Concentric Double Quantum Rings Research Articles

Effect of spin-orbit coupling and impurities on the optical properties of double quantum rings under magnetic fields

In this paper, the energy spectrum of concentric double quantum rings (CDQRs) with double well confinement under the influence of spin-orbit interactions (SOIs), magnetic fields, and hydrogen donor impurities is studied by using diagonalization. The light absorption coefficient is then calculated in the form of a compactness density-matrix. Key findings from this study include the ability to manipulate fine energy level splitting and optical absorption coefficients by tuning the magnetic field, impurity position, spin-orbit coupling, and inner rings size. Furthermore, this study reveals the bleaching effect of the optical absorption coefficient under certain conditions and demonstrates the anisotropic nature of the system through changes in the optical absorption coefficients (OACs) for different incident light polarization angles.

Electrically induced direct to indirect exciton transition in CdTe/CdMnTe concentric double quantum ring rooted in SiO2 matrix

Effect of electric field and ring dimension on the localization of direct and indirect exciton formed inside a CdTe/CdMnTe Concentric Double Quantum Ring (CDQR) has been studied using variational technique. The whole double ring structure is embedded inside a SiO2 matrix to enhance exciton localization. Electric field applied along the axial (z) axis tilts the band structure which promotes the carrier localization more in the edges than in the center of the ring. Thus, the stronger electric field gives a triangular confinement to the carrier in the edges which enhances the carrier stability. Along with the effect of electric field the ring dimension which determines the stability of the exciton retains has been estimated through the mean square distance. The effect of the location of the constituent carriers on the excitonic stability and the efficient carrier transfer from inner ring to outer ring has been calculated for both symmetric and asymmetric ring dimensions. Height of the ring has been varied to understand its effect on the localization of both direct and indirect exciton.

Energy spectrum and optical properties in a double parabolic quantum ring: Effects of the impurity, the magnetic field and the spin–orbit interaction

In this letter, the energy spectrum in a concentric double quantum rings(CDQRs) with a parabolic confinement under the influence of the spin–orbit interactions (SOIs), the magnetic field and the hydrogenic donor impurity has been investigated by diagonalizing the Hamiltonian matrix and applying the perturbation theory. Then the optical absorption coefficients (OACs) are calculated through the scheme of compact density-matrix formalism. A discussion of the different intensity of the two SOIs is conducted. Meanwhile, the effects of different factors on the total OACs with different incident light polarization angles have also been investigated, in which the bleaching effects are found.

Optical absorption in concentric double quantum rings

Optical absorption in concentric double quantum rings

Diluted magnetic concentric double quantum rings embedded in a quantum well: effect of magnetic field and ring dimension

Diluted magnetic concentric double quantum rings embedded in a quantum well: effect of magnetic field and ring dimension

Tuning the dynamics of Bound Magnetic Polaron in asymmetric Cd1-x1Mnx1Te/Cd1-x2Mnx2Te Semimagnetic Concentric Double Quantum Rings: Through magnetic ion concentration and magnetic field

Rigorous analysis on the formation of Bound Magnetic Polaron (BMP) in Cd1-x1Mnx1Te/Cd1-x2Mnx2Te Semimagnetic Concentric Double Quantum Ring (SCDQR) for various magnetic field and ring dimensions has been made by estimating Spin Polaronic Shift (SPS) using variational technique. The concentration of Mn2+ has been varied in both the ring and barrier material to estimate the shift in the BMP formation from the inner ring to outer ring with respect to the distribution of Mn2+ ions and impurity location. It is found that the SPS of Inner Ring Centered Donor (IRD) and Outer Ring Centered Donor (ORD) are opposite to each other at lower concentrations. Strengthening the magnetic field and increasing the concentration of magnetic ions increases the SPS because of the large magnetic ordering. Thus the concentration of magnetic ions and magnetic field is of paramount importance in determining the BMP formation and its usage in devices.

Influence of electric field on direct and indirect exciton in a concentrically coupled quantum ring heterostructure embedded in SiO2 matrix

Abstract Excitonic states of Concentric Double Quantum Ring (CDQR) heterostructures embedded in SiO2 matrix under the influence of electric field has been calculated theoretically using variational technique. CDQR has been confined in a quantum well of suitable width which serve as the height of the quantum ring along which the electric field is applied. Central barrier width, outer ring width and carrier location has been tuned to understand the localization of both direct and indirect excitons. Effect of ring dimension and electric field on the energy states of Inner Ring Exciton (IRE) and Outer Ring Exciton (ORE) has been determined. Tilting of conduction and valence bands due to the application of electric field invokes a triangular confinement to the carriers and the same has been discussed. Diamagnetic susceptibility which is merely an indicator of electron hole separation and intersubband transition energy has also been calculated for both symmetric and asymmetric quantum ring. Height of the QR has also been changed to study its effect on the diamagnetic susceptibility of an exciton.

Simultaneous Effects of Hydrostatic Pressure and Temperature on Donor Impurity States in Wurtzite Inx Ga1 – x N/GaN Concentric Double Quantum Rings

Simultaneous Effects of Hydrostatic Pressure and Temperature on Donor Impurity States in Wurtzite In<i>_x</i> Ga₁ –<i> x</i> N/GaN Concentric Double Quantum Rings

Tailoring the electronic properties of concentric double quantum rings in the presence of a donor impurity

The possibility of tailoring III-V GaAs/Al0.3Ga0.7As concentric double quantum rings (CDQRs) by tuning the central barrier, width, and height of the rings brings out carrier confinement in different regimes, which make them suitable for specific applications. Using variational ansatz, Coulomb interaction and carrier localization in the rings are investigated through calculation of diamagnetism and binding energy of donor for different ring dimensions (both axial and radial). Conspicuous changes occur in the behavior of both binding energy and diamagnetism for different dimensions of the inner and outer rings. Tuning the central barrier below a critical limit favors coupling of the two rings and promotes tunneling. The effect of these modifications on the carrier localization has been investigated, which gives a good insight into selective design of CDQRs for specific applications.

Effect of Ga droplet deposition rate on morphology of concentric quantum double rings

For the fabrication of particular nanostructures, Stranski-Krastanov (SK) growth mode driven by strain is most widely used. Meanwhile, another technique that is used to form the complex nanostructures is the droplet epitaxy technique, which is based on the deposition of group III element nanoscale droplets onto substrate and followed by the reaction with group V element for crystallization into III-V compound nanostructures. Droplet epitaxy technique is simple and flexible, and it does not require additional complicated processing and has potential to develop various quantum nanostructures. It, unlike standard MBE growth, exploits the sequential supply of group-III and group-V elements to form quantum nanostructures. Quantum rings are a special class of quantum-confinement structure that can be fabricated by the droplet epitaxy technique and have attracted wide attention due to the Aharonov-Bohm effect, which is specific to the topology of a ring. In this paper, GaAs/GaAs (001) concentric quantum double rings (CQDRs) are prepared by droplet epitaxy technique at different Ga droplet deposition rates in monolayer per second (ML/S). The 2 μm × 2 μm atomic force microscope images are obtained to show the morphologies of CQDRs. We study the effects of Ga droplet deposition rates (0.09 ML/s, 0.154 ML/s, 0.25 ML/s, 0.43 ML/s) on CQDRs. The results show that with the increase of Ga droplet deposition rate, the density of CQDRs increases and the radius of inner ring and the radius of outer ring decrease. According to the nucleation theory, through the relationship between the maximum cluster density and the Ga droplet deposition rate, the critical number of atom nucleations is found to be 5, which suggests that the stable Ga atom crystal nucleus should contain at least 5 Ga atoms in the process of forming Ga droplet, and a nucleation state transformation diagram is drawn in order to obtain an insight into the process of forming Ga droplet according to the nucleation theory and fitting results. The research results could be instructive for preparing the GaAs concentric quantum double rings that the density can be controlled by droplet epitaxy.

Ultrafast mixing of radial and spin states driven by transient magnetic field in concentric double quantum ring in presence of Dresselhaus spin-orbit interaction

We consider the single- and two-electron systems confined in concentric double quantum ring (DQR) nanostructure which interacts with transient perpendicular magnetic field oscillating with THz frequency. For this purpose we find the solutions of time-dependent Schrödinger equation utilizing Configuration Interaction method. We found that the transient magnetic field mixes the radial states, what trigger off the charge density oscillations in this direction, and these may be accompanied by spin oscillations, provided that a spin-orbit interaction (SOI) is involved. The pattern of density oscillations is more regular than those obtained for single- and two-electron spins what results from less complicated excitation energy spectra if SOI is not present in the system. The latter can be made more regular, i.e. the fluctuations of frequency and amplitude of spin oscillations shall be smaller, provided that the sign of the magnetic field at the beginning of time evolution is chosen appropriately since not all energy levels corresponding to Jˆz−=Lˆz−Sˆz eigenstates have mirror symmetry in relation to B → −B transformation. Our outcomes indicate, that the combined effect of a few-picosecond-long magnetic pulse and SOI on electron spin in DQR would enable one to perform transition between spin-up and spin-down states within 2 ps.

Modeling of anisotropic properties of double quantum rings by the terahertz laser field

The rendering of different shapes of just a single sample of a concentric double quantum ring is demonstrated realizable with a terahertz laser field, that in turn, allows the manipulation of electronic and optical properties of a sample. It is shown that by changing the intensity or frequency of laser field, one can come to a new set of degenerated levels in double quantum rings and switch the charge distribution between the rings. In addition, depending on the direction of an additional static electric field, the linear and quadratic quantum confined Stark effects are observed. The absorption spectrum shifts and the additive absorption coefficient variations affected by laser and electric fields are discussed. Finally, anisotropic electronic and optical properties of isotropic concentric double quantum rings are modeled with the help of terahertz laser field.

External electric field effect on the binding energy of a hydrogenic donor impurity in InGaAsP/InP concentric double quantum rings

Within the framework of effective-mass envelope-function theory, the ground state binding energy of a hydrogenic donor impurity is calculated in the InGaAsP/InP concentric double quantum rings (CDQRs) using the plane wave method. The effects of geometry, impurity position, external electric field and alloy composition on binding energy are considered. It is shown that the peak value of the binding energy appears in two rings with large gap as the donor impurity moves along the radial direction. The binding energy reaches the peak value at the center of ring height when the donor impurity moves along the axial direction. The binding energy shows nonlinear variation with the increase of ring height. With the external electric field applied along the z-axis, the binding energy of the donor impurity located at z[Formula: see text] decreases while that located at z[Formula: see text] increases. In addition, the binding energy decreases with increasing Ga composition, but increases with the increasing As composition.

LASER DRIVEN IMPURITY STATES IN TWO DIMENSIONAL CONCENTRIC DOUBLE QUANTUM RINGS

Hydrogenic donor impurity states in the 2D GaAs=Ga0.7Al0.3As concentric double quantum rings have been investigated under the action of intense laser field. An analytical expression of the effective lateral confinement potential induced by the laser field is obtained. The laser dressed effect has been considered both on electron confining and electron-impurity Coulomb interaction potentials. The single electron energy spectrum and wave functions have been found using the effective mass approximation and exact diagonalization technique. The accidental degeneracy of the impurity states has been observed.

The electronic properties of concentric double quantum ring and possibility designing XOR gate

In this paper I have investigated the Aharonov–Bohm oscillation in concentric double quantum ring. The outer ring attached to leads while the inner ring only tunnel-coupled to the outer ring. The effect of inner ring on electron transport properties through outer ring studied and found that the conductance spectrum consists of two types of oscillations. One is the normal Aharonov–Bohm oscillation, and other is a small oscillations superposed above AB oscillation. The AB oscillation utilized to designing nanoscale XOR gate by choosing the magnetic flux and tuning the gate voltages which realization XOR gate action.

Influence of lateral electric field on intraband optical absorption in concentric double quantum rings

The influence of lateral electric field on one-electron states and intraband absorption in two-dimensional concentric double quantum rings is investigated. The confining potential of the rings is modeled as a double harmonic central potential. Using the exact diagonalization technique, we calculate the dependence of the electron energy spectrum as a function of the electric field strength as well as the inner ring radius. Also, different values of confinement strength are considered. Selection rule is obtained for intraband transitions, caused by the direction of incident light polarization. The intraband absorption coefficient is calculated for different values of electric field strength, inner ring radius, confinement strength and incident light polarization direction. The combined influence of electric field strength and change of confining strength show that while the increment of the first one leads only to blueshift of absorption spectrum, the augment of the second one makes the redshift. In addition, both blueshift and redshift of the spectrum have been obtained with the enlargement of inner ring radius. Finally, we show that the absorption spectrum undergoes redshift by changing the polarization of incident light from x- to y-axis.

Donor-impurity related photoionization cross section in [formula omitted] concentric double quantum rings: Effects of geometry and hydrostatic pressure

The donor-impurity related photoionization cross section in GaAs/Ga1−xAlxAs three-dimensional concentric double quantum rings is investigated. The photoionization cross section dependence on the incident photon energy is studied considering the effects of hydrostatic pressure, variations of aluminum concentration, geometries of the structure, and impurity position. The interpretation of the dipole matrix element, which reflects the photoionization probability, is also given. We have found that these parameters can lead to both redshift and blueshift of the photoionization spectrum and also influence the cross section peak value.

States of an on-axis two-hydrogenic-impurity complex in concentric double quantum rings

Abstract The energy structure of an on-axis two-donor system ( D 2 0 ) confined in GaAs concentric double quantum rings under the presence of magnetic field and hydrostatic pressure was analyzed. Based on structural data for the double quantum ring morphology, a rigorous adiabatic procedure was implemented to separate the electrons' rapid in-plane motions from the slow rotational ones. A one-dimensional equation with an effective angular-dependent potential, which describes the two-electron rotations around the common symmetry axis of quantum rings was obtained. It was shown that D 2 0 complex characteristic features are strongly dependent on the quantum ring geometrical parameters. Besides, by changing the hydrostatic pressure and magnetic field strengths, it is possible to tune the D 2 0 energy structure. Our results are comparable to those previously reported for a single and negative ionized donor in a spherical quantum dot after a selective setting of the geometrical parameters of the structure.

Donor impurity-related linear and nonlinear optical absorption coefficients in [formula omitted] concentric double quantum rings: Effects of geometry, hydrostatic pressure, and aluminum concentration

The linear and nonlinear optical absorption associated with the transition between 1s and 2s states corresponding to the electron-donor-impurity complex in GaAs/Ga1−xAlxAs three-dimensional concentric double quantum rings are investigated. Taking into account the combined effects of hydrostatic pressure and the variation of the aluminum concentration, the energies of the ground and first excited s-like states of a donor impurity in such a system have been calculated using the effective mass approximation and a variational method. The energies of these states and the corresponding threshold energy of the optical transitions are examined as functions of hydrostatic pressure, aluminum concentration, radial impurity position, as well as the geometrical dimensions of the structure. The dependencies of the linear, nonlinear and total optical absorption coefficients as functions of the incident photon energy are investigated for different values of those mentioned parameters. It is found that the influences mentioned above lead to either redshifts or blueshifts of the resonant peaks of the optical absorption spectrum. It is particularly discussed the unusual property exhibited by the third-order nonlinear of becoming positive for photon energies below the resonant transition one. It is shown that this phenomenon is associated with the particular features of the system under study, which determine the values of the electric dipole moment matrix elements.

Biexciton energies in concentric double quantum rings

The energies of biexciton confined in one-dimensional concentric double quantum rings are studied using an exact diagonalization approach. An external magnetic field perpendicular to the ring plane is also included. The energies of L = 0 and L≠0 states of the concentric double quantum rings are calculated as functions of the ring radii and the intensity of the external magnetic field. We explore the effects of the Coulomb electron-hole correlation and of the magnetic field confinement for several ring distances and total angular momentum combinations. It is found that when the inner ring radius is very small (less than about 4 nm) the charges are distributed in both of the rings. However, if the inner ring is large, all carriers tend to localize in the inner ring. The angular momentum of the ground state depends on the magnitude of the magnetic field as well as on the distribution of particles in the rings. Likewise, the dependence of the energy on the angular momentum is different for a strong magnetic field than for a weak magnetic field.