of disordered configurations.

The spin Hamiltonian parameters, g factors g_‖ and g_⊥, for Nd³⁺ in ThGeO₄ are theoretically investigated from the perturbation formulas of the g factors for a 4f³ ion in tetragonal symmetry. In these formulas, the contributions to the g factors from the second-order perturbation terms and the admixture of various states are taken into account. It is found that the calculated g factors of this work are smaller than the experimental results, but close to those in the previous theoretical studies by Gutowska et al. Moreover, the calculated hyperfine structure constants A_‖ and A_⊥ as well as the energies of the ⁴I_9/2 Stark levels in this work are also consistent with those in the previous investigations. The discrepancy between theoretical and experimental g factors is discussed.

The capacitance between any two arbitrary lattice sites in an infinite square lattice is studied when one bond is removed (i.e. perturbed). A connection is made between the capacitance and the lattice Green's function of the perturbed network, where they are expressed in terms of those of the perfect network. The asymptotic behavior of the perturbed capacitance is investigated as the separation between the two sites goes to infinity. Finally, numerical results are obtained along different directions and a comparison is made with the perfect capacitances.

A Hamiltonian which describes the interaction of a single atom with two photon modes is introduced. It is shown that the Hamiltonian can be diagonalized in a particular basis. The energies and an eigenvector basis set are obtained. Some quasi-probability densities are calculated using amplitudes determined with respect to the rotated basis. Some of the physical phenomena which are manifested in the calculations are discussed.

Cross-sections for positron-rubidium (³⁷Rb) scattering have been calculated using the Clementi–Roetti wavefunctions and a combination of the coupled-static and frozen-core approximations. The total cross-sections, calculated with eight partial waves corresponding to the total angular momentum ℓ=0 to ℓ=7, are determined over a wide region of scattering energies ranging from 2.7 to 300 eV. The resulting total cross-sections are compared with experimental results and those calculated by other authors. Our total collisional cross-sections display a pronounced peak at 5 eV, nearly consistent with the measurements of Parikh et al. [Phys. Rev. A47, 1535 (1993)] and also reveal another peak at 7 eV, consistent with the experimental cross-section of Stein et al.²³ in the neighborhood of 7 eV. The oscillating behavior of our total collisional cross-sections supports the possible existence of resonance, especially at low energy region. The effect of positronium formation on the total collisional cross-sections diminishes when the incident energy is larger than 100 eV.

We focus on the definition of the unitary transformation leading to an effective second order Hamiltonian, inside degenerate eigensubspaces of the non-perturbed Hamiltonian. We shall prove, by working out in detail the Su–Schrieffer–Heeger Hamiltonian case, that the presence of degenerate states, including fermions and bosons, which might seemingly pose an obstacle toward the determination of such "Fröhlich-transformed" Hamiltonian, in fact does not: we explicitly show how degenerate states may be harmlessly included in the treatment, as they contribute with vanishing matrix elements to the effective Hamiltonian matrix. In such a way, one can use without difficulty the eigenvalues of the effective Hamiltonian to describe the renormalized energies of the real excitations in the interacting system. Our argument applies also to few-body systems where one may not invoke the thermodynamic limit to get rid of the "dangerous" perturbation terms.

The out-of-plane resistivity ρ_c(T) is measured for Bi₂Sr₂CaCu₂O₈ superconducting thin films successfully deposited by in situ magnetron RF–sputtering on vicinal (100) SrTiO₃ substrates. Oriented film with c-axis inclined with respect to the basal plane by a tilt angle α≈4° was obtained. The oxygen content of the film was changed from maximally overdoped state to strongly underdoped nonsuperconducting states. It was found that the temperature corresponding to the minimum between metallic and semiconductive behavior appears to reflect the opening of the pseudogap for transport along c-axis. We found that the opening of the pseudogap along c-axis does not coincide with pseudogap seen in ab plane.

In this paper, a new Bosonic Gutzwiller projection approach is proposed to study the strongly correlated bosons in optical lattice. In this method, there exist many variational parameters which make us calculate the physical characters of states, including the double occupation rate and the higher occupation rates. Based on this approach, a quantum phase transition from superfluid state to Mott insulator state is obtained for the homogenous phase at unit filling.