Effect of Super-exchange Interaction on Ground state Magnetic Properties of Spin-dependent Falicov-Kimball Model on a Triangular Lattice

Ground state magnetic properties are studied by incorporating the super-exchange interaction (J(se)) in the spin-dependent Falicov-Kimball model (FKM) between localized (f-) electrons on a triangular lattice for half filled case. Numerical diagonalization and Monte-Carlo simulation are used to study the ground state magnetic properties. We have found that the magnetic moment of (d-) and (f-) electrons strongly depend on the value of Hund's exchange (J), super-exchange interaction (J(se)) and also depends on the number of (d-) electrons (N-d). The ground state changes from antiferromagnetic (AFM) to ferromagnetic (FM) state as we decrease (N-d). Also the density of d electrons at each site depends on the value of J and J(se).

Polaronic correction to the first excited electronic energy level in an anisotropic semiconductor quantum dot

Within the framework of second-order Rayleigh-Schrodinger perturbation theory, the polaronic correction to the first excited state energy of an electron in an quantum dot with anisotropic parabolic confinements is presented. Compared with isotropic confinements, anisotropic confinements will make the degeneracy of the excited states to be totally or partly lifted. On the basis of a three-dimensional Frohlich's Hamiltonian with anisotropic confinements, the first excited state properties in two-dimensional quantum dots as well as quantum wells and wires can also be easily obtained by taking special limits. Calculations show that the first excited polaronic effect can be considerable in small quantum dots.

Mechanical and chemical bonding properties of ground state BeH2

The crystal structure, mechanical properties and electronic structure of ground state BeH2 are calculated employing the first-principles methods based on the density functional theory. Our calculated structural parameters at equilibrium volume are well consistent with experimental results. Elastic constants, which well obey the mechanical stability criteria, are firstly theoretically acquired. The bulk modulus B, Shear modulus G, Young's modulus E, and Poisson's ratio upsilon are deduced from the elastic constants. The bonding nature in BeH2 is fully interpreted by combining characteristics in band structure, density of states, and charge distribution. The ionicity in the Be-H bond is mainly featured by charge transfer from Be 2s to H 1s atomic orbitals while its covalency is dominated by the hybridization of H 1s and Be 2p states. The Bader analysis of BeH2 and MgH2 are performed to describe the ionic/covalent character quantitatively and we find that about 1.61 (1.6) electrons transfer from each Be (Mg) atom to H atoms.

First-principles study of ground-state properties and high pressure behavior of ThO2

The mechanical properties, electronic structure and phonon dispersion of ground state ThO2 as well as the structure behavior up to 240 GPa are studied using first-principles density-functional theory. Our calculated elastic constants indicate that both the ground-state fluorite structure and high pressure cotunnite structure of ThO2 are mechanically stable. The bulk modulus, shear modulus, and Young's modulus of cotunnite ThO2 are all smaller by approximately 25% compared with those of fluorite ThO2. The Poisson's ratios of both structures are approximately equal to 0.3 and the hardness of fluorite ThO2 is 22.4 GPa. The electronic structure and bonding nature of fluorite ThO2 are fully analyzed, and show that the Th-O bond displays a mixed ionic/covalent character. The phase transition from the fluorite to cotunnite structure is calculated to occur at the pressure of 26.5 GPa, consistent with recent experimental measurement by ldiri et al. [1]. For the cotunnite phase it is further predicted that an isostructural transition takes place in the pressure region of 80-130 GPa.

Electronic energy levels in an asymmetric quantum-dots-in-a-well structure for infrared photodetectors

Theoretical calculation of electronic energy levels of an asymmetric InAs/InGaAS/GaAS quantum-dots-in-a-well (DWELL) structure for infrared photodetectors is performed in the framework of effective-mass envelope-function theory. Our calculated results show that the electronic energy levels in quantum dots (QDs) increase when the asymmetry increases and the ground state energy increases faster than the excited state energies. Furthermore, the results also show that the electronic energy levels in QDs decrease as the size of QDs and the width of quantum well (QW) in the asymmetric DWELL structure increase. Additionally, the effects of asymmetry, the size of QDs and the width of QW on the response peak of asymmetry DWELL photodetectors are also discussed.

A method to obtain ground state electroluminescence from 1.3 mu m emitting InAs/GaAs quantum dots grown by molecular beam epitaxy

1.3 mum emitting InAs/GaAs quantum dots (QDs) have been grown by molecular beam epitaxy and QD light emitting diodes (LEDs) have been fabricated. In the electroluminescence spectra of QD LEDs, two clear peaks corresponding to the ground state emission and the excited state emission are observed. It was found that the ground state emission could be achieved by increasing the number of QDs contained in the active region because of the state filling effect. This work demonstrates a way to control and tune the emitting wavelength of QD LEDs and lasers.

Room-temperature, ground-state lasing for red-emitting vertically aligned InAlAs/AlGaAs quantum dots grown on a GaAs(100) substrate

The effect of the growth temperature on the properties of InAlAs/AlGaAs quantum dots grown on GaAs(100) substrates is investigated. The optical efficiency and structural uniformity are improved by increasing the growth temperature from 530 to 560 degreesC. The improvements of InAlAs/AlGaAs quantum-dot characteristics could be explained by suppressing the incorporation of oxygen and the formation of group-III vacancies. Furthermore, edge-emitting laser diodes with six quantum-dot layers grown at 560 degreesC have been fabricated. Lasing occurs via the ground state at 725 nm, with a room-temperature threshold current density of 3.9 kA/cm(2), significantly better than previously reported values for this quantum-dot systems. (C) 2002 American Institute of Physics.

Ground-state properties of an artificial molecule: a simple model calculation for the double quantum dot

The ground state of a double quantum-dot structure is studied by a simplified Anderson-type model. Numerical calculations reveal that the ground-state level of this artificial molecule increases with the increasing single particle level of the dot, and also increases with the decreasing transfer integrals. We show the staircase feature of the electron occupation and the properties of the ground-state eigenvector by varying the;single particle level of the dot.

A broadband external cavity tunable InAs/GaAs quantum dot laser by utilizing only the ground state emission

A broadband external cavity tunable laser is realized by using a broad-emitting spectral InAs/GaAs quantum dot (QD) gain device. A tuning range of 69 nm with a central wavelength of 1056 nm, is achieved at a bias of 1.25 kA/cm(2) only by utilizing the light emission from the ground state of QDs. This large tunable range only covers the QD ground-state emission and is related to the inhomogeneous size distribution of QDs. No excited state contributes to the tuning bandwidth. The application of the QD gain device to the external cavity tunable laser shows its immense potential in broadening the tuning bandwidth. By the external cavity feedback, the threshold current density can be reduced remarkably compared with the free-running QD gain device.

Single nucleon potential and effective mass with ground state correlations in hot nuclear matter

In the framework of the finite temperature Brueckner-Hartree-Fock approach including the contribution of the microscopic three-body force, the single nuclear potential and the nucleon effective mass in hot nuclear matter at various temperatures and densities have been calculated by using the hole-line expansion for mass operator, and the effects of the three-body forces and the ground state correlations on the single nucleon potential have been investigated. It is shown that both the ground state correlations and the three-body force affect considerably the density and temperature dependence of the single nucleon potential. The rearrangement correction in the single nucleon potential is repulsive and it reduces remarkably the attraction of the single nucleon potential in the low-momentum region. The rearrangement contribution due to the ground state correlations becomes smaller as the temperature rises up and becomes larger as the density increases. The effect of the three-body force on the ground state correlations is to reduce the contribution of rearrangement. At high densities, the single nucleon potential containing both the rearrangement correction and the contribution of the three-body force becomes more repulsive as the temperature increases.