Pawel Hawrylak

Addition spectrum of a lateral dot from Coulomb and spin-blockade spectroscopy

Transport measurements are presented on a class of electrostatically defined lateral dots within a high mobility two dimensional electron gas (2DEG). The new design allows Coulomb Blockade(CB) measurements to be performed on a single lateral dot containing 0, 1 to over 50 electrons. The CB measurements are enhanced by the spin polarized injection from and into 2DEG magnetic edge states. This combines the measurement of charge with the measurement of spin through spin blockade spectroscopy. The results of Coulomb and spin blockade spectroscopy for first 45 electrons enable us to construct the addition spectrum of a lateral device. We also demonstrate that a lateral dot containing a single electron is an effective local probe of a 2DEG edge.

Stability Diagram of a Few-Electron Triple Dot

Individual and coupled quantum dots containing one or two electrons have been realized and are regarded as components for future quantum information circuits. In this Letter we map out experimentally the stability diagram of the few-electron triple dot system, the electron configuration map as a function of the external tuning parameters, and reveal experimentally for the first time the existence of quadruple points, a signature of the three dots being in resonance. In the vicinity of these quadruple points we observe a duplication of charge transfer transitions related to charge and spin reconfigurations triggered by changes in the total electron occupation number. The experimental results are largely reproduced by equivalent circuit analysis and Hubbard models. Our results are relevant for future quantum mechanical engineering applications within both quantum information and quantum cellular automata architectures.

Correlated few-electron states in vertical double-quantum-dot systems

The electronic properties of semiconductor, vertical, double quantum dot systems with few electrons are investigated by means of analytic, configuration-interaction, and mean-field methods. The combined effect of a high magnetic field, electrostatic confinement, and inter-dot coupling, induces a new class of few-electron ground states absent in single quantum dots. In particular, the role played by the isospin (or quantum dot index) in determining the appearance of new ground states is analyzed and compared with the role played by the standard spin.

Magnetism and Correlations in Fractionally Filled Degenerate Shells of Graphene Quantum Dots

We show that the ground state and magnetization of the macroscopically degenerate shell of electronic states in triangular gated graphene quantum dots depends on the filling fraction of the shell. The effect of degeneracy, finite size, and electron-electron interactions are treated nonperturbatively using a combination of density functional theory, tight-binding, Hartree-Fock and configuration interaction methods. We show that electronic correlations play a crucial role in determining the nature of the ground state as a function of filling fraction of the degenerate shell at the Fermi level. We find that the half-filled charge neutral shell leads to full spin polarization but this magnetic moment can be completely destroyed by adding a single electron.

Physics of lateral triple quantum-dot molecules with controlled electron numbers

We review the recent progress in theory and experiments with lateral triple quantum dots with controlled electron numbers down to one electron in each dot. The theory covers electronic and spin properties as a function of topology, number of electrons, gate voltage and external magnetic field. The orbital Hunds rules and Nagaoka ferromagnetism, magnetic frustration and chirality, interplay of quantum interference and electron-electron interactions and geometrical phases are described and related to charging and transport spectroscopy. Fabrication techniques and recent experiments are covered, as well as potential applications of triple quantum-dot molecule in coherent control, spin manipulation and quantum computation.

Response spectra from mid- to far-infrared, polarization behaviors, and effects of electron numbers in quantum-dot photodetectors

Photoresponse characteristics of InAs/GaAs self-assembled quantum-dot infrared photodetectors in a wide spectral region from the mid- to far-infrared are reported. Clear polarization behaviors with a dominant P-polarized response in the mid-infrared and a strong S-response in the far infrared are shown. These behaviors can be qualitatively understood in view of the quantum-dot shape of a large in-plane diameter and a small height in the growth direction. With a set of three samples, effects of the number of electrons per dot on the spectra are investigated.

Excitonic absorption in gate-controlled graphene quantum dots

We present a theory of excitonic processes in gate controlled graphene quantum dots. The dependence of the energy gap on shape, size and edge for graphene quantum dots with up to a million atoms is predicted. Using a combination of tight-binding, Hartree-Fock and configuration interaction methods, we show that triangular graphene quantum dots with zigzag edges exhibit optical transitions simultaneously in the THz, visible and UV spectral ranges, determined by strong electron-electron and excitonic interactions. The relationship between optical properties and finite magnetic moment and charge density controlled by an external gate is predicted. Two-dimensional graphene monolayer exhibits fascinating electronic [1–6] and optical properties[7–15] due to the zero energy gap and relativistic-like nature of quasiparticle dispersion close to the Fermi level. With recent improvements in nanofabrication techniques[16] the zero energy gap of bulk graphene can be opened via engineering size, shape, character of the edge and carrier density, and this in turn offers possibilities to simultaneously control electronic[17–25], magnetic[16, 23–30] and optical[30–32] properties of a single-material nanostructure. In this paper, we present a theory and results of numerical calculations predicting the dependence of the energy gap on shape, size and edge for graphene quantum dots with up to a million atoms. We show that triangular graphene quantum dots with zigzag edges combine magnetic and optical properties tunable with carrier density, with optical transitions simultaneously in the THz, visible and UV spectral ranges. We describe one electron properties of graphene quantum dots with N atoms and

Zero-energy states in triangular and trapezoidal graphene structures

We derive analytical solutions for the zero-energy states of degenerate shell obtained as a singular eigenvalue problem found in tight-binding (TB) Hamiltonian of triangular graphene quantum dots with zigzag edges. These analytical solutions are in agreement with previous TB and density-functional theory results for small graphene triangles and extend to arbitrary size. We also generalize these solutions to trapezoidal structure which allow us to study bowtie graphene devices.

Topological Hunds rules and the electronic properties of a triple lateral quantum dot molecule

We analyze theoretically and experimentally the electronic structure and charging diagram of three coupled lateral quantum dots filled with electrons. Using the Hubbard model and real-space exact diagonalization techniques we show that the electronic properties of this artificial molecule can be understood using a set of topological Hunds rules. These rules relate the multi-electron energy levels to spin and the inter-dot tunneling

Multiband theory of multi-exciton complexes in self-assembled quantum dots

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