We report theoretical studies on the plasmon resonances in linear Au atomic chains by using ab initio timedependent density functional theory. The dipole responses are investigated each as a function of chain length. They converge into a single resonance in the longitudinal mode but split into two transverse modes. As the chain length increases, the longitudinal plasmon mode is redshifted in energy while the transverse modes shift in the opposite direction (blueshifts). In addition, the energy gap between the two transverse modes reduces with chain length increasing. We find that there are unique characteristics, different from those of other metallic chains. These characteristics are crucial to atomic-scale engineering of single-molecule sensing, optical spectroscopy, and so on.

The binding energy of a hydrogenic impurity in self-assembled double quantum dots * Zhang Hong( ) a) † , Wang Xue( ) a) , Zhao Jian-Feng( ) a) , and Liu Jian-Jun( ) b)

This paper presents a systematic study of the ground-state binding energies of a hydrogenic impurity in quantum dots subjected to external electric and magnetic fields. The quantum dot is modeled by superposing a lateral parabolic potential, a Gaussian potential and the energies are calculated via the finite-difference method within the effectivemass approximation. The variation of the binding energy with the lateral confinement, external field, position of the impurity, and quantum-size is studied in detail. All these factors lead to complicated binding energies of the donor, and the following results are found: (1) the binding energies of the donor increase with the increasing magnetic strength and lateral confinement, and reduce with the increasing electric strength and the dot size; (2) there is a maximum value of the binding energies as the impurity placed in different positions along the z direction; (3) the electric field destroys the symmetric behaviour of the donor binding energies as the position of the impurity.

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