Zeeman splitting of shallow donors in GaN

The Zeeman splitting of the donor spectra in cubic- and hexagonal-GaN are studied using an effective mass theory approach. Soft-core pseudopotentials were used to describe the chemical shift of the different substitutional dopants. The donor ground states calculated range from 29.5 to 33.7 meV, with typically 1 meV higher binding in the hexagonal phase. Carbon is found to produce the largest donor binding energy. The ionization levels and excited states are in excellent agreement with Hall and optical measurements, and suggest the presence of residual C in recent experiments.Comment: REVTEX file - 2 figure

Aharonov-Bohm phase as quantum gate in two-electron charge qubits

We analyze the singlet-triplet splitting on a planar array of quantum dots coupled capacitively to a set of external voltage gates. The system is modelled using an extended Hubbard Hamiltonian keeping two excess electrons on the array. The voltage dependence of the low-energy singlet and triplet states is analyzed using the Feshbach formalism. The formation of a well decoupled two-level system in the ground state is shown to rely on the fact of having two particles in the system. Coherent operation of the array is studied with respect to single quantum bit operations. One quantum gate is implemented via voltage controls, while for the necessary second quantum gate, a uniform external magnetic field is introduced. The Aharonov-Bohm phases on the closed loop tunnel connections in the array are used to effectively suppress the tunneling, despite a constant tunneling amplitude in the structure. This allows one to completely stall the qubit in any arbitrary quantum superposition, providing full control of this interesting quantum system.Comment: 6 pages, 5 figures (submitted to PRB

Coulomb effects in artificial molecules

We study the capacitance spectra of artificial molecules consisting of two and three coupled quantum dots from an extended Hubbard Hamiltonian model that takes into account quantum confinement, intra- and inter-dot Coulomb interaction and tunneling coupling between all single particle states in nearest neighbor dots. We find that, for weak coupling, the interdot Coulomb interaction dominates the formation of a collective molecular state. We also calculate the effects of correlations on the tunneling probability through the evaluation of the spectral weights, and corroborate the importance of selection rules for understanding experimental conductance spectra.Comment: dvi file and 4 postscript figures, all included in uu file. To appear in Superlatt. and Microstr. Also available at http://www.phy.ohiou.edu/~ulloa/ulloa.htm