C. Livermore

The Coulomb Blockade in Coupled Quantum Dots

Individual quantum dots are often referred to as “artificial atoms.” Two tunnel-coupled quantum dots can be considered an “artificial molecule.” Low-temperature measurements were made on a series double quantum dot with adjustable interdot tunnel conductance that was fabricated in a gallium arsenide-aluminum gallium arsenide heterostructure. The Coulomb blockade was used to determine the ground-state charge configuration within the “molecule” as a function of the total charge on the double dot and the interdot polarization induced by electrostatic gates. As the tunnel conductance between the two dots is increased from near zero to 2e2/h (where e is the electron charge and h is Plancks constant), the measured conductance peaks of the double dot exhibit pronounced changes in agreement with many-body theory.

Evolution of the Coulomb gap in tunnel-coupled quantum dots

We report differential conductance measurements through a double quantum dot with adjustable interdot tunneling rate. The Coulomb gap of the double dot decreases continuously as the interdot tunnel conductance Gint is increased, from the gap of two isolated dots (when Gint∼0) to the gap of a single large dot of twice the total capacitance (when Gint=2e2/h). Excited electronic states in differential conductance measurements on single dots show a spectrum which is essentially independent of the number of electrons in the dot, but is not uniform.

DIRECT MEASUREMENT OF THE DESTRUCTION OF CHARGE QUANTIZATION IN A SINGLE-ELECTRON BOX

We report here direct measurements of the destruction of charge quantization in a single-electron box, the first over the full range of box-to-lead conductance values from G≅0 to the conductance quantum GQ=2e2/h, using a sensitive single-electron transistor (SET) electrometer. The sensitivity of the electrometer is measured to be δq≅6×10−5e/√Hz and its superiority to conductance measurements of charge fluctuations is clearly demonstrated. As the rate of quantum mechanical tunneling from the box to its lead is increased, the quantization of charge is destroyed, disappearing entirely at G=GQ in agreement with theory.

EVOLUTION OF COULOMB BLOCKADE SPECTRA IN PARALLEL COUPLED QUANTUM DOTS

We report low-temperature conductance measurements in the Coulomb blockade regime on two nominally identical tunnel-coupled quantum dots in parallel defined electrostatically in the two-dimensional gas of a GaAs/AlGaAs heterostructure. We find that the Coulomb blockade spectra of such devices exhibit two distinct sets of peaks, each of which behaves differently with varying interdot tunnel conductance and with temperature. The results conform to recent theories regarding the role of interdot quantum charge fluctuations, and provide evidence for the possible role of inelastic cotunneling between dots at finite interdot conductances.

Measuring interactions between tunnel-coupled quantum dots in the quantum Hall regime

We present measurements of the relaxation of frustrated charge configurations via tunneling in a double quantum dot in the quantum Hall regime. We studied transport through two quantum dots in series at each of three Landau level filling factors: ν=2, 3, and 4. The double dot conductance was measured as a function of the induced charge on each dot and of the interdot tunnel conductance to demonstrate the evolution of the charging diagram with increasing interdot electron tunneling. At all three filling factors, we find that the evolution from well separated to joined dots is complete at an interdot tunnel conductance Gint≅e2/h, in contrast with the zero magnetic field case. We also observe that the residual interaction energy relative to the charging energy increases above the zero field value.

Coulomb gap of coupled quantum dots with adjustable interdot tunneling

Abstract We report low-temperature tunneling measurements at zero magnetic field through a double quantum dot with adjustable interdot tunneling rate, fabricated in a GaAs/AlGaAs heterostructure. We have measured the dependence of the current-voltage characteristic on interdot tunneling, and find that the Coulomb gap decreases continuously from that of two dots in series to that of a single large dot with twice the total capacitance as the interdot tunneling is increased from near zero to 2 e 2 / h . These results are in agreement with many-body charge fluctuations due to tunneling between the dots.

CONDUCTANCE OSCILLATIONS IN TUNNEL-COUPLED QUANTUM DOTS IN THE QUANTUM HALL REGIME

We present measurements of transport through two tunnel-coupled quantum dots of different sizes connected in series in a strong, variable, perpendicular magnetic field. Double dot conductance was measured both as a function of magnetic field, which was varied across the filling factor nu = 4 quantum Hall plateau, and as a function of charge induced evenly on the two dots. The conductance peaks undergo position shifts and height modulations as the magnetic field is varied. These shifts and modulations form a pattern that repeats over large ranges of magnetic field and with the addition of double dot charge. The robust pattern repetition is consistent with a frequency locking effect.