Influence of the rate of increase in the induction on the spectral characteristics of luminescence of ZnS: Mn under treatment in a magnetic field

This paper reports on the results of the investigation of photoluminescence and luminescence excitation spectra of a ZnS powder thermally doped with MnCl · 4H₂O or MnS and subjected to a series of treatments in a pulsed magnetic field with the same maximum amplitude of the magnetic field induction (B _max = 0.3 T) but with different rates of its increase $ \dot B $ \dot B (t) to a maximum value. It has been revealed that pulsed magnetic fields with different rates of increase in the induction B nonlinearly affect the spectral characteristics of the ZnS: Mn compounds. The processes occurring in the material, which can lead to a change in its spectral characteristics due to the manifestation of the magnetoplastic effect, and the influence of the parameter $ \dot B $ \dot B (t) of the magnetic field on the efficiency of the external action have been discussed. It has been demonstrated that a variation in the parameter $ \dot B $ \dot B (t) makes it possible to control the location of defects in the nearest environment of Mn²⁺ ions in the ZnS compound after the completion of treatment of the material in a magnetic field. The mechanisms responsible for the different effects exerted by energetically identical external disturbances on the system have been proposed.     

Use of quantum dots as a switch for single-electron transfer free from cotunneling

Based on symmetry constraint that leads to the appearance of nodes in the wave functions of 3-electron systems at regular triangle configurations, it was found that, if the parameters of confinement are skillfully chosen and if a magnetic field is tuned around the first critical point of the single-triplet transition, a 2-electron quantum dot can be used as a switch for single-electron transport free from cotunneling.     

Optical properties of individual manganese-doped quantum dots

The magnetic state of a single magnetic ion (Mn²⁺) embedded in an individual quantum dot is optically probed using micro-spectroscopy. The fine structure of a confined exciton in the exchange field of a single Mn²⁺ ion (S=) is analyzed in detail. The exciton–Mn²⁺ exchange interaction shifts the energy of the exciton depending on the Mn²⁺ spin component and six emission lines are observed at zero magnetic field. The emission spectra of individual quantum dots containing a single magnetic Mn atom differ strongly from dot to dot. The differences are explained by the influence of the system geometry, specifically the in-plane asymmetry of the quantum dot and the position of the Mn atom. Depending on both these parameters, one has different characteristic emission features which either reveal or hide the spin state of the magnetic atom. The observed behavior in both zero field and under magnetic field can be explained quantitatively by the interplay between the exciton–Mn²⁺ exchange interaction (dependent on the Mn position) and the anisotropic part of the electron–hole exchange interaction (related to the asymmetry of the quantum dot).     

A charged ellipsoidal bunch in a magnetic field

The states of a long rotating charged ellipsoidal bunch in a longitudinal uniform magnetic field are studied. The states are described using two integrals of motion that couple the transverse velocities [(x)\dot]\dot x and [(y)\dot]\dot y with the x and y coordinates; the frequency ω_H=eH/mc (where H is the total magnetic field); and the quantities ω₁ and ω₂, which characterize the Coulomb repulsion in the x and y directions. It is shown that equilibrium states with a high charge density per unit length (ν≳1) can exist.     

Field dependent shape variation of magnetic fluid droplets on magnetic dots

The morphology of magnetic fluid droplets on magnetic thin film dots is studied experimentally, including the aspect ratio and the contact angle variation of the droplets. Under a uniform external magnetic field, the droplet's aspect ratio increases with the external field and with the diameter of the magnetic dot due to the concentrated magnetic flux inside the magnetic fluid droplet. Similar to the electrical wetting phenomenon, the induced magnetic dipoles in the magnetic film and in the magnetic fluid near the solid–liquid interface change the solid–liquid interfacial tension, and in consequence reduce the apparent contact angle of the magnetic fluid droplet.     

The Green's function of confined electrons in an external magnetic field: Analytical formulation

We derive the analytical form of the Green's function of 2-dimensional electrons with lateral confinement in a perpendicular magnetic field. The confinement potentials considered are infinite barriers at radius R (quantum dot) and at r and R (quantum ring).     

Evidence for charging effects in an open dot at zero magnetic field

We have measured the low-temperature transport properties of an open quantum dot formed in a clean one-dimensional channel. At zero magnetic field, continuous and periodic oscillations superimposed upon ballistic conductance steps are observed when the conductance through the dot G exceeds 2e²/h. We ascribe the observed conductance oscillations to evidence for charging effects in an open dot. This is supported by the evolution of the oscillating features for G>2e²/h as a function of both temperature and barrier transparency.     

Energy spectra and quantum crystallization in two-electron quantum dots in a magnetic field

Quantum crystallization of electrons in a quantum dot (QD) subjected to an external magnetic field is considered. Two-electron QDs with two-dimensional (2D) parabolic confining potential in an external transverse magnetic field are calculated. The Hamiltonian is numerically diagonalized in the basis of one-particle functions to find the energy spectra and wave functions for the relative motion of electrons with inclusion of electron-electron interaction for a broad range of the confining-potential steepness (α) and external magnetic fields (B). The region of the external parameters (α, B) within which a gradual transition to quantum crystalline order occurs is numerically determined. In contrast to a 2D unbounded system, a magnetic field acts nonmonotonically on “crystallization” in a quantum dot with several electrons because of a competition between two effects taking place with increasing B, namely, decreasing spread of the electron wave functions and increasing effective steepness of the confining potential, which reduces the average separation between electrons. Fiz. Tverd. Tela (St. Petersburg) 40, 1753–1759 (September 1998)     

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.     

The energy level ordering in two-electron quantum dot spectra

The spectra of a two-electron quantum dot in a magnetic field of arbitrary strength is obtained by using the shifted 1/Nexpansion method. The level ordering as well as the transitions in the angular momenta of the quantum dot are studied. The dependence of the electron absorption spectra on the applied magnetic field is also calculated. Comparisons show that our results are in good agreement with the exact ones.     

2D-Electron Heating in Electric and Magnetic Crossed Fields

2D-electron heating in a potential well of a single n-(AlAs) _x (GaAs)_1–x /i-GaAs (x = 0.28) heterojunction is studied for the cases of a classical (weak) magnetic field B and constant and pulsed electric fields at fixed temperatures 77 and 4.2 K. It is shown that the heating of two-dimensional electrons is similar to that of the bulk ones. The magnetic field cools electrons, and this is manifested in the shifts of the characteristic critical electric fields E _{c 1} and E _{c 2} and in the regions of nonlinearity of voltage-current characteristics. The dependence of the effective electron temperature on the electric field T _e(E)_B is determined.     

Single electron charging effects in semiconductor quantum dots

We have studied charging effects in a lateral split-gate quantum dot defined by metal gates in the two dimensional electron gas (2 DEG) of a GaAs/AlGaAs heterostructure. The gate structure allows an independent control of the conductances of the two tunnel barriers separating the quantum dot from the two 2 DEG leads, and enables us to vary the number of electrons that are localized in the dot. We have measured Coulomb oscillations in the conductance and the Coulomb staircase in current-voltage characteristics and studied their dependence on the conductances of the tunnel barriers. We show experimentally that at zero magnetic field charging effects start to affect the transport properties when both barrier conductances are smaller than the first quantized conductance value of a point contact at 2e ²/h. The experiments are described by a simple model in terms of electrochemical potentials, which includes both the discreteness of the electron charge and the quantum energy states due to confinement.     

Effect of a magnetic field on the yield point of NaCl crystals

A constant magnetic field is found to have a substantial effect on the macroplasticity of NaCl crystals when they are being actively strained at a constant rate during magnetic treatment. We have measured the dependence of the yield point σ _y on the magnetic induction B=0–0.48 T and the strain rate . It is shown that this magnetic effect has a threshold character and is observed only for B>B _c , where B _c grows with increasing as . The lower the strain rate , the larger the relative decrease in the yield point σ _y (B)/σ _y (0) at fixed field B>B _c . At small enough strain rates the threshold field B _c ceases to depend on and goes constant. A theoretical model is proposed which is in good agreement with the observed regularities. The model is based on the competition between thermally activated and magnetically stimulated depinning of dislocations from paramagnetic impurity centers. Zh. éksp. Teor. Fiz. 115, 951–958 (March 1999)     

Negative magnetoresistance in the regime of hopping conduction through <Emphasis Type="Italic">p</Emphasis> states at quantum dots

The magnetoresistance in the system of quantum dots with hopping conduction and filling factor 2 < ν < 3 in the limit of small quantum dots has been considered. In this case, hopping conduction is determined by p states. It has been shown that the system exhibits negative magnetoresistance associated with a change in the wavefunctions of p states in a magnetic field. This mechanism of magnetoresistance is linear in magnetic field in a certain range of fields and can compete with the known interference mechanism of magnetoresistance. The magnitude of this magnetoresistance is independent of the temperature at fairly low temperatures and increases with a decrease in the size of a quantum dot.     

The binding energy of hydrogen-like impurity in quantum dots with convex bottom in magnetic field

Within the framework of effective mass approximation and variational method, the electronic and impurity states in spherical quantum dots with convex bottom in magnetic field are calculated. Calculations are carried out both for on-center and off-center impurities. The impurity binding energy dependencies on radius, measure of convexity of quantum dot bottom, impurity position and magnetic field induction are obtained for the Ga_1-xAl_xAs/Ga_1-yAl_yAs system.     

Cancellation of fine-structure splitting in quantum dots by a magnetic field

We demonstrate control of the fine-structure splitting of the exciton emission lines in single InAs quantum dots by the application of an in-plane magnetic field. The composition of the barrier material and the size and symmetry of the quantum dot are found to determine decrease or increase in the linear polarization splitting of the dominant exciton emission lines with increasing magnetic field. This enables the selection of dots for which the splitting can to be tuned to zero, within the resolution of our experiments. General differences in the g-factors and exchange splittings are found for different types of dot.     

Nuclear-induced time evolution of entanglement of two-electron spins in anisotropically coupled quantum dot

We study the time evolution of entanglement of two spins in an anisotropically coupled quantum dot interacting with the unpolarised nuclear spins environment. We assume that the exchange coupling strength in the z direction J _z is different from the lateral one J _l . We observe that the entanglement decays as a result of the coupling to the nuclear environment and reaches a saturation value, which depends on the value of the exchange interaction difference J = ‖J _l ? J _z ‖ between the two spins and the strength of the applied external magnetic field. We find that the entanglement exhibits a critical behaviour controlled by the competition between the exchange interaction J and the external magnetic field. The entanglement shows a quasi-symmetric behaviour above and below a critical value of the exchange interaction. It becomes more symmetric as the external magnetic field increases. The entanglement reaches a large saturation value, close to unity, when the exchange interaction is far above or below its critical value and a small one as it closely approaches the critical value. Furthermore, we find that the decay rate profile of entanglement is linear when the exchange interaction is much higher or lower than the critical value but converts to a power law and finally to a Gaussian as the critical value is approached from both directions. The dynamics of entanglement is found to be independent of the exchange interaction for an isotropically coupled quantum dot.     

Magneto-optical single dot spectroscopy of GaSb/GaAs type II quantum dots

We have investigated magneto-optical properties of GaSb/GaAs self-assemble type II quantum dots by single dot spectroscopy in magnetic field. We have observed clear Zeeman splitting and diamagnetic shift of GaSb/GaAs quantum dots. The diamagnetic coefficient ranges from 5 to 30 μeV/T². The large coefficient and their large distribution are attributed to the size inhomogeneity and electron localization outside the dot. The g-factor of GaSb/GaAs quantum dots is slightly larger than that of similar type I InGaAs/GaAs quantum dots. In addition, we find almost linear relationship between the diamagnetic coefficient and the g-factor. The linear increase of g-factor with diamagnetic coefficient is due to an increase of spin-orbit interaction with dot size.     

Multiply-charged magnetoexcitons in low-dimensional structures

The frequencies and intensities of absorption lines of a “hole + Nelectrons” complex in a magnetic field are found. The motion of all particles is assumed to be two-dimensional, and the electron and hole quantum wells are assumed to be spatially separated. It is shown how Kohn’s theorem can be extended to the case of a system with a finite total mass. The energy of a N-electron complex in a quantum ring oscillates as a function of the magnetic flux with a period that depends on N and the ratio of the masses. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 6, 423–427 (25 September 1997)     

On the effect of a magnetic field on the yield point and kinetics of macroplasticity in LiF crystals

A strong effect of a static magnetic field B on active deformation kinetics ( =const) in LiF crystals is observed. This is a threshold effect with respect to B and ; it is observed only for B> B _c (B≃0.4 T) and . Magnetic sensitivity is exhibited by the yield stress τ _y , which decreases by approximately a factor of 1.5 for B=0.48 T, and by the stage-II and stage-III hardening coefficients θ_II and θ _III, the former decreasing and the latter increasing in a magnetic field. A physical interpretation is proposed for the observed behavior. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 6, 470–474 (25 March 1997)