Excitons and many-electron effects in the optical response of single-walled boron nitride nanotubes

We report first-principles calculations of the effects of quasiparticle self-energy and electron-hole interaction on the optical properties of single-walled boron nitride nanotubes. Excitonic effects are shown to be even more important in BN nanotubes than in carbon nanotubes. Electron-hole interactions give rise to complexes of bright (and dark) excitons, which qualitatively alter the optical response. Excitons with a binding energy larger than 2 eV are found in the BN nanotubes. Moreover, unlike the carbon nanotubes, theory predicts that these exciton states are comprised of coherent supposition of transitions from several different subband pairs, giving rise to novel behaviors.     

Plasmons and optical excitations in graphene rings

A simple semiclassical drude-like conductivity of graphene is employed to describe plasmon excitations of graphene in the ring structures. A quasi-static self-consistent integral equation approach is performed, allowing the calculation of all the plasmon modes with different angular momentum l. Among them only the dipole modes (l?=?1) will couple out to the radiation modes, which in turn can be excited optically by the plane waves, and the excitation energies as a function of the ratio of the radius of the inner hole to that of the outer ring have also been investigated. It is demonstrated that the energy of symmetric modes will monotonically decrease as the ratio rises, and the energy of antisymmetric modes does not exhibit a monotonically increasing behavior as in a three-dimensional metallic ring, but first reduces and then increases. These predictions are tested by full-wave simulations using the optical conductivity of graphene that was obtained from the random phase approximation (RPA).     

Collectivity in the optical response. of small metal clusters

The question of whether the linear absorption spectra of metal clusters can be interpreted as density oscillations (collective “plasmons”) or can only be understood as transitions between distinct molecular states is still a matter of debate for clusters with only a few electrons. We calculate the photo-absorption spectra of Na₂ and Na₅ ⁺ comparing two different methods: quantum fluid dynamics and time-dependent density functional theory. The changes in the electronic structure associated with particular excitations are visualized in “snapshots” via transition densities. Our analysis shows that even for the smallest clusters, the observed excitations can be interpreted as intuitively understandable density oscillations. For Na₅ ⁺, the importance of self-interaction corrections to the adiabatic local density approximation is demonstrated. Received: 1 July 2001 / Published online: 10 October 2001     

Spherical electron-ion systems in semiclassical approximation: Atoms and atomic clusters

A semiclassical method for calculating the total energy and spatial distribution of electron density in spherically symmetric electron-ion systems is applied to atoms and both solid and hollow atomic clusters. Both exchange-correlation interaction and second-order gradient correction are taken into account. The contribution due to the fourth-order gradient correction is discussed. An expression is proposed for the oscillating correction to the averaged electron density. An expression is obtained for the equilibrium radius of a hollow cluster. The dependence of the equilibrium radius of an endohedral cluster on the valence of the central atom is analyzed.     

Response of finite many-electron systems beyond the time-dependent local density approximation: Application to small metal clusters

A linear response formalism is developed which is based on density functional theory within the local density approximation, but which is now corrected for its spurious self-interaction errors, in the way originally proposed by Perdew and Zunger for ground state calculations. The original formulation is extended to incorporate self-interaction corrections in the scrrening terms. The general formalism is then applied to the calculation of the static and dynamic response of the metal clusters {Na₈, Na₉⁺}, {Na₂₀, Na} and {Na₄₀ Na} within the jellium model. Comparison with experimental data and with other theoretical calculations indicates that the present formalism accounts for the overall (and most of the fine) features of the photoabsorption spectrum of these systems, providing a systematic improvement with respect to previous approaches. The remaining discrepancies are rationalized in terms of the effects to be expected by correctly accounting for the discrete structure of the ionic cores.     

由相干制备原子系统构成的理想光学介质

电磁感应透明（electromagnetically induced transparency,EIT）发生时所伴随的效应(如可控的线性吸收和色散,可控的非线性增强)是理想光学介质所必需的重要特性。通过理解这种相干制备的多能级原子系统的线性和非线性特征,人们能够更好地设计和利用这种新颖的光学介质,并将其应用到光通信和量子信息处理中。文章简单介绍了与EIT有关的原子介质的基本（线性和非线性）光学特性,并简单评价了几个令人感兴趣的应用。     

由相干制备原子系统构成的理想光学介质

电磁感应透明(electromagnetically induced transparency,EIT)发生时所伴随的效应(如可控的线性吸收和色散,可控的非线性增强)是理想光学介质所必需的重要特性。通过理解这种相干制备的多能级原子系统的线性和非线性特征,人们能够更好地设计和利用这种新颖的光学介质,并将其应用到光通信和量子信息处理中。文章简单介绍了与EIT有关的原子介质的基本(线性和非线性)光学特性,并简单评价了几个令人感兴趣的应用。     

The effect of many-electron correlations on the alignment of ions in atomic photoionization

It is shown that the inclusion of many-electron correlations and electron shell rearrangement can significantly change the alignment of the ion arising from the photoionization of atomic subshells with $\text{j >}\text{1}\text{2}$. The degree of alignment for photoionization of the 4d¹⁰ subshells of Xe, Cs and Ba atoms and the 5p⁶ subshell of Ba is calculated.     

Quasicrystals and atomic clusters

Historically, two principal approaches exist to determine the quasicrystal atomic structures: one is derived directly from diffraction experiments, using cut and projection of higher dimensional space-representation techniques; the other one takes use of the similarity between so-called approximant crystalline phases, observed to co-exist with quasicrystals. The known structure of these phases is shown to be the starting point of building a quasicrystal model in terms of constitutive polyatomic clusters of icosahedral symmetry. It requires to apply inflation properties of quasiperiodicity, and decoration of elementary building tiles. One example is detailed and discussed, in terms of a two-cluster model, in the case of AlLiCu.     

Temperature and size dependence of the optical response of small Ag clusters

The absorption spectra of small Ag ⁺ n clusters are calculated at finite vibrational temperature by using a microscopic tight-binding RPA method. We consider free clusters with sizes between n = 3 and n =13 and take into account explicitly the degrees of freedom corresponding to the 4 d-electrons. We analyze the optical absorption as a function of the cluster size. We show that the contribution of the d-electrons has an important influence on the size dependence of the energy of the Mie plasmon. We also perform ensemble averages to obtain the absorption spectra for different vibrational temperatures. We obtain relatively good agreement with experiment for a temperature . The dynamics of the 4 d-electrons, which shows in small clusters an incipient delocalized character for n >7, yields an important contribution to the absorption spectrum already for n =13. We find that the strength of this contribution can be controlled by varying the vibrational temperature. Received: 4 January 1999 / Received in final form: 12 May 1999     

Nonreciprocal transmission of multi-band optical signals in thermal atomic systems

Multi-band signal propagation and processing play an important role in quantum communications and quantum computing.In recent years,optical nonreciprocal devices such as an optical isolator and circulator are proposed via various configurations of atoms,metamaterials,nonlinear waveguides,etc.In this work,we investigate all-optical controlled nonreciprocity of multi-band optical signals in thermal atomic systems.Via introducing multiple strong coupling fields,nonreciprocal propagation of the prob...     

Magneto-electric response functions for simple atomic systems

We consider a simple atomic two-body bound state system that is overall charge neutral and placed in a static electric and magnetic field, and calculate the magneto-electric response function as a function of frequency. This is done from first principles using a two-particle Hamiltonian for both an harmonic oscillator and Coulomb binding potential. In the high frequency limit, the response function falls off as 1/ω ² whilst at low frequencies it tends to a constant value.     

Three-step model for high-harmonic generation in many-electron systems

The three-step model (TSM) of high-harmonic generation (HHG) is generalized to atomic and molecular many-electron systems. Using many-body perturbation theory, corrections to the standard TSM due to exchange and electron-electron correlations are derived. It is shown that canonical Hartree-Fock orbitals represent the most appropriate set of one-electron states for calculating the HHG spectrum. To zeroth order in many-body perturbation theory, a HHG experiment allows direct access, in general, to a combination of occupied Hartree-Fock orbitals rather than to the highest occupied molecular orbital by itself.