Spin Fine Structure in Optically Excited Quantum Dot Molecules

The interaction between spins in coupled quantum dots is revealed in distinct fine structure patterns in the measured optical spectra of InAs/GaAs double quantum dot molecules containing zero, one, or two excess holes. The fine structure is explained well in terms of a uniquely molecular interplay of spin exchange interactions, Pauli exclusion and orbital tunneling. This knowledge is critical for converting quantum dot molecule tunneling into a means of optically coupling not just orbitals, but spins.Comment: 10 pages, 7 figures, added material, (published

Photoluminescence Spectroscopy of the Molecular Biexciton in Vertically Stacked Quantum Dot Pairs

We present photoluminescence studies of the molecular neutral biexciton-exciton spectra of individual vertically stacked InAs/GaAs quantum dot pairs. We tune either the hole or the electron levels of the two dots into tunneling resonances. The spectra are described well within a few-level, few-particle molecular model. Their properties can be modified broadly by an electric field and by structural design, which makes them highly attractive for controlling nonlinear optical properties.Comment: 4 pages, 5 figures, (v2, revision based on reviewers comments, published

Electrically tunable g-factors in quantum dot molecular spin states

We present a magneto-photoluminescence study of individual vertically stacked InAs/GaAs quantum dot pairs separated by thin tunnel barriers. As an applied electric field tunes the relative energies of the two dots, we observe a strong resonant increase or decrease in the g-factors of different spin states that have molecular wavefunctions distributed over both quantum dots. We propose a phenomenological model for the change in g-factor based on resonant changes in the amplitude of the wavefunction in the barrier due to the formation of bonding and antibonding orbitals.Comment: 5 pages, 5 figures, Accepted by Phys. Rev. Lett. New version reflects response to referee comment

Sequential and co-tunneling behavior in the temperature-dependent thermopower of few-electron quantum dots

We have studied the temperature dependent thermopower of gate-defined, lateral quantum dots in the Coulomb blockade regime using an electron heating technique. The line shape of the thermopower oscillations depends strongly on the contributing tunneling processes. Between 1.5 K and 40 mK a crossover from a pure sawtooth- to an intermitted sawtooth-like line shape is observed. The latter is attributed to the increasing dominance of cotunneling processes in the Coulomb blockade regime at low temperatures.Comment: 4 pages, 4 figures, submitted to Phys. Rev.

Thermopower of a Kondo-correlated quantum dot

The thermopower of a Kondo-correlated gate-defined quantum dot is studied using a current heating technique. In the presence of spin correlations the thermopower shows a clear deviation from the semiclassical Mott relation between thermopower and conductivity. The strong thermopower signal indicates a significant asymmetry in the spectral density of states of the Kondo resonance with respect to the Fermi energies of the reservoirs. The observed behavior can be explained within the framework of an Anderson-impurity model. Keywords: Thermoelectric and thermomagnetic effects, Coulomb blockade, single electron tunneling, Kondo-effect PACS Numbers: 72.20.Pa, 73.23.HkComment: 4 pages, 4 figures, revised version, changed figure

Thermoelectric effects in quantum dots

We report a numerical renormalization-group study of the thermoelectric effect in the single-electron transistor (SET) and side-coupled geometries. As expected, the computed thermal conductance and thermopower curves show signatures of the Kondo effect and of Fano interference. The thermopower curves are also affected by particle-hole asymmetry.Comment: 8 pages with 3 figures; accepted for publication in Physica B (special issue 'Strongly Correlated Electron Systems-SCES2008'

Characteristic molecular properties of one-electron double quantum rings under magnetic fields

The molecular states of conduction electrons in laterally coupled quantum rings are investigated theoretically. The states are shown to have a distinct magnetic field dependence, which gives rise to periodic fluctuations of the tunnel splitting and ring angular momentum in the vicinity of the ground state crossings. The origin of these effects can be traced back to the Aharonov-Bohm oscillations of the energy levels, along with the quantum mechanical tunneling between the rings. We propose a setup using double quantum rings which shows that Aharonov-Bohm effects can be observed even if the net magnetic flux trapped by the carriers is zero.Comment: 16 pages (iopart format), 10 figures, accepted in J.Phys.Cond.Mat

Measuring Temperature Gradients over Nanometer Length Scales

When a quantum dot is subjected to a thermal gradient, the temperature of electrons entering the dot can be determined from the dot's thermocurrent if the conductance spectrum and background temperature are known. We demonstrate this technique by measuring the temperature difference across a 15 nm quantum dot embedded in a nanowire. This technique can be used when the dot's energy states are separated by many kT and will enable future quantitative investigations of electron-phonon interaction, nonlinear thermoelectric effects, and the effciency of thermoelectric energy conversion in quantum dots.Comment: 6 pages, 5 figure

Optically-controlled single-qubit rotations in self-assembled InAs quantum dots

We present a theory of the optical control of the spin of an electron in an InAs quantum dot. We show how two Raman-detuned laser pulses can be used to obtain arbitrary single-qubit rotations via the excitation of an intermediate trion state. Our theory takes into account a finite in-plane hole $g$-factor and hole-mixing. We show that such rotations can be performed to high fidelities with pulses lasting a few tens of picoseconds.Comment: 6 pages, 4 figures; minor changes, J-ref adde