M. Tomak

Excitonic effects on the nonlinear optical properties of small quantum dots

The excitonic effects on the nonlinear optical properties of small quantum dots with a semiparabolic confining potential are studied under the density matrix formalism. First, within the framework of the strong confinement approximation, we present the excitonic states and then calculate the excitonic effects on the nonlinear optical properties, such as second harmonic generation, third harmonic generation, nonlinear absorption coefficient and refractive index changes. We find the explicit analytical expressions between the corresponding nonlinear optical properties with and without considering the excitonic effects. It is seen that these analytical expressions are related only to ratios of the effective masses of electron and hole. These explicit expressions indicate that the excitonic effects on the corresponding nonlinear optical properties become more important with increasing orders of the optical susceptibilities. In addition, we suggest a scaling rule for the nth-order susceptibility as γ(n+1)/2. The effect of the confining potential frequency on the corresponding nonlinear optical properties is also studied. Our results show a remarkable dependence of nonlinear optical properties on both the excitonic effects and the confining potential frequency.

A parabolic quantum dot with N electrons and an impurity

The two dimensional Thomas-Fermi approximation is applied to the problem of parabolic quantum dot composed of N electrons and an on-center impurity. Change induced by impurity on electron density, chemical potential and total energy is discussed and it is found that existence of impurity makes significant changes on the determination of above properties especially for the small number of electrons and strong confinements.

PHOTOIONIZATION OF IMPURITIES IN INFINITE-BARRIER QUANTUM WELLS

Abstract The dependence of the photoionization cross-section on photon energy is calculated for shallow donors in infinite-barrier GaAs/Ga1−xAlx As quantum well as a function of well width. The effect of a magnetic field is also considered.

Intensity-dependent refractive index of a Pöschl-Teller quantum well

The linear and nonlinear changes in the refractive index of a Poschl-Teller quantum well are studied. The Poschl-Teller potential can easily become asymmetric by a correct choice of its parameter set. We use this feature to investigate the intensity-dependent refractive index of a GaAs quantum well within the density matrix formalism. Numerical results reveal large values of linear and nonlinear changes in the refractive index.

Photoionization of impurities in quantum wells

The dependence of photoionization cross-section on photon energy is calculated for shallow donors in quantum wells. The resulting spectra is completely different for light polarized along and perpendicular to the growth direction.

Bismuth doping of graphene

In this work, we have studied bismuth as a dopant in graphene using density functional theory (DFT). We find that bismuth is weakly physisorbed within DFT. On the other hand, we show that bismuth n-dopes graphene when it is substitutional. We observe that local density approximation results give higher substitutional energy than that of generalized gradient approximation. The electronic structure of graphene is changed when Bi is substitutional.

Thermal and mechanical properties of Cu–Au intermetallic alloys

The thermal and mechanical properties of Cu, Au pure metals and their ordered intermetallic alloys of Cu3Au(L12), CuAu(L10) and CuAu3(L12) are studied by using the molecular dynamics simulation. The effects of temperature and concentration on the physical properties of CuxAu1−x are analysed. Sutton–Chen (SC) and quantum Sutton–Chen (Q-SC) many-body potentials are used. The simulation results such as cohesive energy, density, elastic constants, bulk modulus, heat capacity, thermal expansion, melting points and phonon dispersion curves are in good agreement with the available experimental data at the various temperatures. Q-SC potential parameter results are usually closer to experimental values than the ones predicted from SC potential parameters.

QUANTUM GENETIC ALGORITHM METHOD IN SELF-CONSISTENT ELECTRONIC STRUCTURE CALCULATIONS OF A QUANTUM DOT WITH MANY ELECTRONS

In this study, we have calculated energy levels of an N-electron quantum dot. For this purpose, we have used two different techniques, matrix diagonalization and quantum genetic algorithm, to obtain simultaneous solutions of the coupled Schrodinger and Poisson equation in the Hartree approximation. We have determined single particle energy levels, total energy, chemical potential and capacitive energy. We have also compared the results, demonstrated the applicability of QGA to many-electron quantum systems and evaluated its effectiveness.

Shallow Donors in a Quantum Well Wire: Electric Field and Geometrical Effects

Subband and shallow donor binding energies are calculated. The emphasis is given on the electric field effects for wires of different shapes. It is shown that the donor binding energy is sensitive to the interplay of the electric field and geometrical effects.

Polaron effects on the energy levels of hydrogenic impurities in GaAs/Ga1−xAlxAs quantum well structures

Abstract The effect of electron phonon-coupling on the binding energies of impurities in GaAs/Ga1−χAlχAs quantum well structures is calculated. The correction becomes sizeable as the electron gets more deeply bound.