Collective motion of polar active particles on a sphere

Collective motion of active particles with polar alignment is investigated on a sphere. We discussed the factors that affect particle swarm motion and define an order parameter that can show the degree of particle swarm motion. In the model, we added a polar alignment strength, along with Gaussian curvature, affecting particles swarm motion. We find that when the force exceeds a certain limit, the order parameter will decrease with the increase of the force. Combined with our definition of order parameter and observation of the model, the reason is that particles begin to move side by side under the influence of polar forces. In addition, the effects of velocity, rotational diffusion coefficient, and packing fraction on particle swarm motion are discussed. It is found that the rotational diffusion coefficient and the packing fraction have a great influence on the clustering motion of particles, while the velocity has little influence on the clustering motion of particles.     

Far-infrared cyclotron resonance study of neutral impurity scattering in GaAs

Far-infrared laser cyclotron resonance with the help of photoexcitation enables us to separate the electron-neutral donor and electron-neutral acceptor scattering rates in GaAs. Allowing for the complication due to the coexistence of donors and acceptors, inaccuracy due to unknown contribution of phonon scatterings etc., one finds that the electron-neutral donor scattering cross-section is 2.9×10^–11 cm² at 4.2 K, that is 23 times as large as the electron-acceptor scattering cross-section.     

Far-infrared absorption in gas-evaporated Al particles: Effects of a fractal structure

We analyze far-infrared absorption in gas evaporated Al particles. Electron microscopy showed that the samples have a fractal structure for length scales below a correlation length. By comparing the experimental complex dielectric function with Bergman-Milton bounds for Al₂O₃-coated particles we extracted a structural parameter which could be reconciled with computations based on the fractal structure. Thus we can explain the allegedly “anomalous” far-infrared absorption semi-quantitatively in terms of particle aggregation.     

Far-infrared absorption in small metal particles: Non-local theory

The non-local theory of electromagnetic response has been applied to study the far-infrared absorption in small particles. The absorption coefficient appears to be described by the same expression as that given by the local theory, except that the effective electronic scattering time in the magnetic-dipole absorption differs strongly from that in the electric-dipole absorption. In the latter case the effective electronic free-path is highly sensitive to the scattering conditions on the particle boundary and changes from about the particle diameter for diffuse scattering to the electronic mean free-path in a bulk metal sample for specular scattering. This can explain observed large far-infrared absorption. Quantum effects are also discussed briefly.     

Transmission properties in a one-dimensional finite extrinsic semiconductor InSb photonic crystal

The properties of wave transmission in a strongly dispersive semiconductor-dielectric photonic crystal (SDPC) are theoretically investigated. We consider a one-dimensional finite SDPC, air/(SD) ^N S/air, where, N is the stack number, S is an extrinsic semiconductor, n-type InSb (n-InSb), and D is a dielectric layer of SiO₂. Transmission peaks can be found in the frequency region where the real part of the complex permittivity of n-InSb is negative. The number of peaks is equal to the stack number N. The positions of peaks can be tuned by the thicknesses of S and D. The peaks are seen to be blue-shifted as the doping density increases, leading to a tunable filter. The locations of peaks are also strongly dependent on the incidence angle, but weakly dependent on the polarization of the incident wave. The results suggest that a tunable multichannel filter can be achieved by using such an SDPC. Filters with tunable and multichannel features are of technical use in photonic applications such as in the optical electronics and communications.     

Binding energy of a polaron in asymmetric polar semiconductor heterostructures

By using a modified Lee-Low-Pines variational method, we have investigated the ground-state binding energy of a polaron confined in asymmetric single and step quantum wells (QWs) due to interface phonons, confined bulk-like LO phonons, and half-space LO phonons. The relative importance of the different phonon modes is analysed in detail. Our results show that the asymmetry and the well width of the QWs have a significant influence on the polaron energy. The polaron binding energy has an intimate relation to the potential parameters of QWs. The subband nonparabolicity has a little influence to the polaron binding energy. Comparing with the results calculated with perturbation theory, a good agreement is found.     

The magneto-optical properties of an ensemble of ellipsoidal dielectric particles in a magnetic fluid

The paper deals with an analytical investigation of magnetic birefringence in a dilute suspension of ellipsoidal dielectric particles in a magnetic fluid. The major axes of particles tend to line up along the field because of the difference in the “demagnetizing” coefficients along and across the ellipsoid axis, which brings about a multiple increase in the optical anisotropy of the system. Formulas are derived which describe the field dependence of magnetization of the system and of the difference between the refractive indices for ordinary and extraordinary rays. It is demonstrated that, given the bulk concentration of impurity particles of several percent, the magnetization of the system varies insignificantly, while the birefringence signal may increase by two orders of magnitude.     

Cyclotron resonance in a semiconductor in a steady electric field

The classical kinetic equation is solved to find the carrier distribution in the valence band for germanium in cyclotron resonance, the carriers being generated by the electric field. Negative power absorption is demonstrated.I am indebted to V. G. Veselago for proposing the topic and to Yu. M. Kagan for a discussion.     

Energy of a bound polaron in a magnetic field in asymmetric polar semiconductor heterostructures

In this paper, a new modified Hamiltonian of a polaron bound to a donor impurity in asymmetric step quantum wells (QWs) in the presence of an arbitrary magnetic field is given, in which the coupling of an electron with confined bulk-like LO phonons, half-space LO phonons and interface phonon modes is included. Especially, the interaction of the impurity with all possible optical-phonon modes is also considered. The ionization energy of a bound polaron in a magnetic field for asymmetric step QWs are studied by using a modified Lee-Low-Pines (LLP) variational method. The effects of the finite electronic confinement potential and the subband nonparabolicity are also considered. The relative importance of the donor impurity located at the well and the step is analyzed. Our results show the interaction between the impurity and the phonon field in screening the Coulomb interaction has a significant influence on the binding energy of bound polaron. The influence of subband non-parabolicity is appreciable on the bound polaron effects for the narrow well. The binding energy of bound polaron given in this paper are excellent agreement with the experimental measurement.     

Relaxation of femtosecond photoexcited electrons in a polar indirect band-gap semiconductor nanoparticle

A model calculation is given for the energy relaxation of a non-equilibrium distribution of hot electrons (holes) prepared in the conduction (valence) band of a polar indirect band-gap semiconductor, which has been subjected to homogeneous photoexcitation by a femtosecond laser pulse. The model assumes that the pulsed photoexcitation creates two distinct but spatially interpenetrating electron and hole non-equilibrium subsystems that initially relax non-radiatively through the electron (hole)-phonon processes towards the conduction (valence) band minimum (maximum), and finally radiatively through the phonon-assisted electron-hole recombination across the band-gap, which is a relatively slow process. This leads to an accumulation of electrons (holes) at the conduction (valence) band minimum (maximum). The resulting peaking of the carrier density and the entire evolution of the hot electron (hole) distribution has been calculated. The latter may be time resolved by a pump-probe study. The model is particularly applicable to a divided (nanometric) polar indirect band-gap semiconductor with a low carrier concentration and strong electron-phonon coupling, where the usual two-temperature model [1-4] may not be appropriate.