Magnetic field dependence of hole levels in InAs quantum dots

We have studied the charging of InAs quantum dots with holes in perpendicular fields up to 16 T by capacitance—voltage spectroscopy. The first two charging peaks show almost no shift with magnetic field which is consistent with the filling of a twofold degenerate s-like state with no orbital angular momentum. The next four charging peaks shift towards lower and higher energy in an alternating fashion. Peaks 5 and 6 shift approximately twice as strong as peaks 3 and 4. This behavior cannot be explained by the charging of a fourfold degenerate p-shell according to Hund′s first rule. We speculate that the p-shell is not completely filled before the filling of the d-shell starts.     

Photocurrent, rectification, and magnetic field symmetry of induced current through quantum dots

We report mesoscopic dc current generation in an open chaotic quantum dot with ac excitation applied to one of the shape-defining gates. For excitation frequencies large compared to the inverse dwell time of electrons in the dot (i.e., GHz), we find mesoscopic fluctuations of induced current that are fully asymmetric in the applied perpendicular magnetic field, as predicted by recent theory. Conductance, measured simultaneously, is found to be symmetric in field. In the adiabatic (i.e., MHz) regime, in contrast, the induced current is always symmetric in field, suggesting its origin is mesoscopic rectification.     

Magnetic properties of quantum dots and rings

Exact many-body methods as well as current-spin-density functional theory are used to study the magnetism and electron localization in two-dimensional quantum dots and quasi-one-dimensional quantum rings. Predictions of broken-symmetry solutions within the density functional model are confirmed by exact configuration interaction (CI) calculations: In a quantum ring the electrons localize to form an antiferromagnetic chain which can be described with a simple model Hamiltonian. In a quantum dot the magnetic field localizes the electrons as predicted with the density functional approach. Received 5 December 2000     

Dimensional effects and magnetic field influence on excitons in coupled quantum dots and coupled quantum wells

Direct and indirect excitons in coupled quantum wells and in coupled quantum dots are studied. We consider excitons with two-dimensional, quasi-two-dimensional and three-dimensional carriers. Problems were investigated for a wide range of characteristic parameters—confining to potential steepness, distances between quantum wells or dots, effective width of wells and magnetic fields. The mutual influence of the controlling parameters of the problem on exciton properties is analyzed. Energy and wave function spectra were calculated and dispersion law and effective masses were obtained.     

Magnetic interaction between coupled quantum dots

We study the magnetic coupling in artificial molecules composed of two and four laterally coupled quantum dots. The electronic ground-state configurations of such systems are determined by applying current spin density functional theory which allows to include effects of magnetic fields. While the ground-state of a two-dot molecule with strong enough inter-dot coupling tends to be antiferromagnetic with respect to the spins of the single dot components, we find that a square lattice of four dots has a ferromagnetic ground state. Received 17 February 1999 and Received in final form 1 June 1999     

Degrees of symmetry in quantum field theories

A hierarchy of possible symmetries in quantum field theory is defined, which reaches from a purely mathematical invariance to the conventional physical invariance, including the commonly discussed type of spontaneously broken symmetry (SBS). It is shown that one type of SBS, which is usually not considered, naturally leads to theories with an algebra of non-conserved currents and a non-linearly transforming phenomenological Lagrangian. An exactly solvable model is given and some general remarks are made.     

Coulomb effects in semiconductor quantum dots

This paper presents a new model for the Internet graph (AS graph) based on the concept of heuristic trade-off optimization, introduced by Fabrikant, Koutsoupias and Papadimitriou in [5] to grow a random tree with a heavily tailed degree distribution. We propose here a generalization of this approach to generate a general graph, as a candidate for modeling the Internet. We present the results of our simulations and an analysis of the standard parameters measured in our model, compared with measurements from the physical Internet graph.Received: 9 February 2004, Published online: 14 May 2004PACS: 89.75.-k Complex systems - 89.75.Hc Networks and genealogical trees - 89.75.Da Systems obeying scaling laws - 89.75.Fb Structures and organization in complex systems - 89.65.Gh Economics; econophysics, financial markets, business and managementLRI: http: //www.lri.fr/~ihameli; CNRS, LIP, ENS Lyon : http: //www.ens-lyon.fr/~nschaban     

Hidden symmetry and correlated states of electrons and holes in quantum dots

We describe hidden symmetry and its application to the construction of exact correlated states of electrons and holes in quantum dots. The hidden symmetry is related to degenerate single particle energy shells and symmetric interactions. Both can be engineered in a quantum dot. We focus on hidden symmetry involving spin singlet pairing of electrons and spin singlet pairing of holes. Detailed calculations for a third shell are presented to illustrate the mechanism of pairing.     

Background charges and quantum effects in quantum dots transport spectroscopy

We extend a simple model of a charge trap coupled to a single-electron box to energy ranges and parameters such that it gives new insights and predictions readily observable in many experimental systems. We show that a single background charge is enough to give lines of differential conductance in the stability diagram of the quantum dot, even within undistorted Coulomb diamonds. It also suppresses the current near degeneracy of the impurity charge, and yields negative differential lines far from this degeneracy. We compare this picture to two other accepted explanations for lines in diamonds, based respectively on the excitation spectrum of a quantum dot and on fluctuations of the density-of-states in the contacts. In order to discriminate between these models, we emphasize the specific features related to environmental charge traps. Finally we show that our model accounts very well for all the anomalous features observed in silicon nanowire quantum dots.     

Signature of symmetry of laterally coupled quantum dots in far-infrared spectrum

We study the effect of structure asymmetry on the energy spectrum and the far-infrared spectrum (FIR) of a lateral coupled quantum dot. The calculated spectrum shows that the parity break of coupled quantum dot results in more coherent superpositions in the low-lying states and exhibits unique anti-crossing in the two-electron FIR spectrum modulated by a magnetic field. We also find that the Coulomb correlation effect can make the FIR spectrum of coupled quantum dot without strict parity deviate greatly from Kohn theorem, which is just contrary to the symmetric case. Our results therefore suggest that FIR spectrum may be used to determine the symmetry of coupled quantum dot and to evaluate the degree of Coulomb interaction.     

Absorption spectra of small semiconductor quantum dots

A density matrix approach has been employed to study analytically the absorption spectra of small semiconductor quantum dots under the strong confinement regime. The results are obtained for a single quantum dot (SQD) as well as for inhomogeneous distribution of quantum dots (IQDs) with Gaussian distribution of quantum dot sizes. A numerical analysis has been made for a SQD and IQDs in a CdS crystal with data taken from recent experimental work. A negative change in the absorption coefficient occurs in the shorter pump wavelength side of the spectrum due to the biexcitonic contribution. The wavelength at which crossover from positive to negative values of the change in absorption coefficient occurs is found to depend upon both the QD size as well as the excitation intensity. The results agree satisfactorily with the experimental observations in small CdS quantum dots.     

Surface-plasmon-assisted electromagnetic field enhancement in semiconductor quantum dots

The temporal development of incident electromagnetic plane waves across semiconductor quantum dots (QDs) is analyzed by the finite-difference time-domain method. By coating the QDs using thin metal films, surface plasmon polaritons (SPPs) can be created. As illustration, our modeling approach is applied to fluorescent multiphoton quantum dots made of cadmium sulphide of particular size (3.7 nm) and energy band gap (2.67 eV). When such a QD is coated by a metal film, a dipole-formed SPP is generated at the external surface of the coated QD by the incident electromagnetic wave with a photon energy of 1.34 eV corresponding to a two-photon process. When the thickness of the metal film is 0.37 nm, the peak intensity of the SPP oscillates through both the thin metal film and the core QD, resulting in an electromagnetic field inside the QD enhanced by a factor of 10, and thus an increased two-photon excitation that can be useful for bioimaging applications. Further increasing the metal film thickness blockades the SPP initially generated at the external surface of the coated QD from penetrating through the metal film, reducing the electromagnetic field inside the QD. PACS 73.22.-f; 78.67.Hc     

Infinite conformal symmetry in two-dimensional quantum field theory

We present an investigation of the massless, two-dimentional, interacting field theories. Their basic property is their invariance under an infinite-dimensional group of conformal (analytic) transformations. It is shown that the local fields forming the operator algebra can be classified according to the irreducible representations of Virasoro algebra, and that the correlation functions are built up of the “conformal blocks” which are completely determined by the conformal invariance. Exactly solvable conformal theories associated with the degenerate representations are analyzed. In these theories the anomalous dimensions are known exactly and the correlation functions satisfy the systems of linear differential equations.     

Local field effects on phonon-induced transparency in quantum dots embedded in a semiconductor medium

We study theoretically the influence of local fields on phonon-induced transparency (PIT) in quantum-dot systems embedded in a semiconductor matrix. As compared with our previous work without local field effects, we present analytical and numerical results from solution of the generalized optical Bloch equations including the local field effects. It is shown that the local field effects broaden the transparency window due to PIT and reduce the group velocity of light. For some specific parameters of the light and quantum dots, fast light can be obtained in such systems. The results also demonstrate that Kerr nonlinearity is enhanced greatly due to the local field effects. PACS 42.50.Gy; 78.67.Hc; 73.21.La; 03.67.-a     

Magnetic and optical properties of self-organized InMnAs quantum dots

Ten layers of self-assembled InMnAs quantum dots with InGaAs barrier were grown on high resistivity (1 0 0) p-type GaAs substrates by molecular beam epitaxy (MBE). The presence of ferromagnetic structure was confirmed in the InMnAs diluted magnetic quantum dots. The ten layers of self-assembled InMnAs quantum dots were found to be semiconducting, and have ferromagnetic ordering with a Curie temperature, T_C=80 K. It is likely that the ferromagnetic exchange coupling of sample with T_C=80 K is hole mediated resulting in Mn substituting In and is due to the bound magnetic polarons co-existing in the system. PL emission spectra of InMnAs samples grown at temperature of 275, 260 and 240 °C show that the interband transition peak centered at 1.31 eV coming from the InMnAs quantum dot blueshifts because of the strong confinement effects with increasing growth temperature.     

Auxiliary field quantum Monte-Carlo simulation of interacting electrons in quantum dots

Accurate auxiliary field quantum Monte-Carlo (AFQMC) simulations of interacting electrons in quantum dots are reported. Two different formulations of this approach are presented both of which have been designed specifically for application to quantum dots. A deflation technique for calculation of anti-symmetrized traces is introduced. The auxiliary field is sampled with a hybrid algorithm and the artificial dynamics needed for use with the present formulation of AFQMC is described. The constrained path approximation is used to control the sign problem. Results for the ground state energy of two spin-polarised, interacting electrons are presented and are found to agree well with exact diagonalization results for a wide range of screening lengths. The sign problem does not appear in the regime of small screening length.     

Magnetic field dependence of electronic structures in a single and double quantum dots by 3D-MHFKS calculation

The 3-dimensional Mesh-Hartree-Fock-Kohn-Sham (3D-MHFKS) calculation is applied to study the magnetic (B-) field dependence of the electronic structures of circular-shaped vertical quantum dot (Q-dot) with electron number (N) in double barrier structure (DBS) and also coupled double Q-dots in triple barrier structure (TBS). One of the advantageous points of the 3D-MHFKS calculation is that the strength of coupling between two dots are explicitly evaluated by introducing the realistic barrier in TBS as a straightforward extension of 3D-MHFKS calculation of the single Q-dot in DBS. The calculated chemical potentials represented in B-N phase diagram are consistently and systematically discussed by showing the B-field dependence of the occupied single particle energy levels from the view point how the electronic states transfer sequentially from Fock-Darwin (FD) to lowest Landau (LL) and from LL to the spin flip (SF) and from SF to spin-polarized maximum density droplet (MDD) domains as increasing B-field in the Q-dots.