Z. Barticevic

Effects of applied magnetic fields and hydrostatic pressure on the optical transitions in self-assembled InAs/GaAs quantum dots

A theoretical study of the photoluminescence peak energies in InAs self-assembled quantum dots embedded in a GaAs matrix in the presence of magnetic fields applied perpendicular to the sample plane is performed. The effective mass approximation and a parabolic potential cylinder-shaped model for the InAs quantum dots are used to describe the effects of magnetic field and hydrostatic pressure on the correlated electron-hole transition energies. Theoretical results are found in quite good agreement with available experimental measurements for InAs/GaAs self-assembled quantum dots.

Flat bands in slightly twisted bilayer graphene: Tight-binding calculations

The presence of flat bands near Fermi level has been proposed as an explanation for high transition temperature superconductors. The bands of graphite are extremely sensitive to topological defects which modify the electronic structure. In this Rapid Communication, we found nondispersive flat bands no farther than 10 meV of the Fermi energy in slightly twisted bilayer graphene as a signature of a transition from a parabolic dispersion of bilayer graphene to the characteristic linear dispersion of graphene. This transition occurs for relative rotation angles of layers around 1.5° and is related to a process of layer decoupling. We have performed ab initio calculations to develop a tight-binding model with an interaction Hamiltonian between layers that include the orbitals of all atoms and takes into account interactions up to third nearest neighbors within a layer.

Electron-hole transitions in self-assembled InAs/GaAs quantum dots: Effects of applied magnetic fields and hydrostatic pressure

A theoretical study of the effects of applied magnetic fields and hydrostatic pressure on the electron-hole transition energies in self-assembled InAs/GaAs quantum dots is presented. The effective-mass approximation and a model of a cylindrical-shaped quantum dot with in-plane parabolic potential have been used to describe the InAs/GaAs quantum dots. Present theoretical results are in quite good agreement with experimental measurements of the magnetic field and pressure dependence of the exciton transition energies in InAs/GaAs self-assembled quantum dots.

Magnetospectroscopy of acceptors in “blue” diamonds

Abstract Naturally occurring, nitrogen-free, p-type diamond—now known to be boron-doped—as well as man-made diamonds deliberately doped with boron display an electronic Raman transition, Δ′, originating in the 1 s ( p 3/2 ) : Γ 8 ground state of the acceptor and terminating in its 1 s ( p 1/2 ) : Γ 7 spin–orbit partner. With magnetic field B along [0 0 1] , [1 1 1] , or [1 1 0] , the electronic Raman spectrum displays eight Zeeman transitions and four Raman lines ascribed to transitions between the Zeeman sublevels of Γ8 (Raman-electron-paramagnetic-resonance: Raman-EPR). They exhibit polarizations expected from the polarizability tensors formulated in terms of the Luttinger parameters γ 1 , γ 2 , and γ3 characterizing the top of the valence band. The selection rules and the relative intensities of the Zeeman components as well as of the Raman-EPR lines, observed in diverse polarization configurations and scattering geometries, have led to: assignments of magnetic quantum numbers; the level ordering of the Zeeman sublevels, or equivalently, the magnitudes and signs of g1 and g2, the orbital and spin g-factors of the acceptor-bound hole; the extreme mass anisotropy as reflected in the ratio (γ2/γ3)=0.08±0.01. Magnetic-field-induced mixing of zero field states, time reversal symmetry, and the diamagnetic contributions which characterize the different sublevels are fully taken into account in the interpretation of the experimental results. These include the striking mutual exclusion of the Stokes spectrum from its anti-Stokes counterpart in specific polarization configurations.

A theoretical study of exciton energy levels in laterally coupled quantum dots

A theoretical study of the electronic and optical properties of laterally coupled quantum dots, under applied magnetic fields perpendicular to the plane of the dots, is presented. The exciton energy levels of such laterally coupled quantum-dot systems, together with the corresponding wavefunctions and eigenvalues, are obtained in the effective-mass approximation by using an extended variational approach in which the magnetoexciton states are simultaneously obtained. One achieves the expected limits of one single quantum dot, when the distance between the dots is zero, and of two uncoupled quantum dots, when the distance between the dots is large enough. Moreover, present calculations-with appropriate structural dimensions of the two-dot system-are shown to be in agreement with measurements in self-assembled laterally aligned GaAs quantum-dot pairs and naturally/accidentally occurring coupled quantum dots in GaAs/GaAlAs quantum wells.

Spectra of acceptors in quantum dots: the effect of a magnetic field

We report calculations of the energy spectra of shallow-acceptor impurities in quantum dots in the presence of external magnetic fields. We calculate the binding energies of the ground and excited acceptor states in the effective-mass approximation using a formalism based on a four-band model that includes the coupling of the spin to the magnetic field as well as the valence-band mixing. The potential of the acceptor is taken to be the screened Coulomb potential of a point charge, and we take into account the mismatch of the dielectric constant through the method of image charges. We present a complete analysis of the acceptor binding energies as a function of the lateral confinement of the quantum dot and as a function of the magnetic field strength. We also discuss the role of the mixing of heavy holes and light holes in determining the acceptor spectrum in the different regimes of confinement.

Magnetic-field effects on transport in carbon nanotube junctions

Here we address a theoretical study on the behavior of electronic states of heterojunctions and quantum dots based on carbon nanotubes under magnetic fields. Emphasis is put on the analysis of the local density of states, the conductance, and on the characteristic curves of current voltage. The heterostructures are modeled by joining zigzag tubes through single pentagon-heptagon pair defects, and described within a simple tight-binding calculation. The conductance is calculated using the Landauer formula in the Green-functions formalism. The theoretical approach used incorporates the atomic details of the topological defects by performing an energy relaxation via Monte Carlo calculation. The effect of a magnetic field on the conductance gap of the system is investigated and compared to those of isolated constituent tubes. It is found that the conductance gap of the studied carbon nanotube heterostructure exhibits oscillations as a function of the magnetic flux. However, unlike the pristine tubes case, they are not Aharonov-Bohm periodic oscillations.

Electronic properties of coupled quantum rings in the presence of a magnetic field

We have studied the energy spectrum of a system formed by two concentric, coupled, semiconductor quantum rings. We have investigated the effect of a uniform magnetic field applied along the rings axis, on the energy spectrum of the system. We found that the spectrum of the coupled rings with similar confinement length corresponds roughly to the superposition of the spectra of two separate rings, although modified by the anticrossings produced among different states of the individual rings with same angular momentum. The Aharonov-Bohm oscillations do not have a definite period.

Intra-donor transitions in triple quantum-well structures under external fields

A theoretical study on shallow donor-impurity states in distinct triple GaAs–(Ga, Al)As quantum-well heterostructures is presented under external magnetic and electric fields applied perpendicular to the interfaces of the semiconducting elements. The emphasis is put on the determination of intra-donor absorption spectra and its dependence on the impurity profile distributions and different photon polarizations are considered. Impurity states are calculated by adopting the effective mass approximation and using a variational scheme in which ground and excited states are obtained simultaneously. Magnetospectroscopy measurements may be used to observe such transitions.

Electronic and impurity states in triple quantum wells

Abstract A detailed theoretical study on spectra of donor-impurities in triple GaAs–(Ga,Al)As quantum wells in the presence of external electric and magnetic fields is presented. Impurity states are calculated, within the effective-mass approximation, by adopting a variational scheme in which ground and excited states are obtained simultaneously. Different geometrical confinement regimes for the impurities are considered and the effects of the induced field-confinement on the binding energies are analyzed. The impurity binding energy presents characteristic features determined fundamentally by the spatial distribution of the electronic wave function within the triple-well structure. We show that by changing the intensity of the external magnetic and electric field, a large spread in the range of the donor binding energy may be obtained.