Phase diagrams of La1?xCaxMnO3 in double exchange model with added antiferromagnetic and Jahn-Teller interaction

Within the effective mass approximation we theoretically studied the electronic properties of CdSe/ZnS and ZnS/CdSe core-shell quantum dots surrounded by wide-gap dielectric materials. The finite element method is used to obtain the lowest impurity levels and the carrier spatial distribution within the dot. We found that in these zero-dimensional semiconductor structures the electron energy is sensitively dependent on the dielectric constants of the embedding and on the heterostructure geometry. The influence of polarization charges on the binding energy of hydrogenic impurities off-center located is also investigated. The results suggest that in dielectrically modulated nanodots the donor energy can be tuned to a large extent by the structure design, the impurity position and a proper choice of the dielectric media.     

Bound polaron in a wurtzite GaN/AlxGa1 − xN ellipsoidal finite-potential quantum dot

A variational method is used to study the ground state of a bound polaron in a weakly oblate wurtzite GaN/Al_xGa_1 − xN ellipsoidal quantum dot. The binding energy of the bound polaron is calculated by taking the electron couples with both branches of LO-like and TO-like phonons due to the anisotropic effect into account. The interaction between impurity and phonons has also been considered to obtain the binding energy of a bound polaron. The results show that the binding energy of bound polaron reaches a peak value as the quantum dot radius increases and then diminishes for the finite potential well. We found that the binding energy of bound polaron is reduced by the phonons effect on the impurity states, the contribution of LO-like phonon to the binding energy is dominant, the anisotropic angle and ellipticity influence on the binding energy are small.     

Binding energy and photoionization cross-section of a hydrogen-like donor impurity in a quantum well-wire in a magnetic field

The effect of a uniform longitudinal magnetic field on the binding energy and photoionization cross-section of a hydrogen-like donor impurity is studied for a semiconductor quantum well-wire approximated by a cylindrical well of finite depth. The selection rules and analytical expressions for the photoionization cross-section are obtained depending on the magnetic field induction, impurity position, and light wave polarization.     

Dielectric mismatch and shallow donor impurities in GaN/HfO2 quantum wells

In this work we investigate electron–impurity binding energy in GaN/HfO₂ quantum wells. The calculation considers simultaneously all energy contributions caused by the dielectric mismatch: (i) image self-energy (i.e., interaction between electron and its image charge), (ii) the direct Coulomb interaction between the electron–impurity and (iii) the interactions among electron and impurity image charges. The theoretical model account for the solution of the time-dependent Schrödinger equation and the results shows how the magnitude of the electron–impurity binding energy depends on the position of impurity in the well-barrier system. The role of the large dielectric constant in the barrier region is exposed with the comparison of the results for GaN/HfO₂ with those of a more typical GaN/AlN system, for two different confinement regimes: narrow and wide quantum wells.     

Simultaneous effects of dielectric mismatch and electric field on the electronic properties in Si nanodots

By using the finite element method within the effective mass approximation, the effects of both dielectric confinement and electric field on the shallow-donor binding energy and polarizability in spherical Si quantum dots are investigated. It is found that: (i) the ground state binding energy is significantly increased by the dielectric mismatch at the dot interface, (ii) in the freestanding nanodot the competition between the electric field, polarization charges induced at interfaces and impurity position determines the symmetry of the electron probability distribution; (iii) the donor polarizability decreases with electric field strength and this effect is more pronounced for large dielectric mismatches. Therefore, the electronic properties of the nanocrystals could be tuned by proper tailoring of the surrounding medium dielectric constant as well as by varying the electric field. The normalized binding energy of an on-center hydrogenic donor is also been estimated and the results are in good agreement with the previous reported values.     

Comparison of external electric and magnetic fields effect on binding energy of hydrogenic donor impurity in different shaped quantum wells

The effects of external electric and magnetic fields on the ground state binding energy of hydrogenic donor impurity are compared in square, V-shaped, and parabolic quantum wells. With the effective-mass envelope-function approximation theory, the ground state binding energies of hydrogenic donor impurity in InGaAsP/InP QWs are calculated through the plane wave basis method. The results indicate that as the quantum well width increases, the binding energy changes most fast in SQW. When the well width is fixed, the binding energy is the largest in VQW for the donor impurity located near the center of QWs. For the smaller and larger well width, the electric field effect on binding energy is the most significant in VQW and SQW, respectively. The magnetic field effect on binding energy is the most significant in VQW. The combined effects of electric and magnetic fields on the binding energy of hydrogenic donor impurity are qualitative consistent in different shaped QWs.     

抛物量子阱中束缚极化子的极化势和结合能

利用改进的Lee-Low-Pines(LLP)方法,用变分法计算了无限深抛物量子阱中同时考虑与体纵光学声子和界面纵光学声子相互作用的束缚极化子的极化势和结合能.数值计算得出:阱宽较大时极化势很小,阱宽较小时极化势较大,所以对于较窄的抛物阱必须考虑极化势.对于给定阱宽的抛物阱,随着远离阱中心极化势迅速减小,当到达阱的界面附近极化势又开始增大.阱宽较小时,束缚极化子的结合能随着阱宽L的增大而急剧减小;阱宽较大时,结合能减小的非常缓慢,最后接近体材料中的三维值.     

抛物阱中极化子效应对D-心的影响

采用改进的LLP方法在考虑体声子影响的情况下研究了抛物量子阱中电子-LO声子相互作用对D^-心能量的影响.研究结果表明,阱宽较小时,D^-心的基态能量和结合能随着阱宽L的增大而急剧减小;阱宽较大时,能量的减小比较缓慢,最后接近体材料中的三维值.并且得出了电子-LO声子相互作用可使D^-心的结合能有显著的提高.     

Donor-impurity related binding energy and photoinization cross-section in quantum dots: electric and magnetic fields and hydrostatic pressure effects

We have studied the behavior of the binding energy and photoionization cross-section of a donor-impurity in cylindrical-shape GaAs-Ga_0.7Al_0.3As quantum dots, under the effects of hydrostatic pressure and in-growth direction applied electric and magnetic fields. We have used the variational method under the effective mass and parabolic band approximations. Parallel and perpendicular polarizations of the incident radiation and several values of the quantum dot geometry have also been considered. Our results show that the photoionization cross-section growths as the hydrostatic pressure is increased. For parallel polarization of the incident radiation, the photoionization cross-section decreases when the impurity is shifted from the center of the dot. In the case of perpendicular polarization of the incident radiation, the photoionization cross-section increases when the impurity is shifted in the radial direction of the dot. For on-axis impurities the transitions between the ground state of the impurity and the ground state of the quantum dot are forbidden. In the low pressure regime (less than 13.5 kbar) the impurity binding energy growths linearly with pressure, and in the high pressure regime (higher than 13.5 kbar) the binding energy growths up to a maximum and then decreases. Additionally, we have found that the applied electric and magnetic fields may favor the increase or decrease in binding energy, depending on the impurity position.     

Hydrogenic impurity states in zinc-blende GaN/AlN coupled quantum dots

Based on the effective-mass approximation, we have calculated the donor binding energy of a hydrogenic impurity in zinc-blende (ZB) GaN/AlN coupled quantum dots (QDs) using a variational method. Numerical results show that the donor binding energy is highly dependent on the impurity position and coupled QDs structural parameters. The donor binding energy is largest when the impurity is located at the center of quantum dot. When the impurity is located at the interdot barrier edge, the donor binding energy has a minimum value with increasing the interdot barrier width.     

Binding energy and photoionizaiton cross-section of hydrogen-like impurity in a Pöschl-Teller quantum well

The effect of the donor impurity position and the form of confining potential on the binding energy and the photoionization cross-section in a semiconductor quantum well with the Pöschl-Teller potential is studied. An analytical expression for the photoionization cross-section is obtained for the case when the polarization vector of light wave is directed along the direction of size quantization. It is shown that the photoionization cross-section has a threshold behavior.     

压力及屏蔽对无限深量子阱中施主结合能的影响

对GaAs/Al_xGa_1-xAs和GaN/Al_xGa_1-xN无限深量子阱系统,考虑压力及屏蔽效应,利用变分方法数值计算这两种系统中的杂质态结合能。给出了结合能随阱宽和压力的变化关系,同时讨论了有无屏蔽时的区别。结果表明,结合能随压力增大而增大,随阱宽增大而减小;屏蔽效应随着压力的增加而增加,并且显著降低了杂质态的结合能。     

Hydrostatic pressure effect on the donor impurity states in asymmetric multiple quantum wells

Based on the effective-mass approximation, hydrostatic pressure effect on the donor binding energy in zinc blende (ZB) InGaN/GaN asymmetric multiple quantum wells (AMQWs) is investigated variationally. Numerical results show that the hydrostatic pressure increases the donor binding energy for any impurity position. Moreover, the hydrostatic pressure effect is more noticeable if the impurity is localized inside the wide well of the AMQWs. For any hydrostatic pressure, the donor binding energy is distributed asymmetrically with respect to the center of the AMQWs. In particular, the donor binding energy of impurity located at the center of the wide well of the AMQWs is insensitive to the increment of the inter-well barrier width if the inter-well barrier width is large.     

Hydrostatic pressure and electric field effects on the normalized binding energy in asymmetrical quantum wells

We have investigated the simultaneous effects of the hydrostatic pressure and electric field on the ground subband level and on normalized binding energy of an on-center donor in asymmetrical GaAs/AlGaAs quantum wells within the effective-mass approximation and a variational approach. We found that the well size at which the impurity energy changes from positive to negative value (turning point) strongly depends on the asymmetry and hydrostatic pressure. As a key result, we suggest that the study of the normalized binding energy for various values of the electric field in direct and inverse polarization regimes can be used to feel the quantum well asymmetry and to unambiguously find out the effective pressure acting on a given heterostructure.     

Hydrogenic impurity in double quantum dots

The ground state binding energy and the average interparticle distances for a hydrogenic impurity in double quantum dots with Gaussian confinement potential are studied by the variational method. The probability density of the electron is calculated, too. The dependence of the binding energy on the impurity position is investigated for GaAs quantum dots. The result shows that the binding energy has a minimum as a function of the distance between the two quantum dots when the impurity is located at the center of one quantum dot or at the center of the edge of one quantum dot. When the impurity is located at the center of the two dots, the binding energy decreases monotonically.     

Binding energies of helium-like impurities in parabolic quantum wells under an applied electric field

We present a variational method to compute the binding energies of helium-like impurities in finite parabolic GaAs- Ga_1 − xAl_xAs quantum wells. The effects of band nonparabolicity in the conduction band are taken into account within the effective mass approximation. The dependence of the impurity binding energy on the applied electric field and the impurity position is also discussed together with the polarization effect for all cases.     

The phonon effects on the impurity states in cylindrical quantum wire with a finite confining potential

The variational method and the effective mass approximation are applied to calculate the binding energies of the hydrogenic impurity states in a cylindrical quantum wire with finite deep potential well. The phonon effects on the impurity states are considered by taking both the couplings of the electron-phonon and the impurity ion-phonon into account. The numerical results for the GaAs cylindrical quantum wire are given and discussed. It is found that the ion-phonon interaction reduces the impurity binding energy and supplies key contribution to the energy shift, but the electron-phonon coupling enhances the binding energy less. Longitudinal optical (LO) phonons play more important role than interface optical (IO) phonons in the impurity potential screening. The polaron effect caused by LO phonons is more important when the wire is thinner, otherwise the LO phonons are dominant for the thicker wires.     

Built-in electric field effect on hydrogenic impurity in wurtzite GaN/AlGaN quantum dot

The binding energy of a hydrogenic donor impurity in a wurtzite (WZ) GaN/AlGaN quantum dot (QD) is investigated, including the strong built-in electric field effect due to the spontaneous and piezoelectric polarizations. Numerical results show that the strong built-in electric field induces an asymmetrical distribution of the donor binding energy with respect to the center of the QD. The donor binding energy is insensitive to dot height when the impurity is located at the right boundary of the QD with large dot height.     

The isotropic-to-nematic transition in a two-dimensional fluid of hard needles: a finite-size scaling study

Laser dependence of binding energy on exciton in a GaAs quantum well wire embedded on an AlGaAs wire within the single band effective mass approximation is investigated. Laser dressed donor binding energy is calculated as a function of wire radius with the renormalization of the semiconductor gap and conduction valence effective masses. We take into account the laser dressing effects on both the impurity Coulomb potential and the confinement potential. The valence-band anisotropy is included in our theoretical model by using different hole masses in different spatial directions. The spatial dielectric function and the polaronic effects have been employed in a GaAs/AlGaAs quantum wire. The numerical calculations reveal that the binding energy is found to increase with decrease with the wire radius, and decrease with increase with the value of laser field amplitude, the polaronic effect enhances the binding energy considerably and the binding energy of the impurity for the narrow well wire is more sensitive to the laser field amplitude.     

Polarization dependence of Fano resonances in impurity photoconductivity of quantum wells doped with shallow donors

The spectrum of impurity photoabsorption of a heterostructure with quantum wells doped with shallow donors has been calculated in the energy range close to the optical phonon energy. It has been shown that the resonant feature of photoconductivity (Fano resonance) caused by the interaction of electrons with polar optical phonons depends strongly on radiation polarization when the resonance state energy is higher than the energy of the bottom of the second quantum-well subband. This dependence on polarization has been experimentally revealed in the impurity photoconductivity spectrum of the AlGaAs/GaAs heterostructure.