Sergio E. Ulloa

Coherent control of tunneling in a quantum dot molecule

Quantum dot ~QD! structures provide a three-dimensional confinement of carriers. Electrons and holes in the QD can occupy only a set of states with discrete energies, just as in an atom, and can thus be used to perform ‘‘atomic physics’’ experiments in solid-state structures. One advantage of QD’s is that they provide different energy scales and physical features which can be easily varied over a wide range of values. Most important, perhaps, is that QD’s also allow the control of direct quantum-mechanical electronic coupling with not only composition but externally applied voltages. These flexible systems represent therefore the ideal for theoretical and experimental investigations, where the interactions between light and matter can be studied in a fully controlled, wellcharacterized environment, and with excellent optical and electrical probes. These features make semiconductor QD’s promising candidates for applications in electro-optical devices such as QD lasers, 1,2 and in quantum information processing. 3‐ 6 In the latter case, one can exploit the optical excitation in a QD, 3,5 or its spin state, 4,6 as qubits. These high

All-electric quantum point contact spin-polarizer

The controlled creation, manipulation and detection of spin-polarized currents by purely electrical means remains a central challenge of spintronics. Efforts to meet this challenge by exploiting the coupling of the electron orbital motion to its spin, in particular Rashba spin-orbit coupling, have so far been unsuccessful. Recently, it has been shown theoretically that the confining potential of a small current-carrying wire with high intrinsic spin-orbit coupling leads to the accumulation of opposite spins at opposite edges of the wire, though not to a spin-polarized current. Here, we present experimental evidence that a quantum point contact -- a short wire -- made from a semiconductor with high intrinsic spin-orbit coupling can generate a completely spin-polarized current when its lateral confinement is made highly asymmetric. By avoiding the use of ferromagnetic contacts or external magnetic fields, such quantum point contacts may make feasible the development of a variety of semiconductor spintronic devices.

Magnetic field effects on quantum ring excitons

We study the effect of magnetic field and geometric confinement on excitons confined to a quantum ring. We use analytical matrix elements of the Coulomb interaction and diagonalize numerically the effective-mass Hamiltonian of the problem. To explore the role of different boundary conditions, we investigate the quantum ring structure with a parabolic confinement potential, which allows the wave functions to be expressed in terms of center of mass and relative degrees of freedom of the exciton. On the other hand, wave functions expressed in terms of Bessel functions for electron and hole are used for a hard-wall confinement potential. The binding energy and electron--hole separation of the exciton are calculated as function of the width of the ring and the strength of an external magnetic field. The linear optical susceptibility as a function of magnetic fields is also discussed. We explore the Coulomb electron--hole correlation and magnetic confinement for several ring width and size combinations. The Aharanov--Bohm oscillations of exciton characteristics predicted for one-dimensional rings are found to not be present in these finite-width systems.

Mobility edge in aperiodic Kronig-Penney potentials with correlated disorder: Perturbative approach

It is shown that a non-periodic Kronig-Penney model exhibits mobility edges if the positions of the scatterers are correlated at long distances. An analytical expression for the energy-dependent localization length is derived for weak disorder in terms of the real-space correlators defining the structural disorder in these systems. We also present an algorithm to construct a non-periodic but correlated sequence exhibiting desired mobility edges. This result could be used to construct window filters in electronic, acoustic, or photonic non-periodic structures.

Spatially Resolved Manipulation of Single Electrons in Quantum Dots Using a Scanned Probe

The scanning metallic tip of a scanning force microscope was coupled capacitively to electrons confined in a lithographically defined gate-tunable quantum dot at a temperature of 300 mK. Single electrons were made to hop on or off the dot by moving the tip or by changing the tip bias voltage owing to the Coulomb-blockade effect. Spatial images of conductance resonances map the interaction potential between the tip and individual electronic quantum dot states. Under certain conditions this interaction is found to contain a tip-voltage induced and a tip-voltage-independent contribution.

Decoherence of rabi oscillations in a single quantum dot.

We develop a realistic model of Rabi oscillations in a quantum-dot photodiode. Based in a multiexciton density matrix formulation we show that for short pulses the two-level model fails and higher levels should be taken into account. This affects some of the experimental conclusions, such as the inferred efficiency of the state rotation (population inversion) and the deduced value of the dipole interaction. We also show that the damping observed cannot be explained using constant rates with fixed pulse duration. We demonstrate that the damping observed is in fact induced by an off-resonant excitation to or from the continuum of wetting layer states. Our model describes the nonlinear decoherence behavior observed in recent experiments.

Selection rules for transport excitation spectroscopy of few-electron quantum dots

Tunneling of electrons traversing a few-electron quantum dot is strongly influenced by the Coulomb interaction leading to Coulomb blockade effects and single-electron tunneling. We present calculations which demonstrate that correlations between the electrons cause a strong suppression of most of the energetically allowed tunneling processes involving excited dot states. The excitation of center-of-mass modes, in contrast, is unaffected by the Coulomb interaction. Therefore, channels connected to these modes dominate the excitation spectra in transport measurements.

Magnetic-field-dependent transmission phase of a double-dot system in a quantum ring.

The Aharonov-Bohm effect is measured in a four-terminal open ring geometry. Two quantum dots are embedded in the structure, one in each of the two interfering paths. The number of electrons in the two dots can be controlled independently. The transmission phase is measured as electrons are added to or taken away from the individual quantum dots. Although the measured phase shifts are in qualitative agreement with theoretical predictions, the phase evolution exhibits unexpected dependence on the magnetic field. Phase lapses are found only in certain ranges of the magnetic field.

Extended states in disordered systems: Role of off-diagonal correlations

We study one-dimensional systems with random diagonal disorder but off-diagonal short-range correlations imposed by structural constrains. We find that these correlations generate effective conduction channels for finite systems. At a certain golden correlation condition for the hopping amplitudes, we find an extended state for an infinite system. Our model has important implications to charge transport in DNA molecules, and a possible set of experiments in semiconductor superlattices is proposed to verify our most interesting theoretical predictions.

Zeeman splitting of shallow donors in GaN

The Zeeman splitting of the donor spectra in cubic and hexagonal GaN is 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.