Small Plasma Frequency Research Articles

Weak line discovered by Voyager 1 in the interstellar medium: Quasi-thermal noise produced by very few fast electrons

A weak continuous line has been recently discovered onboard Voyager 1 in the interstellar medium, whose origin raised two major questions. First, how can this line be produced by plasma quasi-thermal noise on the Voyager short antenna? Second, why does this line emerge at some distance from the heliopause? We provide a simple answer to these questions, which elucidates the origin of this line. First, a minute quantity of supra-thermal electrons, as generally present in plasmas – whence the qualifier ‘quasi-thermal’ – can produce a small plasma frequency peak on a short antenna, of amplitude independent of the concentration of these electrons; furthermore, the detection required long spectral averages, alleviating the smallness of the peak compared to the background. We therefore attribute the observed line to a minute proportion of fast electrons that contribute negligibly to the pressure. Second, we suggest that, up to some distance from the heliopause, the large compressive fluctuations ubiquitous in this region prevent the line to emerge from the statistical fluctuations of the receiver noise because it is blurred out by the averaging required for detection, especially in the presence of short-wavelength density fluctuations. These results open up novel perspectives for interstellar missions, by showing that a minute proportion of fast electrons may be sufficient to measure the density even with a relatively short antenna, because the quietness of the medium enables a large number of spectra to be averaged.

A solar tube: Efficiently converting sunlight into electricity and heat

A solar cell, capable of directly converting solar energy into electricity, bears a Shockley-Queisser limit of about 31% based on a single pn junction. In general, less than 50% of the solar irradiation spectrum (mostly in the ultraviolet and visible regions) can be utilized for the conversion. As such, the theoretical utilization rate is less than 15% in a single device, with most of the extra energy being lost as heat. It is thus appealing to enhance the energy utilization with devices that could collect the heat loss. In this study, we design and demonstrate a solar tube to realize photo-electric and photo-thermal conversions simultaneously. The key point is the use of titanium tube: (1) it has a small plasma frequency to enable wide absorption for thermal conversion; (2) it accommodates TiO2 nanotube arrays to solve the cracking problem under tensile stress. A sandwiched membrane of high transparency and conductivity is developed for hole transport and collection. Eventually, a total energy efficiency of about 25.2% is obtained. Such a solar tube is anticipated to highly boost the utilization rate of solar energy with judiciously designed structure.

Effects of optical polarization on hybridization of radiative and evanescent field modes

A back action from Dirac electrons in graphene on the hybridization of radiative and evanescent fields is found as an analogy to Newton's third law. Here, the back action appears as a localized polarization field which greatly modifies an incident surface-plasmon-polariton (SPP) field. This yields a high sensitivity to local dielectric environments and provides a scrutiny tool for molecules or proteins selectively bounded with carbons. A scattering matrix is shown with varied frequencies nearby the surface-plasmon (SP) resonance for the increase, decrease and even a full suppression of the polarization field, which enables accurate effective-medium theories to be constructed for Maxwell-equation finite-difference time-domain methods. Moreover, double peaks in the absorption spectra for hybrid SP and graphene-plasmon modes are significant only with a large conductor plasma frequency, but are overshadowed by a round SPP peak at a small plasma frequency as the graphene is placed close to conductor surface. These resonant absorptions facilitate the polariton-only excitations, leading to polariton condensation for a threshold-free laser.

Manifestation of multiband optical properties of MgB 2

The optical conductivity of MgB 2 has been determined on a dense polycrystalline sample in the spectral range 6 meV–4.6 eV using a combination of ellipsometric and normal incidence reflectivity measurements. σ 1( ω) features a narrow Drude peak with anomalously small plasma frequency (1.4 eV) and a very broad ‘dome’ structure, which comprises the bulk of the low-energy spectral weight. This fact can be reconciled with the results of band structure calculations by assuming that charge carriers from the 2D σ-bands and the 3D π-bands have principally different impurity scattering rates and negligible interband scattering. This also explains a surprisingly small correlation between the defect concentration and T c, expected for a two-gap superconductor. The large 3D carrier scattering rate suggests their proximity to the localization limit.

Evolution of the conducting state of polyaniline from localized to mesoscopic metallic to intrinsic metallic regimes

The results of microwave dielectric constant ( ϵ mw ) and dc conductivity ( σ dc ) of doped polyaniline (PAN) samples in different charge delocation regimes are reported. With camphor culfonic acid doped PAN prepared in m-cresol solvent (PAN-CSA ( m-cresol)) respresenting the most delocalized material, a negative ϵ mw is measured indicating an intrinsic metallic state. The Drude model at low frequency is used to estimate a relatively small plasma frequency ω p ⋍ 0.015 eV and an anomalously long scattering time τ ⋍ 1.2 × 10 −11 s. This τ implies the importance of phonon back scattering for the most delocalized electrons as expected for the open Fermi surface of a highly anisotropic metal. Depending on the crystallinity, there are two different charge localization regimes in PAN samples: uncoupled metallic islands and coupled “mesoscopic” metallic state. The three-dimensional nature of metallic state is demonstrated by the correlation of low temperature dielectric response with the parallel and perpendicular crystalline domain lengths obtained from x-ray data. The charge localization to delocalization transition dependence on the polymer processing will be discussed. We present the temperature dependent nonmetal-metal transition for some preparations of PAN-CSA ( m-cresol). The intrinsic conductivity of metallic PAN is estimated to be ∼ 10 7 S/cm.

Heavy-fermion behavior in the optical functions of CeCu5

Optical reflectivity of the compound CeCu5 was investigated over a wide spectral range (1 meV-6 eV) at 5 and 300 K. Anomalous features were detected at low temperature, corresponding to a small plasma frequency (scaled with the Kondo temperature), and to the opening of a small gap of a few meV.

Optical evidence for strong anisotropy in the normal and superconducting states in Bi2Sr2CaCu2O8+z.

The optical reflectivity spectra for the bc surface of a large ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathit{z}}$ crystal were investigated over a wide energy range (30--30 000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) at room temperature and 6 K. In the spectrum for E\ensuremath{\parallel}c, only the phonon peaks were observed without any electronic contribution down to 30 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, whereas for E\ensuremath{\parallel}b a Drude-like spectrum was observed. Even below ${\mathit{T}}_{\mathit{c}}$ the spectrum for E\ensuremath{\parallel}c does not show any remarkable change except for a small change in the phonon spectrum. This indicates an extremely small plasma frequency for E\ensuremath{\parallel}c, less than 30 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, and thus quite a large mass anisotropy ${\mathit{m}}_{\mathit{c}}^{\mathrm{*}}$/${\mathit{m}}_{\mathit{b}}^{\mathrm{*}}$g${10}^{4}$. Such a small plasma frequency supports the conduction model along the c axis, that the supercurrent flows by tunneling through insulating (semiconductor) layers such as the Bi-O layers.

Optical study of U 2PTC 2: Evidence of heavy fermion behavior

Optical reflectivity spectra of a monocrystal of U 2PtC 2 are presented. The measurements have been performed over the whole spectral range from 1 meV to 12 eV. Particular attention was paid to the infra red part where we measured both at room temperature and at the temperature of 6 K. At room temperature the spectrum shows metallic behavior with a plasma frequency of about 2 eV. Instead, at low temperature the material behaves in a similar way as the heavy fermion compounds such as UPt 3. A small minimum appears in the optical conductivity spectrum at 4 meV and evidence is found of the existence of a gap of about 10 meV. A small plasma frequency of 0.44 eV can be associated with the formation of heavy quasiparticles with an effective mass of some tens of electronic masses.

The influence of a background plasma on the diocotron instability of a relativistic hollow electron beam

The influence of a background plasma on the diocotron instability of a relativistic hollow electron beam is investigated within the framework of a cold fluid model. The background plasma is taken to have a uniform density except at the mean radius of the electron beam where the density is assumed to be discontinuous. Of the charged species comprising the plasma, only the electrons influence the dynamics of the beam-plasma system; the effect of the ions is negligible because of their small plasma frequency. A dispersion relation for the eigenfrequencies of the system is derived and employed to examine instabilities. It is found that the fundamental (l = 1) mode, which is unconditionally stable for a hollow electron beam in a vacuum, is now unstable for certain parameter regimes. The appearance or growth of instabilities with increases in the plasma density discontinuity is observed, while increases in the relativistic factor γb are seen to exert a stabilizing influence. It is also found that thin beams are generally more unstable than thick ones.