A numerical analysis of an electron waveguide coupler based on two quantum wires coupled by a magnetically defined barrier is presented with the use of the scattering-matrix method. For different geometry parameters and magnetic fields, tunneling transmission spectrum is obtained as a function of the electron energy. Different from that of conventional electron waveguide couplers, the transmission spectrum of the magnetically coupled quantum wires does not have the symmetry with regard to those geometrically symmetrical ports. It was found that the magnetic field in the coupling region drastically enhances the coupling between the two quantum wires for one specific input port while it weakens the coupling for the other input port. The results can be well understood by the formation of the edge states in the magnetically defined barrier region. Thus, whether these edge states couple or decouple to the electronic propagation modes in the two quantum wires, strongly depend on the relative moving directions of electrons in the propagating mode in the input port and the edge states in the magnetic region. This leads to a big difference in transmission coefficients between two quantum wires when injecting electrons via different input ports. Two important coupler specifications, the directivity and uniformity, are calculated which show that the system we considered behaves as a good quantum directional coupler. © 1997 American Institute of Physics.

Sheng Wei-Dong;Gu Ben-Yuan;Xia Jian-Bai;Wang Jian

Journal of Applied Physics

1997

The effect of electric field on the electronic structure of a spherical quantum dot is studied in the framework of the effective-mass envelope-function theory. The dependence of the energy of electron states and hole states on the applied electric field and on the quantum dot size is investigated; the mixing of heavy holes and light holes is taken into account. The selection rule for the optical transition between the conduction band and valence band states is obtained. The exciton binding energies are calculated as functions of the quantum dot radius and the strength of the electric field. © 1998 American Institute of Physics.

Chang Kai;Xia Jian-Bai

Journal of Applied Physics

1998

Radiative transition in δ-doped GaAs superlattices with a weak coupling was investigated at low temperature. The transitions from both electron ground state and excited state to hole state have been observed. Based on the effective mass approximation theory, the structures of energy band and photoluminescence spectra for the samples were calculated. The theoretical calculating values are in good agreement with the experimental data.

Cheng Wenchao;Xia Jianbai;Li Guohua;Tan Pingheng;Zheng Houzhi

Hongwai Yu Haomibo Xuebao Journal of Infrared and Millimeter Waves

1999

In the framework of effective-mass envelope function theory, the valence energy subbands and optical transitions in lateral superlattices (LSLs) have been calculated by the plane-wave expansion method. The effects of finite offset and valence band mixing are taken into account. The modulations of several types of lateral potential are also evaluated; they indicate that the out-of-phase modulation on either side of the wells is the strongest while the in-phase modulation is the weakest. The lateral modulation periods have a weak effect on the lowest hole energy levels. When one is making LSLs, the fabrication can be tailored to make the lateral modulation period fairly large, which is favourable for technological applications. Our calculations also show that the effect of the difference between the effective masses of holes in different materials on the valence subband structures is significant. Our theoretical results are in agreement with the available experimental data and have great significance as regards investigating and making low-dimensional semiconductor devices.

Li Shu-Shen;Zhu Bang-Fen;Xia Jian-Bai

Journal of Physics Condensed Matter

1999

We have studied the hole levels and exciton states in CdS nanocrystals by using the hole effective-mass Hamiltonian for wurtzite structure. It is found that the optically passive Px state will become the ground hole state for small CdS quantum dots of radius less than 69 Å. It suggests that the "dark exciton" would be more easily observed in the CdS quantum dots than that in CdSe quantum dots. The size dependence of the resonant Stokes shift is predicted for CdS quantum dots. Including the Coulomb interaction, exciton energies as functions of the dot radius are calculated and compared with experimental data.

Li Jingbo;Xia Jian-Bai

Physical Review B Condensed Matter and Materials Physics

2000

We report on the theoretical study of the interaction of the quantum dot (QD) exciton with the photon waveguide models in a semiconductor microcavity. The InAs/GaAs self-assembled QD exciton energies are calculated in a microcavity. The calculated results reveal that the electromagnetic field reduces the exciton energies in a semiconductor microcavity. The effect of the electromagnetic field decreases as the radius of the QD increases. Our calculated results are useful for designing and fabricating photoelectron devices. © 2001 Chin. Phys. Soc. and IOP Publishing Ltd.

Pan Liu-Xian;Li Shu-Shen;Xia Jian-Bai

Chinese Physics

2001

Optical spectra of CdSe nanocrystals are measured at room temperature under pressure ranging from 0 to 5.2 GPa. The exciton energies shift linearly with pressure below 5.2 GPa. The pressure coefficient is 27 meV GPa^-1 for small CdSe nanocrystals with the radius of 2.4 nm. With the approximation of a rigid-atomic pseudopotential, the pressure coefficients of the energy band are calculated. By using the hole effective-mass Hamiltonian for the semiconductors with wurtzite structure under various pressures, we study the exciton states and optical spectra for CdSe nanocrystals under hydrostatic pressure in detail. The intrinsic asymmetry of the hexagonal lattice structure and the effect of spin-orbit coupling on the hole states are investigated. The Coulomb interaction of the exciton states is also taken into account. It is found that the theoretical results are in good agreement with the experimental values.

Li Jingbo;Li Guo-Hua;Xia Jian-Bai;Zhang Jing-Bo;Lin Yuan;Xiao Xu-Rui

Journal of Physics Condensed Matter

2001

In the framework of effective mass envelope function theory, the electronic states of the InAs/GaAs quantum ring are studied. Our model can be used to calculate the electronic states of quantum wells, quantum wires, and quantum dots. In calculations, the effects due to the different effective masses of electrons in rings and out rings are included. The energy levels of the electron are calculated in the different shapes of rings. The results indicate that the inner radius of rings sensitively changes the electronic states. The energy levels of the electron are not sensitively dependent on the outer radius for large rings. If decreasing the inner and outer radii simultaneously, one may increase the energy spacing between energy levels and keep the ground state energy level unchanged. If changing one of two radii (inner or outer radius), the ground state energy level and the energy spacing will change simultaneously. These results are useful for designing and fabricating the double colors detector by intraband and interband translations. The single electron states are useful for studying the electron correlations and the effects of magnetic fields in quantum rings. Our calculated results are consistent with the recent experimental data of nanoscopic semiconductor rings. © 2001 American Institute of Physics.

Li Shu-Shen;Xia Jian-Bai

Journal of Applied Physics

2001

A theoretical model accounting for the macropolarization effects in wurtzite III-V nitrides quantum wells (QWs) is presented. Energy dispersions and exciton binding energies are calculated within the framework of effective-mass theory and variational approach, respectively. Exciton-associated transitions (EATs) are studied in detail. An energy redshift as high as 450 meV is obtained in Al0.25GaN0.75/GaN QWs. Also, the abrupt reduction of optical momentum matrix elements is derived as a consequence of quantum-confined Stark effects. EAT energies are compared with recent photoluminescence (PL) experiments and numerical coherence is achieved. We propose that it is the EAT energy, instead of the conduction-valence-interband transition energy that is comparable with the PL energy. To restore the reduced transition rate, we apply an external electric field. Theoretical calculations show that with the presence of the external electric field the optical matrix elements for EAT increase 20 times. © 2001 American Institute of Physics.

Wan Shou-Pu;Xia Jian-Bai;Chang Kai

Journal of Applied Physics

2001

The time evolution of the quantum mechanical state of an electron is calculated in the framework of the effective-mass envelope function theory for an InAs/GaAs quantum dot. The results indicate that the superposition state electron density oscillates in the quantum dot, with a period on the order of femtoseconds. The interaction energy E ij between two electrons located in different quantum dots is calculated for one electron in the ith pure quantum state and another in the jth pure quantum state. We find that E 11 〉E 12 〉E 22 , and E ij decreases as the distance between the two quantum dots increases. We present a parameter-phase diagram which defines the parameter region for the use of an InAs/GaAs quantum dot as a two-level quantum system in quantum computation. A static electric field is found to efficiently prolong the decoherence time. Our results should be useful for designing the solid-state implementation of quantum computing. © 2001 American Institute of Physics.

Li Shu-Shen;Xia Jian-Bai;Liu Jin-Long;Yang Fu-Hua;Niu Zhi-Chuan;Feng Song-Lin;Zheng Hou-Zhi

Journal of Applied Physics

2001