Nonresonant Regime Research Articles

Postshock turbulence and diffusive shock acceleration in young supernova remnants

<i>Context. <i/>Thin X-ray filaments are observed in the vicinity of young supernova remnants (SNR) blast waves. Identifying the process that creates these filaments would provide direct insight into the particle acceleration occurring within SNR and in particular the cosmic ray yield.<i>Aims. <i/>We investigate magnetic amplification in the upstream medium of a SNR blast wave through both resonant and non-resonant regimes of the streaming instability. We attempt to understand more clearly of the diffusive shock acceleration (DSA) efficiency by considering various relaxation processes of the magnetic fluctuations in the downstream medium. Multiwavelength radiative signatures originating in the SNR shock wave are used to test various downstream turbulence relaxation models.<i>Methods. <i/>Analytical and numerical calculations that couple stochastic differential equation schemes with 1D spherical magnetohydrodynamics simulations are used to investigate, in the context of test particles, turbulence evolution in both the forshock and post-shock regions. Stochastic second-order Fermi acceleration induced by resonant modes, magnetic field relaxation and amplification, and turbulence compression at the shock front are considered to model the multiwavelength filaments produced in SNRs. The <i>γ<i/>-ray emission is also considered in terms of inverse Compton mechanism.<i>Results. <i/>We confirm the result of Parizot and collaborators that the maximum CR energies should not go well beyond PeV energies in young SNRs where X-ray filaments are observed. To reproduce observational data, we derive an upper limit to the magnetic field amplitude and so ensure that stochastic particle reacceleration remains inefficient. Considering various magnetic relaxation processes, we then infer two necessary conditions to achieve efficient acceleration and X-ray filaments in SNRs: (1) the turbulence must fulfil the inequality 2 - <i>β<i/> - <i>≥<i/> 0; where <i>β<i/> is the turbulence spectral index and is the relaxation length energy power-law index; (2) the typical relaxation length must be of the order the X-ray rim size. We find that Alvénic/fast magnetosonic mode damping fulfils all conditions; while non-linear Kolmogorov damping does not. By confronting previous relaxation processes with observational data, we deducte that among our SNR sample, data for the older ones (SN1006 and G347.3-0.5) does not comply with all conditions, which means that their X-ray filaments are probably controlled by radiative losses. The younger SNRs, Cassiopeia A, Tycho, and Kepler pass all tests and we infer that the downstream magnetic field amplitude is in the range of 200–300 <i>μ<i/>Gauss.

Radio emission and nonlinear diffusive shock acceleration of cosmic rays in the supernova SN 1993J

The extensive observations of the supernova SN 1993J at radio wavelengths make this object a unique target for the study of particle acceleration in a supernova shock. To describe the radio synchrotron emission we use a model that couples a semianalytic description of nonlinear diffusive shock acceleration with self-similar solutions for the hydrodynamics of the supernova expansion. The synchrotron emission, which is assumed to be produced by relativistic electrons propagating in the postshock plasma, is worked out from radiative transfer calculations that include the process of synchrotron self-absorption. The model is applied to explain the morphology of the radio emission deduced from high-resolution VLBI imaging observations and the measured time evolution of the total flux density at six frequencies. Both the light curves and the morphology of the radio emission indicate that the magnetic field was strongly amplified in the blast wave region shortly after the explosion, possibly via the nonresonant regime of the cosmic-ray streaming instability operating in the shock precursor. The turbulent magnetic field was not damped behind the shock but carried along by the plasma flow in the downstream region. Cosmic-ray protons were efficiently produced by diffusive shock acceleration at the blast wave. We find that during the first ~8.5 years after the explosion, about 19% of the total energy processed by the forward shock was converted to cosmic-ray energy. However, the shock remained weakly modified by the cosmic-ray pressure. The high magnetic field amplification implies that protons were rapidly accelerated to energies well above 1 PeV. The results obtained for this supernova support the scenario that massive stars exploding into their former stellar wind are a major source of high-energy Galactic cosmic rays.

Non-Resonant Third-Order Nonlinearity of Nanometric and Subnanometric Silver Particles in Aqueous Solution

We have measured and analyzed the behavior of the nonlinear refractive index of silver spheres in water in a non-resonant femtosecond excitation regime. Two different diameter silver nanosphere (0.65 and 9 nm) suspensions were used in the experiments. Thermal and nonthermal contributions to the nonlinear properties of the samples were determined exploring a novel high sensitivity thermal managed eclipse Z-scan technique. The dependence of nonthermal third order nonlinear susceptibility of the colloid with the silver nanoparticles filling factor was described using the generalized Maxwell-Garnett model. The nanoparticle size dependence of the colloid nonlinear refractive index was observed.

Nonlinear interaction of a two-level atom with counterpropagating radiation beams of different frequencies

The theoretical and numerical results on the nonlinear dynamics of an atom in the fields of two counterpropagating radiation beams of different frequencies are presented. Both resonant and nonresonant interaction regimes are investigated. The atom center-of-mass energy dependence on the field amplitudes exhibits the nonlinear threshold effect of reflection of an atom in the interference field. This phenomenon leads to the atom acceleration or deceleration depending on its initial state. This acceleration or deceleration is base of a shock character because of the impact with the moving potential barrier; it occurs at ultrashort distances on the order of radiation field wavelengths. Furthermore, the role of initial conditions is discussed and analyzed numerically.

Comparison of the OKα x-ray emission bands in micro- and mesoporous silica materials and in α-quartz

X-ray emission spectroscopy (XES) at the O Kalpha threshold has been used to investigate the electronic structure of a microporous pure calcined zeolite with the crystal structure of the MFI-type framework (silicalite), a deboronated MFI zeolite (DB-MFI), a pure mesoporous cubic MCM-48 material, a MCM-48 loaded with copper and zinc oxide nanoparticles (CuZnO-MCM-48), and a crystalline layered silicic acid H-RUB-18. For comparison, the XES O Kalpha spectrum of pure alpha-quartz has also been recorded. In the nonresonant energy regime the XES O Kalpha spectra for all these compounds look very similar indicating that the electronic structure of the micro- and mesoporous silica materials is very similar to that of quartz. In the resonant regime, however, the spectra exhibit significant differences. In all the materials under study, the resonant XES O Kalpha spectra recorded at photon energies close to the positions of the O K edges show Raman-type inelastic peaks with an energy loss of 11 eV, originating from electronic excitations within these insulating materials. The prominent features in the XES O Kalpha spectra of alpha-quartz and H-RUB-18 are analyzed by means of quantum chemical ab initio cluster calculations.

Effects of Metal−Molecule Contact and Molecular Structure on Molecular Electronic Conduction in Nonresonant Tunneling Regime: Alkyl versus Conjugated Molecules

The effect of metal−molecule contacts and molecular structures on the electronic conduction of alkyl and conjugated molecular junctions in nonresonant tunneling regime was investigated based on a proposed multibarrier tunneling (MBT) model, where the molecular junction was decomposed into three individual barriers through the molecular body and metal−molecule contacts on either side of the molecule. The resistance−area product (RA), specific contact resistance (RC), and decay coefficients (βC, βP, βBody, and βo) were estimated for alkyl and two selected conjugated molecules (oligoacene and oligophenylene) between Au contacts, and the result of RA (alkyl) > RA (oligoacene) ≥ RA (oligophenylene) for a given molecular length was obtained from different decay coefficients for different molecules. By assuming the tunneling around the highest occupied molecular orbital of alkyl molecules (like a hole type tunneling), RC for alkanemonothiol and alkanedithiol junctions with various metallic (Ag, Cu, and Au) conta...

Simulation of Inelastic Electron Tunnelling Spectroscopy on Different Contact Structures in 4,4′-Biphenyldithiol Molecular Junctions

A first-principles computational method is developed to study the inelastic electron tunnelling spectroscopy (IETS) of 4,4′-biphenyldithiol molecular junction with three different contact structures between the molecule and electrodes in the nonresonant regime. The obtained distinct IETS can be used to resolve the geometrical structure of the molecular junction. The computational results demonstrate that the IETS has certain selection rule for vibrational modes, where the longitudinal modes with the same direction as the tunnelling current have greatest contribution to the IETS. The thermal effect on the IETS is also displayed.

Effect of energy losses on the amplitude-frequency characteristic of the electrodynamic system of optical modulators

The results of calculations of main parameters of a slot-type electrodynamic system taking into account losses in a single-mode nonresonance regime of excitation and in the regime of responance excitation in the vicinity of the cutoff frequency are presented. Analytic expressions obtained for the transmission coefficient, resonance frequency, and transmission band can be used in designing wide-band optical modulators. Experimental verification of laboratory prototypes of electrodynamic systems and their application in available modulators confirms the possibility of employment of such analysis for choosing optimal designs of ultrahigh-frequency modulators of optical radiation.

Turbulence and particle acceleration in collisionless supernovae remnant shocks

This paper investigates the nature of the MHD turbulence excited by the streaming of accelerated cosmic rays in a shock wave precursor. The two recognised regimes (non-resonant and resonant) of the streaming instability are taken into account. We show that the non-resonant instability is very efficient and saturates through a balance between its growth and non-linear transfer. The cosmic-ray resonant instability then takes over and is quenched by advection through the shock. The level of turbulence is determined by the non-resonant regime if the shock velocity $V_{\rm sh}$ is larger than a few times $\xi_{\rm CR} c$, where $\xi_{\rm CR}$ is the ratio of the cosmic-ray pressure to the shock kinetic energy. The instability determines the dependence of the spectrum with respect to $k_\parallel$ (wavenumbers along the shock normal). The transverse cascade of Alfv\'en waves simultaneously determines the dependence in $k_{\perp}$. We also study the redistribution of turbulent energy between forward and backward waves, which occurs through the interaction of two Alfv\'en and one slow magneto-sonic wave. Eventually the spectra at the longest wavelengths are found almost proportional to $k_{\parallel}^{-1}$. Downstream, anisotropy is further enhanced through the compression at shock crossing.

Cyclotron resonance maser operating in a nonresonant electron bunching regime

A new scheme of the cyclotron resonance maser (CRM) based on the phenomenon of electron bunching in a nonresonant wave field is proposed. In this scheme, the electron-wave interaction space is divided into two regions due to magnetic field profiling. In the first (input) region, the magnetic field is such that the working traveling wave is relatively far from the resonance with electrons. It is established that, under certain conditions, the motion of electrons in a nonresonant wave field is accompanied by their effective bunching without significant changes in the energy. In the second (output) region, the magnetic field is close to the resonance and the bunched electron beam effectively radiates at the working wave frequency. Thus, the regular electrodynamic system features an electron-wave interaction of the klystron type with spatially separated processes of the electron bunching and wave emission. The proposed scheme can be used for increasing the efficiency of CRMs with various configurations. It is especially advantageous in CRMs with frequency multiplication, where the electron bunching in a low-frequency nonresonant wave field is accompanied by wave emission at a multiple frequency.

Mesoscopic multistability with atomic coherence effects

The steady state behaviour describing the interaction of a mesoscopic system of coherently injected two-level Rydberg atoms with a coherently driven single-mode cavity is investigated in both resonant and non-resonant regimes. Multiple-switching effects between the output field states are predicted with the atomic coherent excitation (θ) and relative coherent phase parameter (Φr) of the atomic coherent state . The relation between the transmitted field (x) and the atomic cooperation parameter (C) shows multistable behaviour for some values of the polar and azimuthal angles (θ,). The transverse field feature in the form of a Gaussian beam profile tends to degrade the multistable structure in the (C–x) and the input–output field (x–Y) relations. Micromaser action is analysed for the system with atomic coherence within and without the plane wave approximation.

Characteristics of dynamic alignment for diatomic and linear triatomic molecules in intense femtosecond laser fields

The process of dynamic alignment for diatomic and linear triatomic molecules is systematically investigated by solving numerically the rotation equation for angle θ between molecular axis and laser polarization direction in the high-frequency non-resonant regime. A two-step Coulomb explosion model of molecules in intense laser fields is used to determine the instant when molecular dynamic alignment is over. A counting approach and a fourth-order Runge–Kutta algorithm are used to calculate the angular distribution of molecules at a particular instant through solving numerically for a series of initial angle. Our computational results show that the majority of dynamic alignment for the light molecules N 2, H 2 and CO 2 takes place before the molecule ionizes and begins to dissociate. However, for the heavy molecules Br 2, I 2 and CS 2, the dynamic alignment occurs mainly during the process of molecular dissociation. The extent of molecular alignment tightly correlates not only with dynamic process of molecular Coulomb explosion, but also with molecular parameters and laser parameters. The effects of these factors to molecular dynamic alignment are extensively calculated and discussed.

Modeling hysteresis in ferroelectric materials with a dry friction concept

Piezoelectric hysteresis understanding is a key toward better displacement control of devices requiring high strain like micropositionners and adaptive optic systems. Large strain, in the non-resonant regime, requires high electric fields, which lead to an enhancement of the nonlinear/hysteresis effect and results in a lack of controllability.The modeling of the ferroic hysteresis proposed here is based on a analogy with the dry friction concept used in mechanics. It is anticipated that the pinning/depinning domain wall motion is similar to a mass motion on a support presenting a dry friction dissipative force. From the model, a relationship showing the equivalence of the electric field and the stress by poling product is derived. It is shown that from the electric field influence on the polarization, the stress behavior is derived.Starting from the model, which is intrinsically hysteretic, the purely linear behaviors are eventually modeled, and the correlation between linear coefficients and the polarization states is presented. Rayleigh region behaviors are easily retrieved from the proposed model.

Inelastic Current−Voltage Characteristics of Atomic and Molecular Junctions

We report first-principles calculations of the inelastic current-voltage (I-V) characteristics of a gold point contact and a molecular junction in the nonresonant regime. Discontinuities in the I-V curves appear in correspondence to the normal modes of the structures. Due to the quasi-one-dimensional nature of these systems, specific modes with large longitudinal component dominate the inelastic I-V curves. In the case of the gold point contact, our results are in good agreement with recent experimental data. For the molecular junction, we find that the inelastic I-V curves are quite sensitive to the structure of the contact between the molecule and the electrodes thus providing a powerful tool to extract the bonding geometry in molecular wires.

P–f mixing in CeSb, studied by resonant X-ray scattering

X-ray magnetic scattering experiments have been performed in CeSb at the Ce and Sb L edges. In the non-resonant regime, we observe charge satellites reflecting the lattice modulation associated with the periodicity of paramagnetic Γ 8 planes. At the Ce L 2 edge we observe strong magnetic resonances, due to the antiferromagnetic stacking of the ferromagnetic Γ 7. The study at the Sb L 1 edge shows a magnetic dipole resonance, which supports the model of strong p–f mixing, used to explain the origin of the long-range magnetic order of CeSb.

Confronting spin flavor solutions of the solar neutrino problem with current and future solar neutrino data

A global analysis of spin flavor precession (SFP) solutions to the solar neutrino problem is given, taking into account the impact of the full set of latest solar neutrino data, including the recent SNO data and the 1496-day Super-Kamiokande data. These are characterized by three effective parameters: $\ensuremath{\Delta}{m}_{\mathrm{SOL}}^{2}\ensuremath{\equiv}\ensuremath{\Delta}{m}^{2},$ the neutrino mixing angle ${\ensuremath{\theta}}_{\mathrm{SOL}}\ensuremath{\equiv}\ensuremath{\theta},$ and the magnetic field parameter $\ensuremath{\mu}{B}_{\ensuremath{\perp}}.$ For the last we adopt a self-consistent magnetohydrodynamics field profile in the convective zone and identify an optimum ${B}_{\ensuremath{\perp}}\ensuremath{\sim}80\mathrm{kG}$ strength for $\ensuremath{\mu}{=10}^{\ensuremath{-}11}{\ensuremath{\mu}}_{B}.$ We find that no low mass (LOW) quasivacuum or vacuum solutions are present at $3\ensuremath{\sigma}.$ In addition to the standard large mixing angle (LMA) oscillation solution, there are two SFP solutions, in the resonant (RSFP) and nonresonant (NRSFP) regimes. These two SFP solutions have a goodness of fit of 84% (RSFP) and 83% (NRSFP), slightly better than the LMA oscillation solution (78%). We discuss the role of solar antineutrino searches in the fit and present a table of best-fit parameters and ${\ensuremath{\chi}}_{\mathrm{min}}^{2}$ values. Should the KamLAND experiment confirm the LMA solution, the SFP solutions may at best be present at a subleading level, leading to a constraint on $\ensuremath{\mu}{B}_{\ensuremath{\perp}}.$ In the event the LMA is not the solution realized in nature, then experiments such as Borexino can help in distinguishing the LMA solution from the NRSFP solution and the simplest RSFP solution with no mixing.

Nonresonant Kerr effect in microporous silicon: Nonbulk dispersive behavior of below band gap χ(3)(ω)

Although in recent years resonant optical nonlinearities in quantum confined silicon generated significant interest, no experimental work has been dedicated to the nonresonant regime, which is the range of interest for optical switching applications. In this article we report a systematic investigation on the different types of optical nonlinearities which can be activated in quantum-sized silicon. In particular, original measurements of nonresonant nonlinear refraction (Kerr effect) are reported at different wavelengths, spanning the infrared middle-gap range. The dispersive scaling rule and values of the nonlinear refractive index are clearly incompatible with those of three-dimensional semiconductors. Hence the quantum confined density of states plays a key role in determining the frequency dispersion of the nonresonant third-order susceptivity χ(3)(ω). Also, this suggests the need of further investigation of the influence of quantum-size effects (and related density of states modifications) on below-gap χ(3).

Investigation on anisotropy of vertical-cavity surface-emitting lasers

We have studied the spontaneous emission of polarized excitons in the GaInP/AlGaInP vertical-cavity surface-emitting lasers from 50 K to room temperature. It is observed that the spontaneous emission peak enters and leaves the resonant regime. At the resonant regime, the emission intensities of the perpendicularly and horizontally polarized excitons are enhanced and their proportions are different from that in nonresonant regime. These experimental results are explained by the dressed exciton theory of the semiconductor microcavity device. Based on this theory, the intensity enhancement and the polarization dependence are understood as cooperative emission and the microcavity anisotropy.

Magnetotransport investigations of structural irregularities in a weakly coupled GaAs/AlAs superlattice under domain formation

Irregular current branches have been observed experimentally in the I-V characteristics, when static electric-field domains are formed in a weakly coupled GaAs/AlAs superlattice. The origin of these irregularities has been investigated by applying a magnetic field B perpendicular to the layers. With increasing B, the maximum value and the width of different current branches become more and more regular. At $B=9\mathrm{T},$ most current branches are extremely regular indicating that the irregularities are induced by scattering processes, which determine the current in the nonresonant tunneling regime. However, some branches are not affected by the magnetic field demonstrating that they are completely induced by the deviation from an ideal periodicity or by the inhomogeneities at the contact.

Observation of bottleneck effects on the photoluminescence from polaritons in II–VI microcavities

The photoluminescence dynamics of microcavity polaritons are investigated by ps time-resolved measurements as a function of the photon–exciton detuning in the low, non-resonant excitation regime. For negative detuning, we observe the appearance of a relaxation slow-down accompanied by a build-up of a non-equilibrium polariton population on the exciton-like part of the lower branch. Finally, the role in the PL dynamics of the dark excitonic states existing in microcavities containing several quantum wells as alternative relaxation channels is addressed.