We present an exact diagonalization approach for atomistic calculation of excitonic properties of semiconductor nanostructures under piezoelectric field. The method allows for efficient treatment of both single particle and many-body states at a small computational cost and results in a very good agreement with the full diagonalization treatment. We illustrate our approach by analyzing the effect of a piezoelectric field on a spectra of a self-assembled InAs/GaAs lens-shaped quantum dot. We study the influence of linear and quadratic piezoelectric terms on the quantum dot electronic structure and importantly we found that the non-linear, density functional based theory of piezoelectricity produces results very similar to those obtained by a well-established linear approach utilizing empirical parameters.
Building on the effective-mass envelope function theory, this paper focuses on the study of the energy band and the absorption coefficient of InAs/In_{x}Ga_{1 - x}As quantum dots in a well (DWELL) structure. In contrast to InAs/In_{0.15}Ga_{0.85}As quantum DWELL, the InAs/In_{0.2}Ga_{0.8}As quantum DWELL has lower ground states. With the thickness of In_{0.15}Ga_{0.85}As layer changing from 7 nm to 9 nm and In_{0.2}Ga_{0.8}As layer changing from 9 nm to 12 nm, the calculation shows that their absorption coefficient spectra takes a red shift in the long-wave infrared and far-infrared ranges, respectively. Moreover, when the thickness of the In_{x}Ga_{1 - x}As layer is defined as 9 nm, the absorption coefficient spectra of InAs/In_{0.2}Ga_{0.8}As DWELL shows a obvious red shift comparing with that of InAs/In_{0.15}Ga_{0.85}As DWELL.
We present calculation results of multi-color spontaneous emission from quantum-dot-quantum-well semiconductor heteronanocrystals. Our theoretical results explain experimental results of onion-like spherical system similar to: CdSe (core), ZnS (shell), CdSe (shell) spherical quantum dots surrounded by ZnS. We demonstrate influence of shell thickness to exciton localization in distinct layers of heteronanocrystals. Multi-color emission of such heterosystem is determined by l=0, n=1 state localization in CdSe core and by l=0, n=2 state localization in CdSe shell.
We theoretically study the optical properties and the electronic structure of highly elongated quantum dots (quantum dashes) and show how geometrical fluctuations affect the excitonic spectrum of the system. The dependence of the absorption intensities on the geometrical properties (depth and length) of the trapping center in a quantum dash is analyzed and the dependence of the degree of the linear polarization on these geometrical parameters is studied.
Binding energy of a hydrogenic impurity located at the center of the CdTe/ZnTe spherical quantum dot has been calculated under the effective mass approximation by solving Schrödinger equation analytically. Eigen energies are expressed in terms of the Whittaker function and Coulomb wave function. The results show that impurity binding energy strongly depends on QD size if it is around one effective Bohr radius.
Recently, there have been increasing demands for controlling individual electrons, photons, and dopants in developing nm scale Si devices. Our most recent results on Si single-electron nano-devices will be presented. We have demonstrated single-electron transfer in random-tunnel-junctions by a cycle of ac gate bias, detection of photons and detection of individual acceptor ions by Si single-hole transistor.
The optical excitonic Aharonov-Bohm effect in type-I three-dimensional (In,Ga)As/GaAs nanorings is theoretically explored. The single-particle states of the electron and the hole are extracted from the effective mass theory in the presence of inhomogeneous strain, and an exact numerical diagonalization approach is used to compute the exciton states and the oscillator strength f_{x} for exciton recombination. We studied both the large lithographically-defined and small self-assembled rings. Only in smaller self-assembled nanorings we found optical excitonic Aharonov-Bohm effect. Those oscillations are established by anticrossings between the optically active exciton states with zero orbital momentum. In lithographically defined rings, whose average radius is 33 nm, f_{x} shows no oscillations, whereas in the smaller self-assembled nanoring with average radius of 11.5 nm oscillations in f_{x} for the ground exciton state are found as function of the magnetic field that is superposed on a linear dependence. These oscillations are smeared out at finite temperature, thus photoluminescence intensity exhibits step-like variation with magnetic field even at temperature as small as 4.2 K.
Studies for a single charge qubit and two capacitively coupled qubits built on triple quantum dots are presented. We show feasibility of implementing two-qubit gate operations, e.g. the CPHASE gate can be implemented with the fidelity higher than 99% for strong couplings.
Two concentric two-dimensional GaAs/(Al,Ga)As nanorings in a normal magnetic field are theoretically studied. The single-band effective mass approximation is adopted for both the electron and the hole states, and the analytical solutions are given. We find that the electronic single particle states are arranged in pairs, which exhibit anticrossings and the orbital momentum transitions in the energy spectrum when magnetic field increases. Their period is essentially determined by the radius of the outer ring. The oscillator strength for interband transitions is strongly reduced close to each anticrossing. We show that an optical excitonic Aharonov-Bohm effect may occur in concentric nanorings.
The rate equation is used for description of photoluminescence dynamics after pulsed excitation of various states of quantum dots. The picosecond dynamics of averaged charge state of quantum dot is described. We compare our simulations with the experiment and come up with the conclusion that probability of carrier capture weakly depends on quantum dot charge state and that electrons and holes are captured non-synchronously.
In order to analyze the strain distribution of InAs/GaAs quantum dot in a pyramidal geometry, the traditional calculation method is based on the single band envelope approximation with the modified band edge from the eight band k·p theory. In this paper, we use the eight band k·p Hamiltonian to calculate, and the piezoelectric effects and the electronic structure are also discussed subsequently. To this end, some necessarily derived formulae in calculations about using the finite element calculation software COMSOL are presented in this paper. The results show the details about strain distributions, piezoelectric effects and electronic structure of an InAs/GaAs pyramidal quantum dot, verify the feasibility and efficiency of the calculation method.
The effect of retrapping on thermoluminescence intensity peak corresponding to each trap of small amorphous silicon quantum dots in three traps - one recombination center model is investigated. For first order kinetics, where there is no effect of retrapping, the thermoluminescence intensity clearly depends on the level of the trap beneath the edge of the conduction band. This energy difference between the edge of the conduction band and the level of the trap is called trap depth (activation energy). The shallowest trap gives the highest thermoluminescence intensity peak for first order kinetics. However, it was clearly observed that for second order and a case beyond second order kinetics, the thermoluminescence intensity peak corresponding to each trap does not depend on the trap depth. In this case, the retrapping probability coefficients are taken into account and most electrons which are detrapped from the shallow trap(s) will be retrapped to the deeper trap(s) resulting in fewer electrons taking part in the recombination process. This significantly reduces the thermoluminescence intensity peaks of the shallower trap(s). It was observed that the deepest trap, with very high concentration of electrons due to the retrapping phenomenon, gives the highest thermoluminescence intensity. In addition, the variation of concentration of electrons in each trap and the intensity of the thermoluminescence are presented. Though we considered the model of three traps and one recombination center, this phenomenon is true for any multiple traps.
We theoretically study the electronic transport through a triple quantum dot system in triangular geometry weakly coupled to external metallic leads. By means of the real-time diagrammatic technique, the current and Fano factor are calculated in the lowest order of perturbation theory. The device parameters are tuned to such transport regime, in which coherent population trapping of electrons in quantum dots due to the formation of dark states occurs. The presence of such states greatly influences transport properties leading to a strong current blockade and enhanced, super-Poissonian shot noise. We consider both one- and two-electron dark states and examine the influence of magnetic field on coherent trapping in aforementioned states. When the system is in one-electron dark state, we observe a small shift of the blockade's region, whereas in the case of two-electron dark state, we show that strong magnetic field can lift the current blockade completely.
Magnetic field and temperature dependent photoluminescence studies on neutral and charged excitons in individual InAs quantum dots allow us to uncover different mechanisms by which the discrete quantum dot states are coupled to delocalized continuum states in a quantum well (the wetting layer). The behaviour of the neutral and singly charged excitons can be explained taking only discrete quantum dot states into account. For doubly and triply charged excitons we have to consider spin dependent coherent and incoherent interactions between discrete quantum dot states and delocalized wetting layer states.
The theoretical investigation of the electron and hole spectra in a quantum dot with a linearly graded composition within the effective mass approximation is presented. The particular example is β-HgS surrounded by CdS. β-HgS core of radius r_C is surrounded by concentric spherical layers each of Hg_{1-x}Cd_{x}S composition (x is function of r) and finally, form radius r_S by CdS. The existence of these intermediate layers, as model of graded composition, influences rapidly electron and hole spectra.
We present the hole-related electrical activity of the InAs quantum dots embedded in the n-type GaAs. We performed our experiments with the use of the Laplace and conventional deep level transient spectroscopies combined with the above GaAs band-gap illumination. We observed that depending on temperature and electric field the hole emission process is an interplay between the pure thermal emission and tunnelling processes. The tunnelling was quantitatively described by a simple model of the potential barrier.
We propose a quantum dot implementation of a quantum state transfer channel. The proposed channel consists of N vertically stacked quantum dots with the nearest neighbor tunnel coupling, placed in an axial electric field. We show that the system supports high-fidelity transfer of the state of a terminal dot both by free evolution and by adiabatic transfer. The protocol is to a large extent insensitive to inhomogeneity of the energy parameters of the dots and requires only a global electric field.
We report on optical orientation of excitons and trions (singly charged exciton) in individual charge-tunable self-assembled InAs/GaAs quantum dots. When the number of electrons varies from 0 to 2, the trion photoluminescence under quasi-resonant excitation gets progressively polarized from zero to ≈100%. We discuss this behavior as the efficient quenching of exciton spin quantum beats in anisotropic quantum dots due to the trion formation. This result indicates a long hole-spin relaxation time larger than the radiative lifetime, confirmed by time-resolved photoluminescence measurements carried out on a quantum dots ensemble.
We consider theoretically a system composed of a quantum dot coupled to a topological superconducting wire. The dot, being in Coulomb blockade (CB) regime is additionally coupled to the normal leads. The topological wire hosts Majorana states, which, as we show, characteristically modifies conductance through the dot. An unpaired Majorana state in the wire causes a unique temperature dependence of zero bias conductance vs. gate voltage. It decreases in-between CB peaks and on the sides of the peaks from the plateau at ~ e²/2h when temperature increases. At the same time conductance increases at the CB peak positions. It is accompanied by zero bias anomaly in differential conductance. For finite overlap of Majorana states in the wire the zero bias anomaly disappears. Instead, two characteristic Fano resonances of opposite symmetry appear, positioned mirror-symmetrically with respect to zero bias.
We present a quasiclassical approach to few-electron quantum dots in strong magnetic fields based on the notion of a collectively rotating Wigner molecule. A quasiclassical many-particle wave function is derived and illustrated by its application to a two-electron quantum dot. In particular, we calculate the density-current correlation function (conditional current) and show that the Wigner crystal in high magnetic fields may be visualized as an ordered system of current vortices.
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