C. Payette

Two-level anti-crossings high up in the single-particle energy spectrum of a quantum dot

Abstract We study the evolution with magnetic field of the single-particle energy levels high up in the energy spectrum of one dot as probed by the ground state of the adjacent dot in a weakly coupled vertical quantum dot molecule. We find that the observed spectrum is generally well accounted for by the calculated spectrum for a two-dimensional elliptical parabolic confining potential, except in several regions where two or more single-particle levels approach each other. We focus on two two-level crossing regions which show unexpected anti-crossing behaviour and contrasting current dependences. Within a simple coherent level mixing picture, we can model the current carried through the coupled states of the probed dot provided the intrinsic variation with magnetic field of the current through the states (as if they were uncoupled) is accounted for by an appropriate interpolation scheme.

Hyperfine-Coupling-Programming of Current through Coupled Quantum Dots with Multiple-Sweep Bias Voltage Waveforms

Fine features in the leakage current within the two-electron spin blockade region in a weakly coupled vertical double dot device induced on application of a moderate out-of-dot-plane magnetic field (up to 2.5 T) are studied. The nature of these features is consistent with an electron-spin nuclear-spin (hyperfine) interaction origin, and furthermore, their appearance depends strongly on the bias voltage sweep direction through the spin blockade region. Applying multi-part 10 mHz bias voltage waveforms, we can program the total current response via the hyperfine interaction. We demonstrate the operation of shifting a down- (up-) bias sweep through three up- (down-) bias sweeps which leads to a unique outcome in the measured current.

Modeling single-particle energy levels and resonance currents in a coherent electronic quantum dot mixer

We present model calculations based on a coherent tunneling picture, which reproduce well both the single-particle energy level position and the resonant current strength at two typical anticrossings, one involving two levels and the other three levels in a coherent mixer composed of two weakly coupled vertical quantum dots. An essential ingredient is the inclusion of higher degree terms to account for deviations from an ideal elliptical parabolic confining potential in realistic dots. We also calculate density plots of the mixed states for the modified potential.

Magnetic field-induced effects in the high source–drain bias current of weakly coupled vertical quantum dot molecules

We report on the basic properties of recently observed magnetic field resonance, induced time-dependent oscillation, and hysteresis effects in the current flowing through two weakly coupled vertical quantum dots at high source-drain bias (up to a few tens of mV). These effects bare some similarity to those reported in the N = 2 spin-blockade regime, usually for weak in-plane magnetic field, of quantum dot molecules and attributed to hyperfine coupling, except here the measurements are conducted outside of the spin-blockade regime and the out-of-plane magnetic field is up to ∼6T.

Gate adjustable coherent three and four level mixing in a vertical quantum dot molecule

We study level mixing in the single-particle energy spectrum of one of the constituent quantum dots in a vertical double quantum dot by performing magneto-resonant-tunneling spectroscopy. The device used in this study differs from previous vertical double quantum dot devices in that the single side gate is now split into four separate gates. Because of the presence of natural perturbations caused by anharmonicity and anisotropy, applying different combinations of voltages to these gates allows us to alter the effective potential landscape of the two dots and hence influence the level mixing. We present here preliminary results from one three level crossing and one four level crossing high up in the energy spectrum of one of the probed quantum dots, and demonstrate that we are able to change significantly the energy dispersions with magnetic field in the vicinity of the crossing regions.

Slow and Fast Electron Channels in a Coherent Quantum Dot Mixer

We describe a means to realize slow and fast electron channels by coherent mixing of single-particle levels in quantum dots. The underlying physics, which gives insight into state superposition, can potentially be realized in multi-dot structures with complex gate control. However, we employ vertical double dot structures and in our scheme the mixing of single-particle levels arises because of natural perturbations in the confining potential of the high-symmetry dots. Additionally, because of the intrinsic properties of a Fock–Darwin-like spectrum, we utilize a magnetic field to bring multiple single-particle energy levels into close proximity. We determine single-electron resonant tunneling times (effectively dwell times when on resonance) that are either extended in the slow channel or shortened in the fast channel. Most dramatically, for the slow channel, slow-down factors of ~10 and single-electron resonant tunneling times extended into the µs range are demonstrated in all systems of two, three, and four mixed single-particle states investigated here.