Tuning spin properties of excitons in single CdTe quantum dots by annealing

Using polarization-resolved photoluminescence imaging in external magnetic fields we measure statistically significant distributions of exchange splitting, diamagnetic shift and effective g-factor of excitons in tens of single CdTe quantum dots. Comparison between the as-grown and annealed structures shows strong suppression of ensemble inhomogeneities, with the average exchange splitting reduced by half and significant narrowing of the g-factor distribution. Remarkably, the average value of the excitonic g-factor remains unchanged. This unique ability to yield highly uniform quantum dot ensembles without hampering the exciton Zeeman splitting makes annealing a highly attractive means for tuning the spin properties of quantum dot excitons.     

Three-pulse photon echo induced by the optical transition of excitons in core-shell CdSe/ZnS nanocrystal quantum dots

The femtosecond three-pulse photon echo phenomena induced by the optical transition of 1s_e1s_h exciton in a core-shell CdSe/ZnS nanocrystal quantum dot (NQD) are theoretically investigated basing on the optical Bloch equations. The parameter dependence of the photon echo signals is discussed. The numerical calculation results reveal that three-pulse photon echo signals are sensitive to the variation of the size and structure of NQD. The corresponding mechanism has been discussed in terms of quantum size confined effect theory.     

Synthesis and photoluminescence of bright water-soluble CdSe/ZnS quantum dots overcoated by hybrid organic shell

Photoluminescence properties from water soluble CdSe/ZnS QDs encapsulated with hybrid trioctylphosphine-poly(acrylamide-co-acrylic acid)-ethanolamine (TOPO-PSMA-EA) shell have been investigated. It was found that PL efficiency of CdSe/ZnS QDs in water was increased 5–30% after introducing PSMA-EA polymers to encapsulate CdSe/ZnS-TOPO QDs. Higher PSMA concentrations were found to enhance the PL efficiency of QDs up to 1.8 folds, which is ascribed to a better packing and passivation of the TOPO-PSMA-EA shell over the QDs. Time-resolved photoluminescence suggested that the mean lifetime of photoexcited carriers in the water-soluble CdSe/ZnS-TOPO-PSMA-EA QDs elongated 2–17 ns compared with that of uncoated samples, indicating that PL quenching defects were effectively removed for CdSe/ZnS QDs with hybrid TOPO-PSMA-EA shell.     

Magnetization plateaus and thermodynamic properties of a ferrimagnetic Kagome-like nanoparticle under an applied magnetic field

Journal of Materials Science - Magnetic properties of a Kagome-like nanoparticle with a ferromagnetic exchange coupling described by the mixed-spin (5/2, 3/2) Ising model were studied, and unique...     

Dependence on temperature of circular polarization and relaxation time in InAlAs/AlGaAs quantum dots

We have studied the variation of photoluminescence circular polarization as a function of temperature in self-organized InAlAs/AlGaAs quantum dots. We have found that the circular polarization depends on temperature. The relaxation time τ_s is deduced from the circular polarization degree. It decreases from 2 ns at 40 K to 100 ps at 85 K.     

Preparation and growth mechanism of nickel nanowires under applied magnetic field

Nano-Micro Letters - Nickel nanowires with large aspect ratio of up to 300 have been prepared by a hydrazine hydrate reduction method under applied magnetic field. The diameter of nickel nanowires...     

The effect of heat-treatment in a magnetic field or under an applied stress on the magnetic properties of silicon-iron

The effects of heat-treatment in a magnetic field or under an applied stress have been studied in this investigation. Magnetic properties (magnetostriction and power loss in particular), measured along the rolling direction in grain-oriented silicon-iron were unaffected by magnetic annealing, but their stress-sensitivities were improved by annealing under tension.Magnetic annealing was found to be effective in non-oriented silicon-iron and also in grain-oriented material if it was annealed with the field applied along directions other than the rolling direction.The magnetic annealing results can be explained largely on the basis of the Néel-Taniguchi theory of directional ordering of atom pairs. The changes obtained by annealing under stress showed that directional order only plays a minor part. The magnetic changes could be explained by assuming that during heat-treatment under stress a process of magnetostriction strain relief occurs, forming a residual internal stress.Other alloys similar to silicon-iron showed no more response to magnetic annealing or annealing under stress than silicon-iron.     

Direct mapping of hot-electron relaxation and multiplication dynamics in PbSe quantum dots

How hot electrons relax in semiconductor quantum dots is of critical importance to many potential applications, such as solar energy conversion, light emission, and photon detection. A quantitative answer to this question has not been possible due in part to limitations of current experimental techniques in probing hot electron populations. Here we use femtosecond time-resolved two-photon photoemission spectroscopy to carry out a complete mapping in time- and energy-domains of hot electron relaxation and multiexciton generation (MEG) dynamics in lead selenide quantum dots functionalized with 1,2-ethanedithiols. We find a linear scaling law between the hot electron relaxation rate and its energy above the conduction band minimum. There is no evidence of MEG from intraband hot electron relaxation for excitation photon energy as high as three times the bandgap (3E(g)). Rather, MEG occurs in this system only from interband hot electron transitions at sufficiently high photon energies (~4E(g)).     

Enhancement of the luminescence intensity of InAs/GaAs quantum dots induced by an external electric field

InAs/GaAs quantum dots have been subjected to a lateral external electric field in low-temperature microphotoluminescence measurements. It is demonstrated that the dot PL signal could be increased several times depending on the magnitude of the external field and the strength of the internal (built-in) electric field, which could be altered by an additional infrared illumination of the sample. The observed effects are explained by a model that accounts for the essentially faster lateral transport of the photoexcited carriers achieved in an electric field.     

The Hanle effect and electron spin polarization in InAs/GaAs quantum dots up to room temperature

The Hanle effect in InAs/GaAs quantum dots (QDs) is studied under optical orientation as a function of temperature over the range of 150-300 K, with the aim of understanding the physical mechanism responsible for the observed sharp increase of electron spin polarization with increasing temperature. The deduced spin lifetime T(s) of positive trions in the QDs is found to be independent of temperature, and is also insensitive to excitation energy and density. It is argued that the measured T(s) is mainly determined by the longitudinal spin-flip time (T(1)) and the spin dephasing time (T(2)*) of the studied QD ensemble, of which both are temperature independent over the studied temperature range and the latter makes a larger contribution. The observed sharply rising QD spin polarization degree with increasing temperature, on the other hand, is shown to be induced by an increase in spin injection efficiency from the barrier/wetting layer and also by a moderate increase in spin detection efficiency of the QD.     

Chaotic motion of an asymmetric gyrostat in the magnetic field of the earth

Summary The attitude motion of an asymmetric magnetic gyrostat satellite in a circular orbit subjected to both gravitational and magnetic forces of the earth is investigated by using the version of Melnikov's method developed for a two-degree of freedom Hamiltonian system withS ¹ symmetry. For this purpose Deptrit's canonical variables are introduced to establish the Hamiltonian structure for this problem. It is found that the motion is chaotic in the sense of Smale's horseshoe under certain conditions. The effects of the magnetic moment of the gyrostat and the speed of the rotor in the gyrostat on the global motion are also studied by numerical computation. It is shown that as the magnetic moment of the gyrostat increases, the chaotic area in the Poincaré map will enlarge; as the rotor speed increases, a chaotic motion will turn into a regular motion.     

Influence of edge and center oxidation configurations on non-radiative relaxation in graphene quantum dots

Journal of Materials Science: Materials in Electronics - The photoluminescence quantum yield of graphene quantum dots (GQDs) is usually very low because the fast electron–hole recombination...     

Magnetic field control of exchange and noise immunity in double quantum dots

We employ density functional calculated eigenstates as a basis for exact diagonalization studies of semiconductor double quantum dots, with two electrons, through the transition from the symmetric bias regime to the regime where both electrons occupy the same dot. We calculate the singlet-triplet splitting J(epsilon) as a function of bias detuning epsilon and explain its functional shape with a simple, double anticrossing model. A voltage noise suppression "sweet spot," where d J(epsilon)/d(epsilon) = 0 with nonzero J(epsilon), is predicted and shown to be tunable with a magnetic field B.     

Influence of an applied magnetic field on shielding current paths in a high TC superconductor

The influence of an applied magnetic field on shielding current paths in a bulk high T_c superconductor was investigated. This issue is very important for the quantitative analysis of levitation force, since the distortion and localization of the current paths have been found to be a major source of discrepancy between computed and experimental results. Furthermore, it has been speculated that the current paths vary, depending on the applied magnetic field. The magnetic field due to superconducting currents was measured by a Hall sensor when a permanent magnet was positioned at different gaps from a superconductor. The current paths were then reconstructed from the measured field data by an inverse analysis. A genetic algorithm, which is a robust probabilistic optimizing method, was used for the inverse analysis. It was found that the current paths merged into larger loops as the permanent magnet was retreating from the superconductor. It implies that a stronger applied magnetic field locally degrades the current density (J_c) and the shielding current paths are localized by these low-J_c areas which act as insulating boundaries.     

Voltage-current characteristics of an arc in an annulus moving under a magnetic field

Voltage-current relationships are presented in dimensionless terms for the high-current arc rotating in an annular gap under the influence of a magnetic field; air passes through the gap. It is found that one can neglect the effects of the air flow if the speed of the air is small.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 20, No. 4, pp. 683–689, April, 1971.     

Chainlike assembly of magnetite coated with SiO2 nanostructures induced by an applied magnetic field

The assembly of silica coated magnetite nanoparticles into chainlike nanostructures is reported in the presence of applied magnetic field. The coating of SiO₂ onto the surface of magnetite nanoparticles was successfully conducted with the hydrolysis and condensation of tetraethyl orthosilicate. X-ray diffraction and transmission electron microscopy were used to characterize the microstructure and morphology. Vibrating sample magnetometry reveals that the coercivity is dependent on the coating thickness and can be controlled to a certain extent.     

Time-resolved photoluminescence of type-II Ga(As)Sb/GaAs quantum dots embedded in an InGaAs quantum well

Optical properties and carrier dynamics in type-II Ga(As)Sb/GaAs quantum dots (QDs) embedded in an InGaAs quantum well (QW) are reported. A large blueshift of the photoluminescence (PL) peak is observed with increased excitation densities. This blueshift is due to the Coulomb interaction between physically separated electrons and holes characteristic of the type-II band alignment, along with a band-filling effect of electrons in the QW. Low-temperature (4?K) time-resolved PL measurements show a decay time of [Formula: see text]?ns from the transition between Ga(As)Sb QDs and InGaAs QW which is longer than that of the transition between Ga(As)Sb QDs and GaAs two-dimensional electron gas ([Formula: see text]?ns).     

Numerical and analytical study of rotating flow in an enclosed cylinder under an axial magnetic field

Summary. A numerical and analytical study of the steady laminar flow driven by a rotating disk at the top of an enclosed cylinder, having an aspect ratio H/R equal to 1, filled with a liquid metal, and submitted to an axial magnetic field B, is presented. The governing equations in cylindrical coordinates are solved by a finite volume method. In the absence of a magnetic field, the numerical method is validated via a comparison with experimental data; the latter was found to be in good agreement with the predictions. In the presence of a magnetic field, the analytical velocity profiles under the rotating disk and on the bottom wall obtained for a high value of the magnetic interaction parameter N are in excellent agreement with those obtained by numerical simulations. The effect of the top, bottom and vertical walls' conductivity on the flow is studied and found to be an important parameter in the control of the flow.     

Spin relaxation phenomena in the 3He-A1 phase. Effects of magnetic field profile,spin counterflow,and 4He coverage

We report observations on the effects of the magnetic field profile and surface coverage by ⁴He on the magnetically driven superflows in the ³He-A₁ phase. When the gradient magnetic field profile is changed: (1) an unexpected reversal in the direction of superflow is observed, and (2) the relaxation time of the induced pressure increases in the warmer region of the A₁, phase but remains constant in the colder region. When the surfaces in the apparatus are covered with ⁴He: (1) the induced pressure amplitude decreases and the relaxation time increases and saturates at about twice that of pure ³He in the warmer side of the phase, and (2) the relaxation time is independent of the ⁴He coverage in the colder side. We give qualitative interpretation of the observed effects in terms of spin density relaxations; in the warmer side of the phase the spin density relaxes via processes in both bulk liquid and surface boundary and in the colder side the spin relaxation is dominated by connective spin counterflows in the A₂/A phase liquid in the fringing field of the static magnetic field.     

The elastic field around an elliptical crack in an anisotropic medium under an applied stress of polynomial forms

A general form is presented for the stress disturbance caused by an elliptical crack in an anisotropic medium under an arbitrary stress field in the form of polynomials. Somigliana's dislocation method is employed for analysis as Willis [4] did, but a different integral process is taken. The results are expressed in the integral forms defined on the subspace of the surface of a unit sphere. The following theorem is proved: If the displacement discontinuity of the elliptical Somigliana's dislocation has the form where is a homogeneous polynomial of degree $\text{N}$ in the coordinates $\text{x′}{}_1$ and $\text{x′}{}_2$ and $\text{a}{}_1$ and $\text{a}{}_2$ are the semi-axes of the elliptical Somigliana's dislocation, the stresses on the plane of the Somigliana's dislocation are inhomogeneous polynomials in the coordinates, whose terms are of degree $\text{N, (N ? 2}$), $\text{(N ? 4),4.}$