Li Shu-Shen

Effective-mass theory for coupled quantum dots grown on (11N)-oriented substrates

The electronic structures of coupled quantum dots grown on (11N)-oriented substrates are studied in the framework of effective-mass envelope-function theory. The results show that the all-hole subbands have the smallest widths and the optical properties are best for the (113), (114), and (115) growth directions. Our theoretical results agree with the available experimental data. Our calculated results are useful for the application of coupled quantum dots in photoelectric devices.

Application of Plane Wave Method to the Calculation of Electronic States of Nano-Structures

The electronic states of nano-structures are studied in the framework of effective-mass envelope-function theory using the plane wave basis. The barrier width and the number of plane waves are proposed to be 2.5 times the effective Bohr radius and 15(n), respectively, for n-dimensional nano-structures (n = 1,2,3). Our proposals can be widely applied in the design of various nano-structure devices.

Binding Energy of Ionized-Donor-Bound Excitons in the GaAs-AlxGa1-xAs Quantum Wells

The binding energy of an exciton bound to an ionized donor impurity (D+,X) located st the center or the edge in GaAs-AlxGa1-xAs quantum wells is calculated variationally for the well width from 10 to 300 Angstrom by using a two-parameter wave function, The theoretical results are discussed and compared with the previous experimental results.

Maximal Heat Generation in Nanoscale Systems

We investigate the heat generation in a nanoscale system coupled to normal leads and find that it is maximal when the average occupation of the electrons in the nanoscale system is 0.5, no matter what mechanism induces the heat generation.

Spin and charge currents through a quantum dot connected to ferromagnetic leads

We investigate the spin polarized current through a quantum dot connected to ferromagnetic leads in the presence of a finite spin-dependent chemical potential. The effects of the spin polarization of the leads p and the external magnetic field B are studied. It is found that both the magnitude and the symmetry of the current are dependent on the spin polarization of the leads. When the two ferromagnetic leads are in parallel configuration, the spin polarization p has an insignificant effect on the spin current, and an accompanying charge current appears with the increase of p. When the leads are in antiparallel configuration, however, the effect of p is distinct. The charge current is always zero regardless of the variation of p in the absence of B. The peaks appearing in the pure spin current are greatly suppressed and become asymmetric as p is increased. The applied magnetic field B results in an accompanying charge current in both the parallel and antiparallel configurations of the leads. The characteristics of the currents are explained in terms of the density of states of the quantum dot.

Optical spectra and exciton state in vertically stacked self-assembled quantum discs

We study the oscillator strengths of the optical transitions of the vertically stacked self-assembled InAs quantum discs. The oscillator strengths change evidently when the two quantum discs are far apart from each other. A vertically applied electric held affects the oscillator strengths severely, while the oscillator strengths change slowly as the radius of one disc increases. We also studied the excitonic energy of the system, including the Coulomb interaction. The excitonic energy increases with the increasing radius of one disc, but decreases as a vertically applied electric field increases.

Quantum-confined stark effect of vertically stacked self-assembled quantum discs

We study the electronic energy levels and probability distribution of vertically stacked self-assembled InAs quantum discs system in the presence of a vertically applied electric field. This field is found to increase the splitting between the symmetric and antisymmetric levels for the same angular momentum. The field along the direction from one disc to another affects the electronic energy levels similarly as that in the opposite direction because the two discs are identical. It is obvious from our calculation that the probability of finding an electron in one disc becomes larger when the field points from this disc to the other one.

N-Doped Zigzag Graphene Nanoribbons on Si(001): a First-Principles Calculation*

The structural and electronic properties of N-doped zigzag graphene nanoribbons (N-ZGNRs) adsorbed on Si(001) substrates are investigated with first-principles density functional calculations. Compared with the free-standing N-ZGNRs, the energy difference between the substitutional doping at the edge and the inner sites is significantly decreased on the Si substrate. The distribution of the extra charge induced by the N substitutional dopant keeps the Friedel oscillation feature, and is a main effect that influences the C–Si bonding strength. When N is doped in regions with high C–Si bond densities, the strain induced by the dopant also plays an important role in determining the C–Si bonding interactions. Similar to the undoped case, the strong N-ZGNR/Si interaction destroys the antiferromagnetic coupling of the edge states in N-ZGNR, leading to a non-magnetic ground state for the N-ZGNR/Si heterostructures.

The Electronic Structure of Coupled Semiconductor Quantum Dots Arranged as a Graphene Hexagonal Lattice under a Magnetic Field

We study the electronic spectrum of coupled quantum dots (QDs) arranged as a graphene hexagonal lattice in the presence of an external perpendicular magnetic field. In our tight-binding model, the effect of the magnetic field is included in both the Peierls phase of the Hamiltonian and the tight-binding basis Wannier function. The energy of the system is analyzed when the magnetic flux through the lattice unit cell is a rational fraction of the quantum flux. The calculated spectrum has recursive properties, similar to those of the classical Hofstadter butterfly. However, unlike the ideal Hofstadter butterfly structure, our result is asymmetric since the impacts of the specific material and the magnetic field on the wavefunctions are included, making the results more realistic.

Orbital magnetization in semiconductors

This paper theoretically investigates the orbital magnetization of electron-doped (n-type) semiconductor heterostructures and of hole-doped (p-type) bulk semiconductors, which are respectively described by a two-dimensional electron/hole Hamiltonian with both the included Rashba spin–orbit coupling and Zeeman splitting terms. It is the Zeeman splitting, rather than the Rashba spin–orbit coupling, that destroys the time-reversal symmetry of the semiconductor systems and results in nontrivial orbital magnetization. The results show that the magnitude of the orbital magnetization per hole and the Hall conductance in the p-type bulk semiconductors are about 10−2–10−1 effective Bohr magneton and 10−1–1 e2/h, respectively. However, the orbital magnetization per electron and the Hall conductance in the n-type semiconductor heterostructures are too small to be easily observed in experiment.