Influence of magnetic field on the energy positions and widths of one electron resonance states in a multi-shell spherical quantum dot is investigated. The one-band effective mass approximation is assumed. The complexeigenvalue Schrodinger equation approach involving complex rotation of coordinates is used to obtain complex eigenenergies, E_{r} - iΓ/2, corresponding to resonance states. We show how the magnetic field changes the resonance energy, E_{r}, and decay rate, Γ, yielding bound states for some particular cases.
Optical transitions between bound and continuum states in multiple-heterojunction quantum structures are numerically investigated. In the conduction band energy range the absorption to continuum is studied within one-band effective-mass approximation. By changing the shape of quantum wells, we can tune the energy corresponding to absorption peak. We also show how the absorption is modified in the case of annealed structures. The inter-subband transitions in the energy range of valence band in Si/Si_{1-x}Ge_{x}/Si quantum well are described with the help of multiband Luttinger-Kohn Hamiltonian. Particular attention is paid to transitions to resonant states.
The empirical tight-binding approach is used to study atomic-scale effects on electronic coupling in vertically stacked, self-assembled InAs/GaAs quantum dots. A model with unstrained dots is first studied to isolate the atomistic coupling effects from the strain effects. The strain effects are next considered by means of the valence force field method. Electron levels in coupled quantum dots follow closely the simple analogy of coupled dots as artificial molecules. The electron ground state of double dot has always bonding-like character. The coupling of hole states is more complicated because the coupling depends both of the hole envelope function and the atomic character of the hole state. It is shown that the character of the hole ground state of double dot changes from antibonding to bonding-like, when the distance between the dots decreases. It reorders hole levels, changes state symmetries, and makes changes in optical spectra. The calculated red-shift of the lowest transition for closely-spaced dots agrees well with experimental data. We present also some preliminary results on strain effects in such nanocrystals.
Erratum: M. Zieliński, W. Jaskólski, J. Aizpurua, G.W. Bryant, Strain and Spin-Orbit Effects in Self-Assembled Quantum Dots, Acta Phys. Pol. A, 108, 929, 2005.
The electronic structure and quantum conductance of rotationally invariant (6,6)/(12,0) and rotationally non-invariant (5,5)/(8,2) superlattices made of metallic carbon nanotubes are investigated. It is shown that, except in the limit of very large periods, the quantum conductance of such superlattices does not critically depend on their rotational invariance, although it does in case of quantum dots and single junctions made of these nanotubes.
The effects of strain and spin-orbit interaction in self-assembled lens-shaped InAs/GaAs quantum dots are investigated. Calculations are performed with empirical tight-binding theory supplemented by the valence force field method to account for effects of strain caused by lattice mismatch at the InAs-GaAs interface. It is shown that both effects influence strongly the electron and hole energy structure: splitting of the energy levels, the number of bound states, density distributions, and transition rates. We show that piezoelectric effects are almost negligible in quantum dots of the size investigated.
We study graphene nanoribbons and carbon nanotubes with divacancies, i.e., local defects composed of one octagon and a pair of pentagons. We show that the presence of divacancies leads to the appearance of gap states, which may act as acceptor or donor states. We explain the origin of those defect-localized states and prove that they are directly related to the zero-energy states of carbon ring forming the octagonal topological defect.
Interface states of all-metallic carbon nanotube quantum dots and superlattices are studied within a tight-binding model. We focus on achiral systems made by connecting armchair (n,n) and zigzag (2n,0) tubes with a full ring of n pentagon-heptagon topological defects. We show that the coupling between interface states, which arise from the topological defects, reflects the existence of the Friedel oscillations in the (n,n) tube, with an unusually large decay exponent. We expect this interaction to be important for the understanding of other physical properties, such as selective dot growth, magnetic interaction through carbon tubes or optical spectroscopy of interface states.
The properties of carbon nanotubes can be dramatically altered by the presence of defects. In this work we address the properties of two different kinds of defective nanotubes: junctions of achiral tubes with topological defects and partially unzipped carbon nanotubes. In particular, we begin by focussing on the interface states in carbon nanotube junctions between achiral tubes. We show that their number and energies can be derived by applying the Born-von Karman boundary condition to an interface between armchair- and zigzag-terminated semi-infinite graphene layers. We show that these interface states, which were thought to be due to the presence of topological defects, are in fact related to the graphene zigzag edge states. Secondly, we study partially unzipped carbon nanotubes, which can be considered as the junction of a carbon nanotube and a graphene nanoribbon, which has edge features giving rise to novel properties. Carbon nanoribbons act as transparent contacts for nanotubes and viceversa, yielding a high conductance. At certain energies, nanoribbons behave as valley filters for carbon nanotubes; this holds considering electron-electron interaction effects. Furthermore, the application of a magnetic field turns the system conducting, with a 100% magnetoresistance. These novel structures may open a way for new carbon-based devices.
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