Impurity Position Research Articles

Binding energy of a hydrogenic impurity in a coaxial quantum wire with an insulator layer

The electronic properties of a coaxial cylindrical quantum well-barrier system constituted about an central insulating wire were determined under an external electric field. The model wire, inside to outside, was considered to be layered as AlAs/GaAs/Alx1Ga1−x1As/GaAs/Alx2Ga1−x2As. Within the framework of the effective mass-approximation, the binding energy of a hydrogenic impurity is calculated by using the combination of the fourth-order Runge-Kutta method and variational approaches. The binding energy exhibits sharp changes depending on the impurity position and the geometrical parameters of the structure such as the well widths of the GaAs wires and the height and thickness of the barrier constituted by Alx1Ga1−x1As. The binding energy of the electron was found to be independent from the impurity position for the specific widths of the well wires. Also, the barrier properties appeared as very effective parameters in controlling the probability distribution of the electron.

Magnetic-field dependence of the impurity states in a dome-shaped quantum dot

Using the finite element method, the effect of magnetic fields on the donor states and transition energies in a InAs/GaAs quantum dot coupled to its wetting layers is investigated. Results are obtained for different impurity locations. We found that the diamagnetic shift of the ground state energy increases monotonously with the applied field and can be described by a simple function which interpolates between the low and high magnetic-field behavior. Frequencies associated to the transitions between the S-like ground state and P− (P+) excited states range in terahertz region and show a magnetic field-induced red (blue) shift, irrespectively of the impurity position.

Pressure effects on the dipole oscillator strength, polarizability, and mean excitation energy of a hydrogen impurity under cylindrical confinement: off-center axis effect

We study the electronic properties of a hydrogen atom under cylindrical confinement as obtained by a numerical solution to the Schrödinger equation by means of a finite-differences approach. In particular we calculate the dipole oscillator strength, static and dynamic dipole polarizabilities, as well as the mean excitation energy as a function of the position of the hydrogen impurity along the symmetry axis for the case of a ‘standard’ cylindrical confinement cavity and several confinement conditions. The effect of the displacement on the electronic properties is reflected in the change of the wave-function as the impurity approaches the cylinder potential lid produced by the surrounding confinement environment. We find that the intensity of the main dipole transition, , is reduced as the atom is displaced off-center along the symmetry axis, reaching a minimum half-way between the center of the cylinder and the lid and then increasing when at the cylinder lid. In the process some other transition lines become more intense with a maximum also at half-way between the center and the cylinder lid. We find that the label assignment on the excitation transitions changes as the impurity is displaced along the symmetry axis due to the polarizability of the impurity electronic cloud. Results for the static and dynamic polarizability for the confined impurity as well as the mean excitation energy for the cases of penetrable and impenetrable confinement are presented. We find that the static polarizability increases as the impurity approaches the cylinder lid meanwhile the mean excitation energy is reduced.

Study of entanglement entropy and exchange coupling in two-electron coupled quantum dots

In the present work, we have studied the electronic properties of two-electron quantum dots with considering a two-dimensional modified Gaussian confinement potential in the presence of an impurity. We have investigated the effects of impurity position and charge of impurity on the entanglement entropy and the exchange coupling of the system. The location and charge screening of the impurity atom modulate the degree of entanglement. According to the results, it is found that the entanglement can be considered in two regions, at low and high entanglement. The magnitude of the charge impurity determines both regions of entanglement.

Linear and nonlinear optical properties of an off-center donor in GaAs/AlGaAs and AlGaAs/GaAs core-shell ellipsoidal quantum dots

In the present work, the linear and nonlinear absorption coefficients and refractive index changes of an off-center donor in both GaAs/Al0.3Ga0.7As and Al0.3Ga0.7As/GaAs core-shell ellipsoidal quantum dots have been studied, emphasizing on the influence of impurity position and the shapes of core and shell. The results show that the impurity position and the shape of core and shell have an important effect on the optical properties of the system, and the influence of core and shell shapes in such two core-shell structures is different. The physical reasons for the results have been discussed in detail.

Off-centre impurity-related nonlinear optical absorption, second and third harmonic generation in a two-dimensional quantum ring under magnetic field

The effects of an off-centre donor impurity on the non-linear optical absorption, second and third harmonic generation in a GaAs two-dimensional disc-shaped quantum ring under magnetic field are investigated within the compact density-matrix formalism and the effective mass approximation. The results reveal that: (i) the absorption spectra extend on larger energy intervals at the increment of the magnetic field; (ii) the possibility of generating second harmonic response from the system is demonstrated for an impurity placed on the repulsive part of the confining potential; (iii) both second harmonic and third harmonic coefficients are one order of magnitude larger for an impurity placed on the repulsive part of the potential and are blue-shifted by the increment of the magnetic field regardless the impurity position.

Magnetic field control of absorption coefficient and group index in an impurity doped quantum disc

The electronic properties and optical characteristics – absorption coefficient, refractive index and group index – in an impurity doped pseudo-harmonic quantum disc subjected to an applied magnetic field are investigated. Numerical calculations are performed using the exact diagonalization technique and the compact density-matrix formalism. It is found that the chosen structure could be switched between a lambda-type and a ladder-type configuration by a proper tailoring of the magnetic field strength. Furthermore, the absorption profile and the associated slow light frequency range can be controlled not only by varying the control field amplitude and its detuning but also by changing the impurity position and the magnetic field strength.

Green’s functions theory for an interstitial magnetic impurity in a transverse Ising ferromagnet

A Green’s function formalism is used to calculate the spectrum of localized states associated with an interstitial magnetic impurity in a semi-infinite ferromagnet described by the transverse Ising model. The nonresonant and resonant excitations of the system due to the impurity (i.e. the defect modes outside and inside, respectively, the region of the bulk and surface spin waves (SW)) are calculated numerically for the high-temperature phase. Two situations are analyzed, depending on the position of the interstitial impurity: the impurity is in the surface ([Formula: see text]) and the impurity is in the bulk region ([Formula: see text]).

Intergranular barrier height fluctuations in polycrystalline semiconductors

The punctual character and random nature of the impurity positions in depletion regions lead to inhomogeneities that can significantly affect the potential intergranular barriers at polycrystalline semiconductors and, in general, of potential barriers at any semiconductor interface. This would reflect in Arrhenius plots for the electrical conductivity that become curved due to fluctuations of the intergranular barrier heights. Experimental results for polycrystalline tin oxide can be fitted assuming thermionic emission conduction at grain boundaries with a Gaussian distribution of barrier height fluctuations. However, resorting to a computational numerical model, we found that spatial fluctuations in barrier heights due to the discreteness of the donors and their statistical distribution at the depletion region differ from a Gaussian distribution. The type of obtained fluctuations, considering thermionic emission conduction, cannot explain the Arrhenius plots for the electrical conductivity found experimentally, especially at low temperature. Conversely, the tunneling contribution to conduction, without resorting to fluctuations, presents the observed trends.

Quantum interference on electron scattering in graphene by carbon impurities in underlying h -BN

Electronic structures and transport properties of graphene on $h$-BN with carbon impurities are investigated by first-principles calculation and the tight-binding model. We show that the coupling between the impurity level and the graphene's Dirac cone sensitively depends on the impurity position, and in particular, it nearly vanishes when the impurity is located right below the center of the six membered ring of graphene. The Bloch phase factor at the Brillouin zone edge plays a decisive role in the cancellation of the hopping integrals. The impurity position dependence on the electronic structures of graphene on $h$-BN is investigated by the first-principles calculation, and its qualitative feature is well explained by a tight-binding model with graphene and a single impurity site. We also propose a simple one-dimensional chain-impurity model to analytically describe the role of the quantum interference in the position-dependent coupling.

Effects of a scattering center on the ground-state energy of quantum-dot lithium

In this paper, the effects of a repulsive scattering center on the ground-state energy and spin properties of a three-electron parabolic quantum dot are investigated theoretically by means of configuration interaction method. Phase transition from a weakly correlated regime to a strongly correlated regime is examined from several strengths and positions of Gaussian impurity. Numerical results reveal that the transition from spin-1/2 to spin-3/2 state depends strongly on the location of the impurity which accordingly states the controllability of the spin polarization. Moreover, broken circular symmetry results in more pronounced electronic charge localization.

ЛОКАЛИЗАЦИЯ ПОЗИЦИЙ ПРИМЕСЕЙ УГЛЕРОДА В КРИСТАЛЛИЧЕСКИХ СТРУКТУРАХ ПОЛИМОРФНЫХ МОДИФИКАЦИЙ Nb5Si3 ПО ДАННЫМ АТОМИСТИЧЕСКОГО КОМПЬЮТЕРНОГО МОДЕЛИРОВАНИЯ

ЛОКАЛИЗАЦИЯ ПОЗИЦИЙ ПРИМЕСЕЙ УГЛЕРОДА В КРИСТАЛЛИЧЕСКИХ СТРУКТУРАХ ПОЛИМОРФНЫХ МОДИФИКАЦИЙ Nb5Si3 ПО ДАННЫМ АТОМИСТИЧЕСКОГО КОМПЬЮТЕРНОГО МОДЕЛИРОВАНИЯ

Photo-ionization cross-section of donor-related in (In,Ga)N/GaN core/shell under hydrostatic pressure and electric field effects

Hydrogenic-like donor-impurity related self and induced polarizations, bending energy and photo-ionization cross section in spherical core/shell zinc blende (In,Ga)N/GaN are computed. Based on the variational approach and within effective-mass and one parabolic approximations, the calculations are made under finite potential barrier taking into account of the discontinuity of the effective-mass and the constant dielectric. The photo-ionization cross section is studied according to the photon incident energy considering the effects of hydrostatic pressure, applied electric field, structure's radius, impurity's position and indium composition in the core. It is obtained that the influences mentioned above lead to either blue shifts or redshifts of the resonant peak of the photo-ionization cross section spectrum. The unusual behavior related to the structure radius is discussed which is as a consequence of the finite potential confinement. We have shown that the photo-ionization cross section can be controlled with adjusting the internal and external factors. These properties can be useful for producing some device applications such as quantum dot infrared photodetectors.

Photoionization and electrostatic multipoles properties of spherical core/shell/shell quantum nanolayer with off-center impurity

The electronic, optical and electrostatic properties of the spherical core/shell/shell quantum nanolayer with an off-centered impurity have been studied. Spherical nanolayers of both “small” and “large” radii have been considered in the framework of perturbation theory and the variational method. Photoionization cross-section that corresponds to the electron transitions from the impurity ground state to the size-quantized levels have been studied. The dependence of the photoionization cross section on the photon energy, impurity position and the geometrical parameters of the spherical nanolayer have been obtained. The electrostatic multipoles of the considered system have been investigated.

Effects of electron–phonon interaction and impurity on optical properties of hexagonal-shaped quantum wires

We have investigated the influence of electron–phonon (e–p) interaction and hydrogenic donor impurity simultaneously on energy difference, binding energy, the linear, nonlinear and total refractive index changes and absorption coefficients of a hexagonal-shaped quantum wire. For this goal, we have used finite-element method (FEM), a compact density matrix approach and an iterative procedure. It is deduced that energy difference and binding energy decrease by changing the impurity position with and without e–p interaction. The dipole matrix elements have complex behaviours in the presence of impurity with and without e–p interaction. The refractive index changes and absorption coefficients increase and shift towards lower energies by enhancing a 1 with central impurity. In the presence of central impurity, the absorption coefficients and refractive index changes enhance and shift toward higher energies when e–p interaction is considered.

Electronic states in low-dimensional nano-structures: Comparison between the variational and plane wave basis method

A comparison is made between the plane wave basis and variational method. Within the framework of effective-mass approximation theory, the variational and plane wave basis method are used to calculate ground state energy and ground state binding energy in low-dimensional nano-structures under the external electric field. Comparing calculation results, the donor binding energies of ground state display the consistent trend, both of them are strongly dependent on the quantum size, impurity position and external electric field. However, the impurity ground state energy calculated using variational method may be larger than the real value and it results in the smaller binding energy for variational method. In addition, the binding energy is more sensitive to the external electric field for the variational method, which can be seen more clearly from Stark shift.

External Electric Field Effect on Shallow Donor Impurity States in Zinc-Blende InxGa1−xN/GaN Symmetric Coupled Quantum Dots

Based on the effective-mass approximation and variational procedure, the ground-state donor binding energy in a cylindrical zinc-blende InxGa1-xN/GaN symmetric coupled quantum dots (SCQDs) is investigated in the presence of the external electric field. Numerical results show that the donor binding energy increases firstly until a maximum value, and then it begins to drop quickly in all the cases with decreasing the dot radius. As the thickness of left dot and right dot decreases, the donor binding energy increases monotonically at first, reaches a maximum value, and then drops rapidly for an impurity ion located at the right dot center and the middle barrier center. Moreover, the donor binding energy for an impurity ion located at the center of the left dot is insensitive to the variation of dot thickness for large dot thickness due to the Stark effect. Meanwhile, the impurity position plays an important role on the change of the donor binding energy under the external electric field. In particular, the impurity position corresponding to the peak value of the donor binding energy is shifted toward the left QD with increasing the external electric field strength.

Atomic structure of the Zr–He, Zr–vac, and Zr–vac–He systems: First-principles calculation

The ab initio investigations have been performed for the atomic structure of the Zr–He, Zr–vac, and Zr–vac–He systems with concentrations of helium atoms and vacancies (vac) of ~6 at %. A heliuminduced instability of the zirconia lattice has been revealed in the Zr–He system, which disappears with the formation of vacancies. The most preferred positions of impurities in the metal lattice have been determined. The energy of helium dissolution and the excess volume introduced by helium have been calculated. It has been established that the presence of helium in the Zr lattice leads to a significant decrease in the energy of vacancy formation.

Donor impurity-related optical absorption coefficients and refractive index changes in a rectangular GaAs quantum dot in the presence of electric field**Project supported by the Science and Technology Project of Education Department of Heilongjiang Province of China (No. 12541070).

Within the quasi-one-dimensional effective potential model and effective mass approximation, we obtain the wavefunctions and energy eigenvalues of the ground (j = 1) and first 2 excited states (j = 2 and 3) of a donor impurity in a rectangular GaAs quantum dot in the presence of electric field. The donor impurity-related linear and nonlinear optical absorption as well as refractive index changes for the transitions j = 1−2 and j = 2−3 are investigated. The results show that the impurity position, incident optical intensity and electric field play important roles in the optical absorption coefficients and refractive index changes. We find that the impurity effect induces the blueshift for j = 1−2 and redshift for j = 3−2 in the absence of the electric field, but it leads to redshift for j = 1−2 and blueshift for j = 3−2 in the existence of the field. Also, the optical coefficient for the higher energy transitions j = 2−3 is insensitive to variation of impurity positions, while that for the low energy transition j = 1−2 depends significantly on the positions of impurity. In addition, the saturation and splitting phenomenon of the optical absorption are observed as the incident optical intensity increases.

Sugar Cube Purity: Capacitive Sensing and Image Processing Approach

Several methods have been implemented to find out if impurity is present in sugar cubes or powders so far. Several high end imaging techniques with X-ray scanners have been used to check the quality of the sugar. In this present study, without disturbing the physical nature of sugar, we have simply used the dielectric property analysis to check the purity of the sugar. This method can detect even infinitesimal amounts of impurity present in sugar with good accuracy. The positional accuracy is derived using artificial neural networks, which is been trained with various capacitance values when the impurity is present and gives the details on the change in capacitance value as the impurity position changes.