Nonequilibrium Spin Fluctuations in Nonmagnetic Single-Electron Transistors and Quantum Dots

It is shown that nonequilibrium spin fluctuations significantly influence electronic transport in a single-electron transistor, when the spin relaxation on the island is slow. To describe spin fluctuations, the orthodox tunneling theory is generalized by taking into account the electron spin. It is shown that the transition between consecutive charge states can occur via high-spin states, which significantly modifies the shape of Coulomb steps and gives rise to additional resonances at low temperatures.     

Spin Injection Processes in ZnSe-Based Double Quantum Dots of Diluted Magnetic Semiconductors

Spin injection processes in the double quantum dots of ZnSe-based diluted magnetic semiconductors are discussed. Double quantum dots are fabricated from ZnSe-based double quantum wells by electron beam lithography and wet etching. In these samples, the photo-excited carriers in the magnetic dots are injected into the non-magnetic dots. The circular polarization degrees of photoluminescence from the non-magnetic dots are measured by micro-photoluminescence measurement system under the magnetic field up to 5 T. The maximum spin polarization degrees of injected carriers determined from our experiment are 10% for double quantum wells and 15% for double quantum dots. The spin injection efficiency was estimated both from the observed circular polarization degree and the diffusion length of carriers. We concluded that the spin injection efficiency is increased in the double quantum dots.     

Charging Effects on Nuclear Spin Relaxation in Quantum Dots

We study the mechanism of nuclear spin relaxation in quantum dots due to the electron exchange with 2D gas. We show that the nuclear spin relaxation rate T ₁ ^–1 is dramatically affected by the Coulomb blockade (CB) and can be controlled by gate voltage. In the case of strong spin–orbit (SO) coupling the relaxation rate is maximal in the CB valleys whereas for the weak SO coupling the maximum of 1/T ₁ is near the CB peaks. The physical mechanism of nuclear spin relaxation rate at strong SO coupling is identified as Debye–Mandelstam–Leontovich–Pollak–Geballe relaxational mechanism.     

Hanle Effect of Charged and Neutral Excitons in Quantum Wells

We measured magnetic field depolarization of charged and neutral exciton cw photoluminescence of a system consisting of two coupled quantum wells with a residual concentration of holes. Using the Hanle expression, we obtained exciton lifetime and electron spin relaxation time, which are in agreement with results of time-resolved experiments. We suppose that the tunneling takes place via an emission of an LO phonon and we find this process spin conserving.     

Spin–Lattice Relaxation of Mn Ions in Nanostructures with Semiconductor Quantum Dots

We use the combination of nonequilibrium phonon and exciton luminescence techniques to study the spin dynamics in diluted magnetic semiconductor structures with (Cd,Mn)Te and (Cd,Mn)Se quantum dots (QDs). We show that the spin–lattice relaxation (SLR) of Mn ions in these structures differs strongly from the SLR in quantum wells. We explain the results by a model where SLR process in structures with QDs is modified by the spin diffusion on Mn ions from the QD to a wetting layer.     

Hyperfine Interactions and Electron Spin-Dynamics in a Quantum Dot

We study the dynamics of a single electron spin in an isolated quantum dot induced by hyperfine interaction with nuclei. The decoherence is caused by the spatial variation of the electron wave function within the dot, leading to a nonuniform hyperfine coupling A. We evaluate the spin correlation function and find that the decay is not exponential but rather power (inverse logarithm) law-like. For polarized nuclei we find an exact solution and show that the precession amplitude and the decay behavior can be tuned by the magnetic field. The decay time is given by N/A, where N is the number of nuclei inside the dot, and the amplitude of precession decays to a finite value. We show that there is a striking difference between the decoherence time for a single dot and the dephasing time for an ensemble of dots.     

Spin Dynamics in CdSe/ZnSe Quantum Dots: Resonant Versus Nonresonant Excitation

We have studied the exciton spin dynamics in CdSe/ZnSe quantum dots, comparing strictly resonant and nonresonant excitation. In case of strictly resonant excitation, excitons are generated in the quantum dot ground state, and because of the zero-dimensionality of the system no transient shift of the photoluminescence signal can be seen. No loss of the spin information is observed within the time window under investigation, if one excites the quantum dot eigenstates. Interestingly, even in case of nonresonant excitation, a high, time-independent polarization degree is obtained. We found maxima in the absolute value of the polarization degree if the laser excess energy amounts to a multiple of LO-phonon energies.     

Thermally Activated Photoluminescence in Lead Selenide Colloidal Quantum Dots

The thermal activation processes in PbSe colloidal quantum dots and their influence on the ground‐state exciton emission are discussed. Activation of a dark exciton occurs at 1.4–7 K, assisted by an acoustic phonon coupling. Activation of a bright exciton occurs at 100–200 K, which appears as a sudden change in the photoluminescence band intensity, energy, and full width at half maximum. This activation overcomes the dark–bright‐state splitting, when the activation temperature increases with the decrease of the dots' size. The dark exciton lifetime is found to be ≈6–12 µs at 1.4 K, while the bright exciton lifetime at 300 K evaluated as 450 ns varies slightly with the change in the size of the dots. In addition, the emission quantum yield of these dots, measured at a variety of temperatures when dissolved in various solvents, reveals information about the influence of the environment on the recombination processes.     

Spin Filter Effect in a Parallel Double Quantum Dot

We present a theoretical study of the spectral and the spin-dependent transport properties of a few electron semiconductor parallel double quantum dot (DQD) in the presence of local induced Zeeman splittings at the quantum dots. Working in an extended Hubbard model and treating the coupled QD as a single coherent system, the linear response spin-dependent conductance is calculated at low temperatures. We analyze the conditions such that the device would operate as a bipolar spin filter by only varying the incident electron Fermi energy from non-magnetic leads.     

Spin Effects in Lateral Quantum Dots

We present the review of our work on spin effects in single lateral quantum dots with the emphasis on the results of Coulomb blockade spectroscopy studies. Realization of a spin-based quantum bit proposal in a lateral quantum dot is discussed. Described are the ways of isolating a single electron spin in a dot containing only one as well as many electrons. Demonstrated is a current readout of spin transitions in a dot by means of spin blockade spectroscopy due to spin polarized injection/detection mechanism in a lateral dot. Discussed are transitions induced both by changing a magnetic field and a number of electrons in a dot with the emphasis on the effects observed close to filling factor in a dot = 2.     

Effects of Magnetic Quantum Dots on Magnetoconductance in Quantum Wires

We present a theoretical study of electronic transport in quantum wires (narrow two-dimensional electron gas) with array of magnetic quantum dots. Each magnetic quantum dot is defined by a small circular region where the strength of perpendicular magnetic field is modulated. By making use of a newly developed calculation method based on the gauge transformations, we calculated the conductance as a function of the external perpendicular magnetic field. Our numerical calculations show that the magnetoconductance is very sensitive to the number of magnetic quantum dots in the field region where the direction of the net magnetic field in dot regions is antiparallel to the external magnetic field.     

Magnetically Engineered Semiconductor Quantum Dots as Multimodal Imaging Probes

Light‐emitting semiconductor quantum dots (QDs) combined with magnetic resonance imaging contrast agents within a single nanoparticle platform are considered to perform as multimodal imaging probes in biomedical research and related clinical applications. The principles of their rational design are outlined and contemporary synthetic strategies are reviewed (heterocrystalline growth; co‐encapsulation or assembly of preformed QDs and magnetic nanoparticles; conjugation of magnetic chelates onto QDs; and doping of QDs with transition metal ions), identifying the strengths and weaknesses of different approaches. Some of the opportunities and benefits that arise through in vivo imaging using these dual‐mode probes are highlighted where tumor location and delineation is demonstrated in both MRI and fluorescence modality. Work on the toxicological assessments of QD/magnetic nanoparticles is also reviewed, along with progress in reducing their toxicological side effects for eventual clinical use. The review concludes with an outlook for future biomedical imaging and the identification of key challenges in reaching clinical applications.     

Magneto-Optical Study of Multiple Layers of Self-Assembled Quantum Dots Involving Diluted Magnetic Semiconductors

Magneto-optical experiments were carried out on structures comprised of multiple layers of self-assembled quantum dots (QDs) involving diluted magnetic semiconductors (DMSs). Photoluminescence (PL) from interband ground state transitions was clearly observed in these DMS-based QD systems. The PL energy from QD multilayers appears at a lower energy than that emitted by a single QD layer, suggesting that there exists electronic coupling between the QD layers. When an external magnetic field is applied, the PL peaks from QDs both in single-layer and in multilayer form exhibit large Zeeman shifts and a significant enhancement of intensity, a behavior that is typical for many low dimensional systems involving DMSs. In contrast to this behavior, however, we have observed a decrease of the PL intensity as a function of magnetic field in multilayer structures where alternating QW layers contain DMS and non-DMS QDs. We will show evidence that this effect arises from carrier transfer between pairs of QDs from adjacent layers (double QDs) due to the large Zeeman shifts of the conduction and valence bands characteristic of DMS QDs.     

Magnetic-Field-Induced Level Crossing and the Spin Dynamics of Excitons in Zn1?xMnxTe/ZnTe Quantum Wells

Magnetic-field-induced level crossing and the spin dynamics of excitons in a Zn_1–x Mn _x Te/ZnTe single quantum well are studied. The circularly-polarized photoluminescence (PL) shows that the down spin branch of the Zn_1–x Mn _x Te exciton overlaps with both the up and down spin branches of the ZnTe exciton at a crossing field (H _c) of 4 T, due to the giant Zeeman shift of Zn_1–x Mn _x Te. The PL intensities and lifetimes in each layer become gradually equal toward H _c, which shows the mixing of wavefunctions of the excitons generated in each layer. Above H _c, each branch of the spin-polarized exciton separates again. The lifetimes of the spin-polarized exciton PL reflect the spin-flip relaxation in ZnTe and the spin mixing between Zn_1–x Mn _x Te and ZnTe layers.     

Growth and Spectroscopy of CdSe: Mn Quantum Dots

In this paper we report the growth and optical properties of ZnSe/CdSe:Mn magnetic quantum dots by Atomic Layer Epitaxy. For the uncapped samples, dot densities of the order of 10⁹ cm^–2 were measured by Atomic Force Microscopy. The ensemble dot photoluminescence (PL) was observed over a range of energies between 2.1 and 2.5 eV, and a spectrally broad emission at 2.15 eV from the internal Mn²⁺ transition was observed at high Mn concentrations. Single dot spectroscopy was carried out by confocal microscopy and the PL line width was measured as a function of Mn concentration. For large Mn contents the temporal change in magnetization causes a broadening of the single dot PL line of up to 4 meV FWHM. However, for low concentrations the single dot PL line widths were resolution limited at <0.2 meV.     

Quantification of Reactive Oxygen Species Generation by Photoexcitation of PEGylated Quantum Dots

Photocatalytic generation of reactive oxygen species (ROS) from quantum dots (QDs) has been widely reported yet quantitative studies of ROS formation and their quantum yields are lacking. This study investigates the generation of ROS by water soluble PEGylated CdSe/ZnS QDs with red emission. PEGylation of QDs is commonly used to confer water solubility and minimise uptake by organs of the reticuloendothelial system; therefore studies of ROS formation are of biomedical relevance. Using non‐photolytic visible wavelength excitation, the superoxide anion radical is shown to be the primary ROS species generated with a quantum efficiency of 0.35%. The yield can be significantly enhanced in the presence of the electron donor, nicotinamide adenine dinucleotide (NADH), as demonstrated by oxygen consumption measurements and electron paramagnetic resonance spectroscopy with in situ illumination. Direct production of singlet oxygen is not detectable from the QDs alone. A comparison is made with ROS generation by the same QDs complexed with a sulfonated phthalocyanine which can generate singlet oxygen via Förster resonance energy transfer between the QDs and the phthalocyanine.     

Laser-Controlled Magnetization in a Single Magnetic Semiconductor Quantum Dot

The photoluminescene signal of individual semimagnetic CdSe–Zn_0.75Mn_0.25Se quantum dots is used to study the magnetization of the Mn²⁺ spin system in the exchange field of a single exciton. We demonstrate that by increasing the laser excitation power a significant blue shift of the photoluminescence signal occurs. This is attributed to a laser-induced demagnetization, i.e. the laser-generated carriers heat the Mn²⁺ spin system via spin–flip exchange scattering.