A new approach to the theory of Cherenkov radiation based on relativistic generalization of the Landau criterion

Relativistic generalization of the Landau criterion is obtained which, in contrast to the classical Tamm-Frank and Ginzburg theories, determines the primary energy mechanism of emission of nonbremsstrahlung Cherenkov radiation. It is shown that Cherenkov radiation may correspond to a threshold energetically favorable conversion of the condensate (ultimately long-wavelength) elementary Bose perturbations of a medium into transverse Cherenkov photons emitted by the medium proper during its interaction with a sufficiently fast charged particle. The threshold conditions of emission are determined for a medium with an arbitrary refractive index n, including the case of isotropic plasma with n<1 for which the classical theory of Cherenkov radiation prohibits such direct and effective nonbremsstrahlung emission of these particular transverse high-frequency electromagnetic waves. It is established that these conditions of emission agree with the data of well-known experiments on the threshold for observation of Cherenkov radiation, whereas the classical theory only corresponds to the conditions of observation of the interference maximum of this radiation. The possibility of direct effective emission of nonbremsstrahlung Cherenkov radiation, not taken into account in the classical theory, is considered for many observed astrophysical phenomena (type III solar radio bursts, particle acceleration by radiation, etc.).     

Characterization of the initial filamentation of a relativistic electron beam passing through a plasma

The linear instability that induces a relativistic electron beam passing through a plasma with return current to filament transversely is often related to some filamentation mode with the wave vector normal to the beam or confused with Weibel modes. We show that these modes may not be relevant in this matter and identify the most unstable mode on the two-stream or filamentation branch as the main trigger for filamentation. This sets both the characteristic transverse and longitudinal filamentation scales in the nonresistive initial stage.     

Indirect nuclear spin coupling between Cu++ pair in copper acetate monohydrate

The methods of molecular dynamics has been used to investigate the behaviour of transverse and longitudinal current-current autocorrelation functions in a model liquid simulating molten strontium chloride. Examples of the spectra of these functions are given and it is shown that the liquid propagates shear waves in a manner not unlike transverse phonons in crystals. The spectra of the mass and charge density, related to the spectra of the longitudinal currents, were computed. In both cases a non-central peak indicative of propagating acoustic and optic-type modes was observed for small wavenumbers, the optic-type mode persisting for larger wavenumbers than the acoustic-type modes. The possibility of observing these modes in inelastic neutron scattering experiments is discussed briefly.     

Dielectric properties of multiband electron systems II. Collective modes

Starting from the tight-binding dielectric matrix in the random phase approximation we examine the collective modes and electron-hole excitations in a two-band electronic system. For long wavelengths (q → 0), for which most of the analysis is carried out, the properties of the collective modes are closely related to the symmetry of the atomic orbitals involved in the tight-binding states. In insulators there are only inter-band charge oscillations. If atomic dipolar transitions are allowed, the corresponding collectivemodes reduce in the asymptotic limit of vanishing bandwidths to Frenkel excitons for an atomic insulator with weak on-site interactions. The finite bandwidths renormalize the dispersion of these modes and introduce a continuum of incoherent inter-band electron-hole excitations. The possible Landau damping of collective modes due to the presence of this continuum is discussed in detail. In conductors the intra-band charge fluctuations give rise to plasmons. If the atomic dipolar transitions are forbidden, the coupling of inter-band collective modes and plasmons tends to zero as q → 0. On the contrary, in dipolar conductors this coupling is strong and nonperturbative, due to the long range monopole-dipole interactions between intra-band and inter-band charge fluctuations. The resulting collective modes are hybrids of intra-band plasmons and inter-band dipolar oscillations. It is shown that the frequency of the lower hybridized longitudinal mode is proportional to the frequency of the transverse dipolar mode when the latter is small. The dielectric instability in a multi-band conductor is therefore characterized by the simultaneous softening of a transverse and a longitudinal mode, which is an important, directly measurable consequence of the present theory.     

Astrophysical applications of the gravitational searchlight

The astrophysical consequences of the blue-shifted radiation emitted in the forward direction by a source moving in an equatorial orbit with radius slightly in excess of 1.5 times the Schwarzschild radius of a highly collapsed central object are examined with special reference to quasistellar objects.     

Second quantization in a waveguide with variable cross section

The motion of a system consisting of noninteracting bosons in a waveguide with variable cross section is studied. These particles have transverse as well as longitudinal degrees of freedom, but only a finite number of transverse modes can propagate in the waveguide. While for a waveguide with constant cross section, the numbers of particles in a given state of longitudinal and transverse modes remain constant, in the case of a waveguide with variable cross section there is conversion between these modes, although the total number of particles is conserved. By considering the equations of motion for the annihilation (or creation) operator, it is shown that the boundaries act as an external force, and thus generate localized transverse modes in the waveguide.     

Charged condensation

We consider Bose–Einstein condensation of massive electrically charged scalars in a uniform background of charged fermions. We focus on the case when the scalar condensate screens the background charge, while the net charge of the system resides on its boundary surface. A distinctive signature of this substance is that the photon acquires a Lorentz-violating mass in the bulk of the condensate. Due to this mass, the transverse and longitudinal gauge modes propagate with different group velocities. We give qualitative arguments that at high enough densities and low temperatures a charged system of electrons and helium-4 nuclei, if held together by laboratory devices or by force of gravity, can form such a substance. We briefly discuss possible manifestations of the charged condensate in compact astrophysical objects.     

Solitary wave emission from a non-linear waveguide with a PTS cladding

We present numerical simulations of solitary wave emission from a waveguide in which the cladding is the non-linear material polydiacetylene para-toluene sulfphonate (PTS). In addition to a self-focusing cubic non-linearity PTS exhibits a defocusing quintic non- linearity, which imposes a limit on the peak non-linear index change. The influence of this limiting on solitary wave emission is studied in detail in one transverse dimension, and we show that it can increase the fraction of the incident power transferred into the emitted solitary wave. Fabrication issues arising from the limiting are also discussed. Numerical simulations in two transverse dimensions are also presented showing stable emitted solitary waves due to the stabilizing effect of the limiting, in contrast to the self-focusing collapse which occurs for a cubic non-linearity.     

Dispersion and the electron–phonon interaction in a single heterostructure

We investigate the electron–phonon interaction in a polar–polar single heterostructure through the use of the linear combination of hybrid phonon modes, considering the role of longitudinal optical, transverse optical and interface modes, using a continuum model that accounts for both mechanical and electrical continuity over a heterostructure interface. We discuss the use of other models for such systems, such as the bulk phonon (3DP) and dielectric continuum (DC) models, using previously developed sum-rules to explain the limitations on their validity. We find that our linear combination (LC) model gives an excellent agreement with scattering rates previously derived using the 3DP and DC models when the lattice dispersion is weak enough to be ignored, however, when there is a noticeable lattice dispersion, the LC model returns a different answer, suggesting that interface modes play a much greater part in the scattering characteristics of the system under certain conditions. We also discuss the remote phonon effect in polar/polar heterostructures.     

Transverse patterns and length-scale selection in vertical-cavity surface-emitting lasers with a large square aperture

The length-scale selection in complex emission patterns spontaneously arising in broad-area vertical-cavity surface-emitting lasers is studied experimentally. The wavenumbers of individual tilted wave modes show a square-root-like dependence on the detuning between emission frequency and longitudinal cavity resonance, in accordance with theoretical expectations. The absolute values of the wavenumbers are in good qualitative agreement with calculations taking into account the reflection properties of the Bragg reflectors and dispersion. PACS 42.60.Jf; 42.65.Sf; 42.55.Sa; 42.55.Px     

Is it possible to explore Peccei-Quinn axions from frequency-dependence radiation dimming?

We explore how the Peccei-Quinn (PQ) axion parameter space can be constrained by the frequency-dependence dimming of radiation from astrophysical objects. To do so we perform accurate calculations of photon-axion conversion in the presence of a variable magnetic field. We propose several tests where the PQ axion parameter space can be explored with current and future astronomical surveys: the observed spectra of isolated neutron stars, occultations of background objects by white dwarfs and neutron stars, the light-curves of eclipsing binaries containing a white dwarf. We find that the lack of dimming of the light-curve of a detached eclipsing white dwarf binary recently observed, leads to relevant constraints on the photon-axion conversion. Current surveys designed for Earth-like planet searches are well matched to strengthen and improve the constraints on the PQ axion using astrophysical objects radiation dimming.     

Generalization of Biot’s equations with allowance for shear relaxation of a fluid

On the basis of the generalized variational principle for dissipative continuum mechanics, a system of generalized Biot’s equations is derived to describe the wave propagation in a two-phase porous permeable medium in the presence of shear relaxation in the pore-filling fluid. It was shown that the inclusion of shear viscoelasticity of the fluid leads to the appearance of two transverse modes in addition to two longitudinal modes described by the Biot theory. One of the transverse modes is an acoustic mode, whereas the other is a diffusion mode characterized by the linear frequency dependence of phase velocity and attenuation coefficient in the low-frequency region.     

Investigation of the Radiation Mechanism of Microwave Excited Cluster Lamps

Experimental and theoretical investigations towards a better understanding of the radiation mechanisms in WO₂Br₂-discharges, so-called “cluster lamps” are presented. The main purpose of the investigation is to find out whether a major part of the continuum emitted from these discharges, which in the past has been interpreted as thermal cluster radiation, could also originate from molecular band radiation, in particular from triatomic molecules in colder discharge regions. Comparing emission spectra of WO₂Br₂-discharges from different plasma regions, using lamps operated at different vapor pressures and power levels (i.e. non-LTE and LTE-discharges), we conclude that by far the major part of the strong continuum radiation emitted from WO₂Br₂ lamps is indeed thermal radiation of hot tungsten cluster particles. WO— (below 550 nm) and WO₂-molecular radiation (above 650 nm), which is superimposed on the cluster continuum, also contribute to the total radiation output, but to a lesser degree. A crude model for WO₂ molecular emission is presented. Unfortunately, the WO₂ emission component greatly impedes proper fitting of the measured cluster continuum to a thermal cluster model.     

The effect of longitudinal-transverse coupling and medium temperature on Cerenkov radiation

The effect of medium properties on the energy loss suffered by a relativistic charged particle in a dielectric medium generating Cerenkov radiation is discussed. Here we have taken into account the effect of the coupling of the longitudinal (σ) mode with the transverse (λ) mode in the electromagnetic interaction. Calculation shows that the inclusion of λ-σ coupling in the transverse interaction affects significantly the radiation output. However the modification of the radiation due to the thermal state of the system becomes important at very high temperatures such as one finds in astrophysical situations. This temperature effect is negligible at ordinary temperatures and thus is undetectable in laboratory experiments.     

激光等离子体相互作用中Weibel不稳定性

利用粒子模拟法对超强激光与等离子体相互作用中产生的Weibel不稳定性及其产生机制进行了详细的研究。给出不稳定性的线性色散关系和饱和磁场与各向异性参数之间的函数关系,发现Weibel不稳定性的存在使超强激光在等离子体中激发的自生磁场饱和,饱和自生磁场的存在使粒子速度分布在激光传播方向上表现出各向异性。讨论了Weibel不稳定性的线性和非线性饱和过程,对更好地理解快点火物理中自生磁场的产生、快电子输运等过程有重要意义。     

激光等离子体相互作用中Weibel不稳定性

利用粒子模拟法对超强激光与等离子体相互作用中产生的Weibel不稳定性及其产生机制进行了详细的研究。给出不稳定性的线性色散关系和饱和磁场与各向异性参数之间的函数关系,发现Weibel不稳定性的存在使超强激光在等离子体中激发的自生磁场饱和,饱和自生磁场的存在使粒子速度分布在激光传播方向上表现出各向异性。讨论了Weibel不稳定性的线性和非线性饱和过程,对更好地理解快点火物理中自生磁场的产生、快电子输运等过程有重要意义。     

Theoretical and numerical analysis of coherent Smith-Purcell radiation

Coherent enhancement of Smith-Purcell radiation has attracted people's attention not only in adopting a better source but also in beam diagnostics aspect. In this paper, we study the intrinsic mechanism of coherent Smith-Purcell radiation on the basis of the van den Berg model. The emitted power of Smith-Purcell radiation is determined by the bunch profile in transverse and longitudinal directions. For short bunch whose longitudinal pulse length is comparable with the radiation wavelength, it can be concluded approximately that the power is proportional to the square number of electrons per bunch.     

Synchrotron radiation spectrum for galactic-sized plasma filaments

The radiation spectrum for synchrotron-emitting electrons in galactic-sized Birkeland current filaments is analyzed. It is shown that the number of filaments required to thermalize the emission spectrum to blackbody is not reduced when a non-Maxwellian electron distribution is assumed. If the cosmic background radiation (CBR) spectrum (T=2.76 K) is due to absorption and re-emission of radiation from galactic-sized current filaments, higher-order synchrotron modes are not as highly self absorbed as lower-order modes, resulting in a distortion of the blackbody curve at higher frequencies. This is especially true for a non-Maxwellian distribution of electrons for which the emission coefficient at high frequencies is shown to be significantly less than that for a Maxwellian distribution. The deviation of the CBR spectrum in the high-frequency regime may thus be derivable from actual astrophysical parameters, such as filamentary magnetic fields and electron energies in the model     

Trapped ion quantum computation with transverse phonon modes

We propose a scheme to implement quantum gates on any pair of trapped ions immersed in a large linear crystal, using interaction mediated by the transverse phonon modes. Compared with the conventional approaches based on the longitudinal phonon modes, this scheme is much less sensitive to ion heating and thermal motion outside of the Lamb-Dicke limit thanks to the stronger confinement in the transverse direction. The cost for such a gain is only a moderate increase of the laser power to achieve the same gate speed. We also show how to realize arbitrary-speed quantum gates with transverse phonon modes based on simple shaping of the laser pulses.     

Earth-based gravitational wave detection from pulsars

The main features of continuous gravitational radiation bathing the Earth has been evaluated for a set of 558 pulsars. In particular, the maximum gravitational wave background and the maximum gravitational wave emission have been evaluated for each source and compared with the projected sensitivities of the planned Earth based very long baseline interferometric antennas for gravitational wave detection, like VIRGO and LIGO. This study shows that such detectors have a good chance of detecting gravitational waves emitted from this class of astrophysical sources.