单层正三角锯齿型石墨烯量子点的电子结构和磁性

采用基于密度泛函理论的广义梯度近似(GGA),对不同尺寸(N=2—11)的单层正三角锯齿型石墨烯量子点(Z_N -GNDs)的结构进行优化,得到与实验数据较好符合的晶格常数,进一步计算得到不同尺寸下体系的自旋多重度、磁矩、电子态密度以及自旋电子密度.结果表明:所有体系都呈现金属性,在尺寸较小的体系中量子尺寸效应对电子结构的影响比较明显;与单层石墨烯片一样,sp²杂化作用和非键态电子在量子点中仍起到非常重要的作用;费米能级上有自旋向上的电子分布,体系的 关键词： 石墨烯 量子点 电子结构 磁性     

Optical non-linearities associated to hydrogenic impurities in InAs/GaAs self-assembled quantum dots under applied electric fields

Abstract

The effects of the hydrogenic impurity on the electron-related non-linear optical processes in a InAs/GaAs dome-shaped quantum dot with a wetting layer under applied electric fields are studied within the density-matrix formalism. The one-electron energy levels and wave functions are calculated using the effective mass approximation and the finite element method. The non-linear optical absorption, relative refractive index change and non-linear optical rectification associated with interlevel transitions are calculated under a strong probe field excitation for both in-plane and z-polarisation of the incident light. According to our results as the electric field increases the absorption and dispersion peaks decrease and exhibit red shift. Hydrogenic impurity located at the origin induces a blue shift in the optical responses. For the optical absorption coefficient the peaks magnitude is enhanced by the impurity presence independent of the electric field strengths, whereas the non-linear optical rectification is larger in the case with impurity only for zero applied electric field.     

Donor impurity-related photoionization cross section in GaAs cone-like quantum dots under applied electric field

The donor impurity-related electron states in GaAs cone-like quantum dots under the influence of an externally applied static electric field are theoretically investigated. Calculations are performed within the effective mass and parabolic band approximations, using the variational procedure to include the electron–impurity correlation effects. The uncorrelated Schrödinger-like electron states are obtained in quasi-analytical form and the entire electron–impurity correlated states are used to calculate the photoionisation cross section. Results for the electron state energies and the photoionisation cross section are reported as functions of the main geometrical parameters of the cone-like structures as well as of the electric field strength.     

Laterally coupled circular quantum dots under applied electric field

The optical response of a system of two laterally coupled quantum dots with circular cross-sectional shape is investigated within the effective mass approximation, taking into account the effects of the change in the geometrical configuration, the application of an external static electric field, and the presence of a donor impurity center. The first-order dielectric susceptibility is calculated in order to derive the corresponding light absorption and relative refractive index coefficients. The possibility of tuning these optical properties by means of changes in the quantum dot symmetry and the electric field intensity is particularly discussed.     

参杂GaAs量子点的电子结构

用有效质量近似和少体物理方法计算了在抛物势中,参杂（一个带正电荷）GaAs量子点中有7个极化电子时的本征能量和本征波函数,并从本征波函数中提取的一体、二体密度函数方法得到了电子结构的直观图像,用对称性对量子点中库仑相互作用能和电子结构进行了分析。     

Effect of electric field on the electronic spectrum and the persistent current of a quantum ring with two electrons

The effect of an electric field E on a narrow quantum ring that contains two electrons and is threaded by a magnetic flux B has been investigated. Localization of the electronic distribution and suppression of the Aharonov--Bohm oscillation (ABO) are found in the two-electron ring, which are similar to those found in a one-electron ring. However, the period of ABO in a two-electron ring is reduced by half compared with that in a one-electron ring. Furthermore, during the variation of B, the persistent current of the ground state may undergo a sudden change in sign. This change is associated with a singlet--triplet transition and has no counterpart in one-electron rings. For a given E, there exists a threshold of energy. When the energy of the excited state exceeds the threshold, the localization would disappear and the ABO would recover. The value of the threshold is proportional to the magnitude of E. Once the threshold is exceeded, the persistent current is much stronger than the current of the ground state at E=0.     

Electronic and optical properties of GaN/AlN quantum dots with adjacent threading dislocations

We present a theory to simulate a coherent GaN QD with an adjacent pure edge threading dislocation by using a finite element method. The piezoelectric effects and the strain modified band edges are investigated in the framework of multi-band $\bm k\cdot \bm p$ theory to calculate the electron and the heavy hole energy levels. The linear optical absorption coefficients corresponding to the interband ground state transition are obtained via the density matrix approach and perturbation expansion method. The results indicate that the strain distribution of the threading dislocation affects the electronic structure. Moreover, the ground state transition behaviour is also influenced by the position of the adjacent threading dislocation.     

垂直耦合自组织InAs双量子点中激子能的计算

在有效质量近似条件下研究了垂直耦合的自组织InAs/GaAs量子点的激子态.在绝热近似条件下，采用传递矩阵方法计算了电子和空穴的能谱.通过哈密顿量矩阵的对角化，对电子和空穴间的库仑相互作用进行了精确处理.讨论了两量子点间的垂直距离对激子基态能的影响.从基态波函数概率分布的角度，讨论了激子的束缚能.计算了重空穴和轻空穴激子的基态能随外部垂直磁场变化的函数关系.计算了量子点大小（量子点半径）对激子能的影响. 关键词： 量子点 激子 对角化     

Piezoelectric effects and electronic structures of InAs/GaAs quantum dots grown along (111) and (011) directions

Piezoelectric effects and electronic structures of InAs/GaAs quantum dots grown along (111) and (011) directions are investigated in this paper. The finite element method is used. Electronic energy levels are calculated by solving the three-dimensional effective mass Schr?dinger equation including a strain modified confinement potential and piezoelectric effects. The difference in electronic structure between quantum dots grown along the (111) direction and the (011) direction are compared. The cubic and truncated pyramidal shaped quantum dots are adopted.     

The influence of electric field on a parabolic quantum dot qubit

This paper calculates the time evolution of the quantum mechanical state of an electron by using variational method of Pekar type on the condition of electric--LO-phonon strong coupling in a parabolic quantum dot. It obtains the eigenenergies of the ground state and the first-excited state, the eigenfunctions of the ground state and the first-excited state This system in a quantum dot may be employed as a two-level quantum system qubit. The superposition state electron density oscillates in the quantum dot with a period when the electron is in the superposition state of the ground and the first-excited state. It studies the influence of the electric field on the eigenenergies of the ground state, the first-excited state and the period of oscillation at the different electron--LO-phonon coupling constant and the different confinement length.     

Effects of Shannon entropy and electric field on polaron in RbCl triangular quantum dot

In this paper, the time evolution of the quantum mechanical state of a polaron is examined using the Pekar type variational method on the condition of the electric-LO-phonon strong-coupling and polar angle in RbCl triangular quantum dot. We obtain the eigenenergies, and the eigenfunctions of the ground state, and the first excited state respectively. This system in a quantum dot can be treated as a two-level quantum system qubit and the numerical calculations are performed.The effects of Shannon entropy and electric field on the polaron in the RbCl triangular quantum dot are also studied.     

施加电场的半抛物量子阱中束缚态的能级结构

采用位移谐振子与数值求解相结合的方法,研究和讨论了施加电场的半抛物量子阱中束缚态的能级结构,得到了体系的本征能量与本征函数的表达式．数值结果显示,随着电场强度的增大,束缚态的能量几乎线性地下降,电场对半抛物束缚势频率较低的系统以及系统的高能级的影响较大,相邻能级间隔也随着电场强度的增大而减小,这与施加电场的抛物量子阱中的情况明显不同．     

Polaron effect on the optical rectification in spherical quantum dots with electric field

The polaron effect on the optical rectification in spherical quantum dots with a shallow hydrogenic impurity in the presence of electric field is theoretically investigated by taking into account the interactions of the electrons with both confined and surface optical phonons. Besides, the interaction between impurity and phonons is also considered. Numerical calculations are presented for typical Zn_(1-x)Cd_xSe/ZnSe material. It is found that the polaronic effect or electric field leads to the redshifted resonant peaks of the optical rectification coefficients. It is also found that the peak values of the optical rectification coefficients with the polaronic effect are larger than without the polaronic effect, especially for smaller Cd concentrations or stronger electric field.     

The influences of thickness of spacing layer and the elastic anisotropy on the strain fields and band edges of InAs/GaAs conical shaped quantum dots

Based on the continuum elastic theory, this paper presents a finite element analysis to investigate the influences of elastic anisotropy and thickness of spacing layer on the strain field distribution and band edges (both conduction band and valence band) of the InAs/GaAs conical shaped quantum dots. To illustrate these effects, we give detailed comparisons with the circumstances of isolated and stacking quantum dot for both anisotropic and isotropic elastic characteristics. The results show that, in realistic materials design and theoretical predication performances of the optoelectronic devices, both the elastic anisotropy and thickness of the spacing layer of stacked quantum dot should be taken into consideration.     

Second-harmonic generation in asymmetric quantum dots in the presence of a static magnetic field

The second-harmonic generation(SHG) coefficient in an asymmetric quantum dot(QD) with a static magnetic field is theoretically investigated.Within the framework of the effective-mass approximation,we obtain the confined wave functions and energies of electrons in the QD.We also obtain the SHG coefficient by the compact-density-matrix approach and the iterative method.The numerical results for the typical GaAs/AlGaAs QD show that the SHG coefficient depends strongly on the magnitude of magnetic field,parameters of the asymmetric potential and the radius of the QD.The resonant peak shifts with the magnetic field or the radius of the QD changing.     

Electronic structure and optical properties of zinc-blende GaN quantum dots

The energy levels of zinc-blende GaN quantum dots (QDs) are studied within the framework of the effective-mass envelope-function approximation. The dependence of the energy of electron and hole states on the quantum dot (QD) size is presented. The selection rules for optical transitions are given and the oscillator strengths of the dipole-allowed transitions for various QD radii are calculated with the wavefunctions of quantized energy levels. The theoretical absorption spectrum of GaN QDs is in good agreement with the existing experimental result.     

Electronic and optical properties of asymmetric GaAs double quantum dots in intense laser fields

In the present work, we investigated the effect of an intense non-resonant laser field on the electronic structure and the nonlinear optical properties (the light absorption, the optical rectification) of a GaAs asymmetric double quantum dot under a strong probe field excitation. The calculations were performed within the compact-density matrix formalism under the steady state conditions with the use of the effective mass approximation. The obtained results show that: (i) the electronic structure and, consequently, the optical properties are sensitive to the dressed potential; (ii) the changes in the incident light polarisation lead to blue or redshifts in the intraband optical absorption spectrum; (iii) for specific values of the structure parameters and under an intense laser illumination, the asymmetric double quantum dots can be a good candidate for NOR emission of THz radiation.     

椭圆柱形量子点的能级结构

采用椭圆柱坐标及无限深势阱模型对椭圆柱形量子点进行了研究，给出了计算椭圆柱形量子点中单电子的能级结构和波函数的表达式. 关键词： 量子点 人造原子 能级 波函数     

Electron transport and shell structures of single InAs quantum dots probed by nanogap electrodes

We have investigated electron transport and electron filling in single InAs quantum dots (QDs) using nanogap electrodes. Elliptic InAs QDs with diameter of 60/80 nm exhibited clear shell filling up to 12 electrons. Shell-dependent charging energies and level quantization energies for the s, p, and d states were determined from the addition energy spectra. Furthermore, it is found that the charging energies and the tunneling conductances strongly depend on the shell, reflecting that the electron wave functions for higher shells are more extended in space.