Longxiang Zhang

Single-electron charging and detection in a laterally coupled quantum-dot circuit in the few-electron regime

We provide a physical analysis of the charging and detection of the first few electrons in a laterally coupled GaAs/AlGaAs quantum-dot ~LCQD! circuit with integrated quantum point contact read-out. Our analysis is based on the numerical solution of the Kohn-Sham equation incorporated into a three-dimensional selfconsistent scheme for simulating the quantum device. Electronic states and eigenenergy spectra reflecting the particular LCQD confinement shape are obtained as a function of external gate voltages. We also derive the stability diagram for the first few electrons in the device, and obtain excellent agreement with experimental data.

Engineering the quantum point contact response to single-electron charging in a few-electron quantum-dot circuit

We show that the design of a quantum point contact adjacent to a quantum dot can be optimized to produce maximum sensitivity to single-electron charging in the quantum dot. Our analysis is based on the self-consistent solution of coupled three-dimensional Kohn-Sham and Poisson equations for the quantum circuit. We predict a detection sensitivity increase by at least 73% over the conventional design.

Photoemission Circular Dichroism and Spin Polarization of the Topological Surface States in Ultrathin Bi 2 Te 3 Films

Circular dichroism (CD) observed by photoemission, being sensitive to the orbital and spin angular momenta of the electronic states, is a powerful probe of the nontrivial surface states of topological insulators, but the experimental results thus far have eluded a comprehensive description. We report a study of Bi2Te3 films with thicknesses ranging from one quintuple layer (two-dimensional limit) to 12 layers (bulk limit) over a wide range of incident photon energy. The data show complex variations in magnitude and sign reversals, which are nevertheless well described by a theoretical calculation including all three photoemission mechanisms: dipole transition, surface photoemission, and spin-orbit coupling. The results establish the nontrivial connection between the spin-orbit texture and CD.

Quantum growth of a metal/insulator system: Lead on sapphire

We report the observation of quantum growth behavior in a metal-on-insulator system. Using insulating substrates, with their large band gaps, should maximize quantum confinement effects. In a study of Pb film growth and thermal processing on sapphire, we have observed robust preferred island height selection over a wide thickness range—a hallmark of quantum confinement effects—up to 250 °C. By contrast, room temperature is the limit for Pb films prepared on Si(111). These results provide the evidence connecting the quantum growth behavior of overlayers with the substrate band gap.

3-D self-consistent simulation of spin-qubit quantum dot circuit with integrated read-out

We present for the first time a numerical analysis of a novel laterally-coupled quantum dot (LCQD) circuit with integrated quantum point contact (QPC) detectors. Our simulation involves the self-consistent solution of three-dimensional (3-D) Poisson and Kohn-Sham equations, using the Slaters Rule for determining the charging voltage. Detailed results on conduction band profiles, eigenenergy spectra and associated wavefunctions in the dots, sensitivity of the QPC, and stability diagram are discussed for the few-electron charging regime.

Engineering Exchange Interaction in Coupled Elongated Quantum Dots

Coupled elongated quantum dots containing up to two electrons are studied with a model potential in magnetic fields. Single and two particle Schrödinger equations are solved using numerical exact diagonolization to obtain the exchange energy and chemical potentials. Special emphasis is placed on the variation of the exchange interaction between the two electrons as the dot shape is modified. As the aspect ratio between the directions perpendicular and parallel to the coupling direction of the double dots increases, the exchange energy at zero magnetic field increases, while the magnetic field at the singlet-triplet transition decreases. By investigating the charge stability diagram, we obtain the inter-dot coupling strength for various configurations. We also show the onset of electron localization into one dot due to inter-dot detuning and increasing magnetic fields.

Stability diagram and exchange interaction in coupled quantum dots in magnetic fields

Properties of the stability diagram and exchange energy of a few-electron laterally coupled quantum dots in magnetic fields are investigated. The calculations are performed by numerically exact diagonalization of the many-body Schroedinger equation. We show variations of the energy separation between the single-particle ground and first excited states, and the exchange energy with biases on the two dots at different magnetic fields. Two-dimensional single-particle wavefunction and electron density profiles show electron localization with magnetic fields. From the extracted double-triple point separation on the stability diagram, we also show that the coupling strength decrease as the magnetic field increases.

Experiments And Simulations On A Few-Electron Quantum Dot Circuit With Integrated Charge Read-Out

We report measurements and simulations on a fully tunable double quantum dot circuit, integrated with two quantum point contacts that serve as charge detectors. The circuit is defined in a two‐dimensional electron gas by means of surface gates on top of a GaAs/AlGaAs heterostructure. Full control over the electron number (down to zero), the dot‐lead coupling and the inter‐dot tunnel coupling is experimentally demonstrated. Computer simulations can map out the double dot charging diagram and show that the charge sensitivity can be significantly enhanced by improving the design of the circuit.