Nikolai B. Zhitenev

PERIODIC AND APERIODIC BUNCHING IN THE ADDITION SPECTRA OF QUANTUM DOTS

We study electron addition spectra of quantum dots in a broad range of electron occupancies starting from the first electron. Spectra for dots containing <200 electrons reveal a surprising feature. Electron additions are not evenly spaced in gate voltage. Rather, they group into bunches. With increasing electron number the bunching evolves from occurring randomly to periodically at about every fifth electron. The periodicity of the bunching and features in electron tunneling rates suggest that the bunching is associated with electron additions into spatially distinct regions within the dots.

Localization in artificial disorder: two coupled quantum dots

Using single electron capacitance spectroscopy, we study electron additions in quantum dots containing two potential minima separated by a shallow barrier. Analysis of the addition spectra in the magnetic field allows us to distinguish between electrons delocalized over the entire dot and those localized in either of the potential minima. We demonstrate that a high magnetic field abruptly splits up a low-density droplet into two smaller fragments, each residing in a potential minimum. An unexplained cancellation of electron repulsion between electrons in these fragments gives rise to paired electron additions.

Single-electron transistor as a charge sensor for semiconductor applications

We describe the use of aluminum single-electron transistors (SETs) to measure, with extremely high sensitivity, the fluctuation of charge in semiconductor quantum dots. Our method of fabricating SETs results in excellent reliability and reproducibility.

Single-electron capacitance spectroscopy of vertical quantum dots using a single-electron transistor

We have incorporated an aluminum single-electron transistor (SET) directly on top of a vertical quantum dot, enabling the use of the SET as an electrometer that is extremely responsive to the motion of charge into and out of the dot. Charge induced on the SET central island from single-electron additions to the dot modulates the SET output, and we describe two methods for demodulation that permit quantitative extraction of the quantum dot capacitance signal. The two methods produce closely similar results for the determined single-electron capacitance peaks.

NEW CLASS OF RESONANCES AT THE EDGE OF THE TWO-DIMENSIONAL ELECTRON GAS

We measure the frequency dependent capacitance of a gate covering the edge and part of a two-dimensional electron gas in the quantum Hall regime. In applying a positive gate bias, we create a metallic puddle under the gate surrounded by an insulating region. Charging of the puddle occurs via electron tunneling from a metallic edge channel. Analysis of the data allows direct extraction of this tunneling conductance. Novel conductance resonances appear as a function of gate bias. Samples with gates ranging from 1-170~

Delocalization in a double quantum dot system

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Localization-Delocalization Transition in Quantum Dots

m along the edge display strikingly similar resonance spectra. The data suggest the existence of unexpected structure, homogeneous over long length scales, at the sample edge.

Paired additions of electrons into quantum dots: How and why does localization destroy Coulomb blockade to producepairing?

We study the delocalization process in a double dot system by merging two quantum dots of different densities. The addition of each electron to the system is detected by a capacitive technique, allowing us to monitor the evolution of the addition spectrum over a very broad range of electron densities. The data indicate that delocalization can occur in different fashions depending on the merging density. While the magnetic field has no effect on the merging of two high-density dots (n⩾2×1011cm−2), it dramatically affects the merging of two dots of lower density (n∼1–2×1011cm−2).