Single-electron charging of quantum-dot atoms

Symmetry selected quantum dynamics of few electrons in nanopillar transistors

Study on single electron tunnel using current-voltage characteristics in nanopillar transistors at 298 K show that the mapping between the Nth electron excited in the central box ∼8.5 × 8.5 × 3 nm3 and the Nth tunnel peak is not in the one-to-one correspondence to suggest that the total number N of electrons is not the best quantum number for characterizing the quality of single electron tunnel in a three-dimensional quantum box transistor. Instead, we find that the best number is the sub-quantum number nz of the conduction z channel. When the number of electrons in nz is charged to be even and the number of electrons excited in the nx and ny are also even at two, the adding of the third electron into the easy nx/ny channels creates a weak symmetry breaking in the parity conserved x-y plane to assist the indirect tunnel of electrons. A comprehensive model that incorporates the interactions of electron-electron, spin-spin, electron-phonon, and electron-hole is proposed to explain how the excited even electrons can be stabilized in the electric-field driving channel. Quantum selection rules with hierarchy for the ni (i = x, y, z) and N = Σni are tabulated to prove the superiority of nz over N.

Magnetic field effect on the energy levels of an exciton in a GaAs quantum dot: Application for excitonic lasers

The problem of an exciton trapped in a Gaussian quantum dot (QD) of GaAs is studied in both two and three dimensions in the presence of an external magnetic field using the Ritz variational method, the 1/N expansion method and the shifted 1/N expansion method. The ground state energy and the binding energy of the exciton are obtained as a function of the quantum dot size, confinement strength and the magnetic field and compared with those available in the literature. While the variational method gives the upper bound to the ground state energy, the 1/N expansion method gives the lower bound. The results obtained from the shifted 1/N expansion method are shown to match very well with those obtained from the exact diagonalization technique. The variation of the exciton size and the oscillator strength of the exciton are also studied as a function of the size of the quantum dot. The excited states of the exciton are computed using the shifted 1/N expansion method and it is suggested that a given number of stable excitonic bound states can be realized in a quantum dot by tuning the quantum dot parameters. This can open up the possibility of having quantum dot lasers using excitonic states.

Phonon-mediated generation of quantum correlations between quantum dot qubits

We study the generation of quantum correlations between two excitonic quantum dot qubits due to their interaction with the same phonon environment. Such generation results from the fact that during the pure dephasing process at finite temperatures, each exciton becomes entangled with the phonon environment. We find that for a wide range of temperatures quantum correlations are created due to the interaction. The temperature-dependence of the level of correlations created displays a trade-off type behaviour; for small temperatures the phonon-induced distrubance of the qubit states is too small to lead to a distinct change of the two-qubit state, hence, the level of created correlations is small, while for large temperatures the pure dephasing is not accompanied by the creation of entanglement between the qubits and the environment, so the environment cannot mediate qubit-qubit quantum correlations.

Infrared transmission spectroscopy of charge carriers in self-assembled InAs quantum dots under surface electric fields

We present a study on the intersublevel spacings of electrons and holes in a single layer of InAs self-assembled quantum dots. We use Fourier transform infrared transmission spectroscopy via a density chopping scheme for direct experimental observation of the intersublevel spacings of electrons without any external magnetic field. Epitaxial, complementary-doped and semi-transparent electrostatic gates are grown within the ultra high vacuum conditions of molecular beam epitaxy to voltage-tune the device, while a two dimensional electron gas (2DEG) serves as a back contact. Spacings of the hole sublevels are indirectly calculated from the photoluminescence spectrum by using a simple model given by Warburton et al [1]. Additionally, we observe that the intersubb and resonances of the 2DEG are enhanced due to the quantum dot layer on top of the device.

Electron spin for classical information processing: a brief survey of spin-based logic devices, gates and circuits

In electronics, information has been traditionally stored, processed and communicated using an electron's charge. This paradigm is increasingly turning out to be energy-inefficient, because movement of charge within an information processing device invariably causes current flow and an associated dissipation. Replacing 'charge' with the 'spin' of an electron to encode information may eliminate much of this dissipation and lead to more energy-efficient 'green electronics'. This realization has spurred significant research in spintronic devices and circuits where spin either directly acts as the physical variable for hosting information or augments the role of charge. In this review article, we discuss and elucidate some of these ideas, and highlight their strengths and weaknesses. Many of them can potentially reduce energy dissipation significantly, but unfortunately are error-prone and unreliable. Moreover, there are serious obstacles to their technological implementation that may be difficult to overcome in the near term. This review addresses three constructs: (1) single devices or binary switches that can be constituents of Boolean logic gates for digital information processing, (2) complete gates that are capable of performing specific Boolean logic operations, and (3) combinational circuits or architectures (equivalent to many gates working in unison) that are capable of performing universal computation.

A study of two confined electrons using the Woods-Saxon potential

In this paper, we studied two electrons confined in a quantum dot with the Woods-Saxon potential by using the method of numerical diagonalization of the Hamiltonian matrix within the effective-mass approximation. The great advantage of our methodology is that it enables confinement regimes by varying two parameters in the model potential. A ground-state behavior (singlet [Formula: see text] triplet state transitions) as a function of the strength of a magnetic field has been investigated. We found that the confinement barrier size and the barrier inclination of a Woods-Saxon potential are important for the singlet-triplet oscillation of a two-electron quantum dot. Based on the computed energies and wavefunctions, the linear and nonlinear optical absorption coefficients have been examined between the (1)S state (L = 0) and the (1)P state (L = 1). The results are presented as a function of the incident photon energy for the different values of the barrier size and height. It is found that the optical properties of the two-electron system in a quantum dot are strongly affected by the barrier height and size.

Fock-Darwin states of dirac electrons in graphene-based artificial atoms

We investigate the Fock-Darwin states of the massless chiral fermions confined in a graphitic parabolic quantum dot. In light of Klein tunneling, we analyze the condition for confinement of the Dirac fermions in a cylindrically symmetric potential. New features of the energy levels of the Dirac electrons as compared to the conventional electronic systems are discussed. We also evaluate the dipole-allowed transitions in the energy levels of the dots. We propose that in the high magnetic field limit, the band parameters can be accurately determined from the dipole-allowed transitions.

Excitation spectrum of two correlated electrons in a lateral quantum dot with negligible Zeeman splitting

The excitation spectrum of a two-electron quantum dot is investigated by tunneling spectroscopy in conjunction with theoretical calculations. The dot made from a material with negligible Zeeman splitting has a moderate spatial anisotropy leading to a splitting of the two lowest triplet states at zero magnetic field. In addition to the well-known triplet excitation at zero magnetic field, two additional excited states are found at finite magnetic field. The lower one is identified as the second excited singlet state on the basis of an avoided crossing with the first excited singlet state at finite fields. The measured spectra are in remarkable agreement with exact-diagonalization calculations. The results prove the significance of electron correlations and suggest the formation of a state with Wigner-molecular properties at low magnetic fields.

Optical signatures of spin-orbit interaction effects in a parabolic quantum dot

We demonstrate here that the dipole-allowed optical absorption spectrum of a parabolic quantum dot subjected to an external magnetic field reflects the interelectron interaction effects when the spin-orbit (SO) interaction is also taken into account. We have investigated the energy spectra and the dipole-allowed transition energies for up to four interacting electrons parabolically confined, and have uncovered several novel effects in those spectra that are solely due to the SO interaction.