Insulating state in open quantum dots and quantum dot arrays

We describe the observation of novel localization in mesoscopic quantum dots and quantum dot arrays, which are realized in high mobility GaAs/AlGaAs heterojunctions using the split‐gate technique. With a sufficient gate voltage applied to form the devices, their resistance diverges as the temperature is lowered below a degree Kelvin, behavior which we attribute to localization. Evidence for the localization is found over the entire range of gate voltage for which the dots are defined, persisting to conductances higher than 50e²/h.     

AC-hopping conductance of self-organized Ge/Si quantum dot arrays

Dense (n=4×10¹¹ cm^-2) arrays of Ge quantum dots in a Si host were studied using attenuation of surface acoustic waves (SAWs) propagating along the surface of a piezoelectric crystal located near the sample. The SAW magneto-attenuation coefficient, ΔΓ=Γ(ω,H)-Γ(ω,0), and change of velocity of SAW, ΔV/V=(V(H)-V(0))/V(0), were measured in the temperature interval T=1.5–4.2 K as a function of magnetic field H up to 6 T for the waves in the frequency range f=30–300 MHz. Based on the dependences of ΔΓ on H, T and ω, as well as on its sign, we believe that the AC conduction mechanism is a combination of diffusion at the mobility edge with hopping between localized states at the Fermi level. The measured magnetic field dependence of the SAW attenuation is discussed based on existing theoretical concepts.     

Enhanced infrared absorption of spatially ordered quantum dot arrays

We have investigated the intersubband absorption for spatially ordered and non-ordered quantum dots (QDs). It is found that the intersubband absorption of spatially ordered QDs is much stronger than that of non-ordered QDs. The enhanced absorption is attributed to the improved size uniformity concurrent with the spatial ordering for the growth condition employed. For the FTIR measurement under normal incidence geometry, using a undoped sample as reference can remove the interference effect due to multiple reflections.     

Time evolution of initially localized states in two-dimensional quantum dot arrays in a magnetic field

The evolution of non-stationary localized states |Ψ(t=0) is investigated in two-dimensional tight binding systems of N potential wells with and without a homogeneous field perpendicular to the plane. Most results are presented in analytical form, what is almost imperative if the patterns are as complex as for rings in a magnetic field, where the qualitatively different features arise depending on rational or irrational numbers. The systems considered comprise finite linear chains (N=2,3), finite rings (N=3–6), infinite chains, finite rings (N=3–6) in a magnetic field, and rings with leads attached to each ring site. The position of the particle at time t is described by the projection of the wave function P_m(t)=|m|Ψ(t)|² onto the localized basis function at site m. For finite chains and rings with N=3,4,6 the time evolution is periodic, whereas it is non-periodic for N=5 and N greater then 6. Rings in a magnetic field show a rich spectrum of different features depending on N and the number of flux quanta through the ring, including periodic oscillation and rotation of the charge as well as non-periodic charge fluctuations.     

Strain-induced quantum dot superlattice

Coupled double quantum dots and quantum dot superlattices are formed by utilizing the strain of an InP island on top of a near-surface multi-quantum-well structure. The number and composition of the quantum wells together with the thickness of the barrier separating the quantum wells are varied to investigate the coupling of the wave functions of the carriers confined in separate vertically stacked dots. Photoluminescence studies show that the reduction of the barrier thickness and the increase of the number of wells enhance the coupling, which is observed as red shift and narrowing of the quantum dot peak. The calculated shifts of the peak positions agree closely with the experimental values.     

Single-photon detection mechanism in a quantum dot transistor

We study the transport mechanisms in a quantum dot MODFET by tuning the localization induced by charge stored on the quantum dots with light. The temperature dependence of the resistivity of a macroscopic sample reveals a hopping transport when the dots contain an excess of electrons. The resistance of a mesoscopic sample however, which is capable of detecting single photons, exhibits a much weaker dependence upon temperature. This points towards source-drain tunnelling as a transport mechanism and is confirmed by a statistical analysis of the single-photon-induced conductance steps. The complexity of the conducting paths increases as the average hopping length reduces.     

Polaronic effects in one-dimensional quantum antidot arrays

We study the effect of polaronic corrections arising from theelectron-longitudinal optical phonon interaction on the energyspectrum of a two-dimensional electron system with a one-dimensionalperiodic antidot array geometry created by a weak electrostaticmodulation potential, and subjected to a weak magnetic fieldmodulation as well as a uniform strong perpendicular staticmagnetic field. To incorporate the effects of electron-phononinteractions within the framework of Fröhlich polaron theory, wefirst apply a displaced-oscillator type unitary transformation todiagonalise the relevant Fröhlich Hamiltonian, and we thendetermine the parameters of this transformation together with theparameter included in the electronic trial wave function . On thebasis of this technique, it has been shown that the polaroniccorrections have non-negligible effects on the electronic spectrumof a two-dimensional electron system with a quantum antidot array,since switching such an interaction results in shifting thedegeneracy restoring points of Landau levels wherein the flatbandcondition is fulfilled, thus suppressing the Weiss oscillations.     

Persistent currents in a mesoscopic ring with a side connected quantum dot

We study the persistent current circulating along a mesoscopic ring with a dot side-coupled to it when threaded by a magnetic field. A cluster including the dot and its vicinity is diagonalized and embedded into the rest of the system. The result is numerically exact. We show that in the Kondo regime, the current can be a smooth or a strongly dependent function of the gate potential according to the structure of occupation of the highest energetic electrons of the system.     

Magnetoconductance of a few-electron open quantum dot

Magnetoconductance of a small open lateral dot is studied both theoretically and experimentally for the conditions when the dot contains down to 15 electrons. We confirm the existence of a new regime for open dots in which the transport through the structure occurs through individual eigenstates of the corresponding closed dot. In particular, at low magnetic fields the characteristic features in the conductance are related to the underlying eigenspectrum shells. When the number of modes in the leads is reduced more detailed structures within the shells due to single eigenlevels becomes discernible. At higher fields Landau level condensation is evident as well as the crossing of levels collapsing to the different Landau levels.     

Damping of coherent oscillations in a quantum dot photodiode

In this letter, we develop a model to describe the Rabi oscillations observed in a quantum-dot photodiode. Using a multi-level density matrix formulation, which includes multi-exciton and single particle states, we show that the damping observed in recent experiments is the result of a non-resonant excitation from or to the continuum of the wetting layer states.     

Intraband photoresponse of SiGe quantum dot/quantum well multilayers

In this contribution we study the intravalence band photoexcitation of holes from self-assembled Ge quantum dots (QDs) in Si followed by spatial carrier transfer into SiGe quantum well (QW) channels located close to the Ge dot layers. The structures show maximum response in the important wavelength range 3–5 μm. The influence of the SiGe hole channel on photo- and dark current is studied depending on temperature and the spatial separation of QWs and dot layers. Introduction of the SiGe channel in the active region of the structure increases the photoresponsivity by up to about two orders of magnitude to values of 90 mA/W at T=20 K. The highest response values are obtained for structures with small layer separation (10 nm) that enable efficient transfer of photoexcited holes from QD to QW layers. The results indicate that Si/Ge QD structures with lateral photodetection promise very sensitive large area mid-infrared photodetectors with integrated readout microelectronics in Si technology.     

Coherent dynamics in InGaAs quantum dots and quantum dot molecules

We measure the dephasing time of the exciton ground state transition in InGaAs quantum dots (QD) and quantum dot molecules (QDM) using a sensitive four-wave mixing technique. In the QDs we find experimental evidence that the dephasing time is given only by the radiative lifetime at low temperatures. We demonstrate the tunability of the radiatively limited dephasing time from 400 ps up to 2 ns in a series of annealed QDs with increasing energy separation of 69–330 meV from the wetting layer continuum. Furthermore, the distribution of the fine-structure splitting δ₁ and of the biexciton binding energy δ_B is measured. δ₁ decreases from 96 to with increasing annealing temperature, indicating an improving circular symmetry of the in-plane confinement potential. The biexciton binding energy shows only a weak dependence on the confinement energy, which we attribute to a compensation between decreasing confinement and decreasing separation of electron and hole. In the QDM we measured the exciton dephasing as function of interdot barrier thickness in the temperature range from 5 to 60 K. At 5 K dephasing times of several hundred picoseconds are found. Moreover, a systematic dependence of the dephasing dynamics on the barrier thickness is observed, showing how the quantum mechanical coupling in the molecules affects the exciton lifetime and acoustic-phonon interaction.     

Control of the entanglement between triple quantum dot molecule and its spontaneous emission fields via quantum entropy

The time evolution of the quantum entropy in a coherently driven triple quantum dot molecule is investigated. The entanglement of the quantum dot molecule and its spontaneous emission field is coherently controlled by the gate voltage and the rate of an incoherent pump field. The degree of entanglement between a triple quantum dot molecule and its spontaneous emission fields is decreased by increasing the tunneling parameter.     

High-fidelity quantum sensing of magnon excitations with a single electron spin in quantum dots

Single-electron spins in quantum dots are the leading platform for qubits, while magnons in solids are one of the emerging candidates for quantum technologies. How to manipulate a composite system composed of both systems is an outstanding challenge. Here, we use spin-charge hybridization to effectively couple the single-electron spin state in quantum dots to the cavity and further to the magnons. Through this coupling, quantum dots can entangle and detect magnon states. The detection efficiency can reach 0.94 in a realistic experimental situation. We also demonstrate the electrical tunability of the scheme for various parameters. These results pave a practical pathway for applications of composite systems based on quantum dots and magnons.     

Spin-flip effects in a parallel-coupled double quantum dot molecule

We investigate theoretically the electronic transport through a parallel-coupled double quantum dot (DQD) molecule attached to metallic electrodes, in which the spin-flip scattering on each quantum dot is considered. Special attention is paid to the effects of the intradot spin-flip processes on the linear conductance by using the equation of motion approach for Green’s functions. When a weak spin-flip scattering on each quantum dot is present, the single Fano peak splits into two Fano peaks, and the Breit–Wigner resonance may be suppressed slightly. When the spin-flip scattering strength on each quantum dot becomes strong, the linear conductance spectrum consists of two Breit–Wigner peaks and two Fano peaks due to the quantum interference effects. The positions and shapes of these resonant peaks can be controlled by using the magnetic flux through the quantum device.     

Bandgap calculation in Si quantum dot arrays using a genetic algorithm

In this work we present a fast and accurate genetic algorithm to determine the envelope functions and eigenenergies of the ground states of electrons and holes in low-dimensional complex semiconductor structures. We have developed the theoretical formalism of the algorithm in a general way in order to make it easy to include arbitrary nonparabolic and anisotropic band profiles in the calculations. From these results, calculation of the bandgaps of nanostructures can be carried out efficiently.Besides presenting and testing the algorithm, we calculate the ground state of electron and holes in two-dimensional quantum dot arrays, taking nonparabolicity and anisotropy into account.     

Non-radiative exciton recombination through excitation energy transfer in quantum dot clusters

Excitation energy transfer (EET) processes in CdSe/CdZnS quantum dot (QD) clusters have been investigated in this study by measuring their time-resolved and spectrally resolved fluorescence intensities. The contributions of radiative and non-radiative exciton recombination through EET are evaluated, where the latter is expected to occur in a large class of QD ensembles because of the presence of nonluminescent QDs. It appears that the fluorescence decay in larger QDs serving as acceptor does not show an initial rise, in addition the lifetime of the acceptor QD is independent of the excitation wavelength, suggesting that an EET is followed mostly by non-radiative recombination.     

Voltage-controlled negative refractive index in vertically coupled quantum dot systems

The authors demonstrate that negative index of refraction can be achieved by tuning the tunneling rate between InGaAs quantum dots layers via simply applying a bias voltage across the layers. As the bias voltage is changed, the index of refraction is tunable from negative through zero to positive. Moreover, the large negative refractive index and little loss can be achieved at the same time.     

Full counting statistics as a probe of quantum coherence in a side-coupled double quantum dot system

We study theoretically the full counting statistics of electron transport through side-coupled double quantum dot (QD) based on an efficient particle-number-resolved master equation. It is demonstrated that the high-order cumulants of transport current are more sensitive to the quantum coherence than the average current, which can be used to probe the quantum coherence of the considered double QD system. Especially, quantum coherence plays a crucial role in determining whether the super-Poissonian noise occurs in the weak inter-dot hopping coupling regime depending on the corresponding QD-lead coupling, and the corresponding values of super-Poissonian noise can be relatively enhanced when considering the spins of conduction electrons. Moreover, this super-Poissonian noise bias range depends on the singly-occupied eigenstates of the system, which thus suggests a tunable super-Poissonian noise device. The occurrence-mechanism of super-Poissonian noise can be understood in terms of the interplay of quantum coherence and effective competition between fast-and-slow transport channels.