Unperturbed Electron Research Articles

Bifurcation analysis of electron-acoustic waves in electron beam-plasma with suprathermal electrons

It has investigated the properties of nonlinear electron-acoustic waves propagating in a four-component collisionless, unmagnetized plasma. The system consists of cool inertial background electrons of temperature T c , the cool inertial electron beam of temperature T b , hot inertialess suprathermal electrons modeled by a κ-distribution of temperature T h , and ions. The nonlinear modified Korteweg-de Vries-Burgers (mKdV-Burgers) equation that characterizes the electron acoustic waves in such a plasma model is derived. The wave solutions of the resultant mKdV-Burgers equation are investigated using planar dynamical system bifurcation theory. The unperturbed hot, cool, and beam electron densities and the superthermal electron parameter are revealed to have a considerable influence on the nonlinear electron-acoustic traveling waves and the associated electric fields. The system evolves also monotonic shock waves when the dissipation term dominates over the dispersion term. The current model might be useful for understanding the generation of nonlinear electron-acoustic waves in a variety of astrophysical plasma settings, including the Earth's magnetosphere.

Role of anharmonicity in the phonon density of states of high temperature superconductors

Owing to the quantum dynamical approach of double time temperature dependent phonon-Green’s functions, the expression for phonon density of states (PDOS) has been developed via an almost complete Hamiltonian which consists of the contributions due to the unperturbed phonons and electrons, anharmonic phonon fields, localized phonon fields and that of electron–phonon interactions. It has been observed that the anhrmonicity play an vital role in the PDOS through energy line width. Enhancement in the PDOS due to electron–phonon interactions has been mainly discussed in this work, which, in turns, will be capable to explain a large number of dynamical properties of crystals even that of superconductors. The temperature dependence of PDOS has been found as a unique feature of the theory, which certainly becomes the outcome of the anharmonic interactions. The presence of electron–phonon interaction parameters in each term is an additional and new feature of the theory.

Evidence of oblique electron acoustic solitary waves triggered by magnetic reconnection in Earth’s magnetosphere

Motivated by the recent Magnetospheric Multiscale (MMS) observations of oblique electron acoustic waves, we addressed the generation mechanism of the observed waves by utilizing the reductive perturbation technique. A nonlinear Zakharov-Kuznetsov (ZK) equation is derived for a collisionless, magnetised plasma composed of cool inertial background electrons, cool inertial electron beam, hot inertialess suprathermal electrons; represented by a κ-distribution, and stationary ions. Moreover, the instability growth rate is derived by using the small-k perturbation expansion method. Our findings revealed that the structure of the electrostatic wave profile is significantly influenced by the external magnetic field, the unperturbed hot, cool, and electron beam densities, the obliquity angle, and the rate of superthermality. Such parameters also have an effect on the instability growth rate. This study clarifies the characteristics of the oblique electron solitary waves that may be responsible for changing the electron and ion distribution functions, which alter the magnetic reconnection process. Moreover, the increase of the growth rate with the plasma parameters could be a source of anomalous resistivity that enhances the rate of magnetic reconnection.

Role of anharmonicity in the electronic heat capacity of superconductors

The role of anharmonicity in the electronic heat capacity (EHC) of high temperature superconductors (HTS) has been investigated via electron density of state (EDOS) using quantum dynamical approach. The EDOS may be obtained using correlation function with the help of Zubarev double time temperature dependent electron Green’s functions (GF) via a generalized Hamiltonian which consists of (i) unperturbed electron, (ii) unperturbed phonon, (iii) electron-phonon, (iv) anharmonic and (v) defect interactions. The model Hamiltonian includes the force constant changes and mass difference caused by the impurities along with the effects of cubic anharmonicity reveals some striking features of EHC of HTS.

Influence of kinetic effects on long wavelength gradient-drift instability in high-frequency range in Hall thruster

A typical for Hall thruster discharges strong inhomogeneity of plasma parameters is a source of various types of instabilities. Some types of such instabilities (long high-frequency waves, in particular) are assumed to contribute to anomalous electron conductivity, which is essential to understand for the development of a reliable modelling tool for Hall thrusters. In this paper, the results of an analytical study of the influence of kinetic effects on dispersion properties of high-frequency perturbations in Hall thruster plasmas are presented, including the effect of the gradient magnetic electron drift on the long wavelength high-frequency instability. It was shown that when the long waves with phase velocity on the order of the unperturbed electron drift motion occur, the electron guiding center deviates significantly from the electric field equipotential line. The observed phenomenon leads to a self-sustained cycle of an intensive energy exchange between the waves and the electrons and a long-wavelength high-frequency instability inciting each other.

On the topology of total and diamagnetic induced electronic currents in molecules.

An application of the continuous transformation of the origin of the current density (CTOCD) scheme to constrain the diamagnetic induced charge current density (Jd) to be divergenceless is introduced. This results in a family of Jd fields perpendicular and proportional to both the gradient of the electron density and the external magnetic field. Since, in the limit of a complete basis set calculation, the paramagnetic component Jp also becomes divergenceless, we call this scheme CTOCD-DC (CTOCD for Divergenceless Components). CTOCD-DC allows for a topological characterization of both Jd and Jp in terms of their stagnation graphs. All stagnation graphs of Jd from CTOCD-DC contain the zero points of the gradient of the unperturbed electron density (∇ρ). In this way, an intimate topological relation between ρ and the diamagnetic current contribution is revealed. Numerical experiments exemplified by the case of LiNHF in point group symmetry C1 suggest that the corresponding paramagnetic current contributions Jp can show tendencies to accumulate pseudo-stagnation lines in proximity of some kind of the zero points of ∇ρ. Common zero points of ∇ρ and the total currents are exactly zero points of the mechanical momentum density.

Modulational instability of acoustic waves in a dusty plasma with nonthermal electrons and trapped ions

Abstract In the present work, employing the nonlinear field equations of a hot dusty plasma in the presence of nonthermal electrons and trapped ions, we studied the amplitude modulation of nonlinear waves in such a plasma medium by use of the reductive perturbation method and obtained the modified nonlinear Schrodinger equation. The modulational instability (MI) was investigated and the effects of the proportion of the fast electrons (α), the trapping parameter (b) and the plasma parameters such as the dust-ion temperature ratio (σd), the partial unperturbed electron to dust density (δ), and the ion-electron temperature ratio (σi) on it was discussed. For the investigation of modulational instability problems three parameters P/Q, Kmax and Γmax play the central role. The variations of these parameters with the wave number k and the other physical parameters are discussed and the possibility of occurence of modulational instability is indicated.

Langmuir wave in a dusty plasma with nonextensive electrons

The effect of the dust charge perturbation on Langmuir waves in a dusty plasma with nonextensive electrons and Maxwellian ions is investigated within the fluid theory framework, where the sink term of electrons in the continuity equation is considered. The dispersion of Langmuir waves shows that the dust charge relaxation introduces an additional source of dissipation in Langmuir waves. The damping rate of Langmuir oscillation caused by the dust charge fluctuations is bigger than the one with Maxwellian electrons when the nonextensively distributed electrons have a suprathermal tail. On the contrary, the excess of low energy electrons for nonextensively distributed electrons can lower the damping rate of Langmuir oscillation. These differences are mainly due to the modification of the unperturbed electron attachment frequency by the nonextensively distributed electrons. Our results may be of wide relevance to space and interstellar plasmas.

Stretching and bending dynamics in triatomic ultralong-range Rydberg molecules

We investigate polyatomic ultralong-range Rydberg molecules consisting of three ground state atoms bound to a Rydberg atom via $s$- and $p$-wave interactions. By employing the finite basis set representation of the unperturbed Rydberg electron Green's function we reduce the computational effort to solve the electronic problem substantially. This method is subsequently applied to determine the potential energy surfaces of triatomic systems in electronic $s$- and $p$-Rydberg states. Their molecular geometry and resulting vibrational structure are analyzed within an adiabatic approach that separates the vibrational bending and stretching dynamics. This procedure yields information on the radial and angular arrangement of the nuclei and indicates in particular that kinetic couplings between bending and stretching modes induce a linear structure in triatomic $l=0$ ultralong-range Rydberg molecules.

Anharmonicity and Impurity Effects on Infrared Absorption in High Temperature Superconductors

Taking into account the effects of anharmonicities and point impurities the quantum dynamics of phonons for high temperature superconductors is developed using Green’s function formalism via an almost complete Hamiltonian (without BCS Hamiltonian) which comprises the effects of (i) unperturbed electrons, (ii) unperturbed phonons, (iii) electron-phonon interactions, (iv) anharmonicities and (v) isotopic impurities. This is utilized to obtain the expressions for infrared absorption coefficient which can be resolved into diagonal and non-diagonal parts. Non-diagonal contribution arises only in impure crystals and vanishes in the case of pure crystal. The investigations are also made to study the dependence of infrared absorption coefficient on various parameters in the superconducting and normal regimes followed by numerical estimates for cuprate superconductor . The temperature dependence of infrared absorption coefficients and automatic emergence of pairons appears as a special feature of the theory.

Kinetic electron and ion instability of the lunar wake simulated at physical mass ratio

The solar wind wake behind the moon is studied with 1D electrostatic particle-in-cell (PIC) simulations using a physical ion to electron mass ratio (unlike prior investigations); the simulations also apply more generally to supersonic flow of dense magnetized plasma past non-magnetic objects. A hybrid electrostatic Boltzmann electron treatment is first used to investigate the ion stability in the absence of kinetic electron effects, showing that the ions are two-stream unstable for downstream wake distances (in lunar radii) greater than about three times the solar wind Mach number. Simulations with PIC electrons are then used to show that kinetic electron effects can lead to disruption of the ion beams at least three times closer to the moon than in the hybrid simulations. This disruption occurs as the result of a novel wake phenomenon: the non-linear growth of electron holes spawned from a narrow dimple in the electron velocity distribution. Most of the holes arising from the dimple are small and quickly leave the wake, approximately following the unperturbed electron phase-space trajectories, but some holes originating near the center of the wake remain and grow large enough to trigger disruption of the ion beams. Non-linear kinetic-electron effects are therefore essential to a comprehensive understanding of the 1D electrostatic stability of such wakes, and possible observational signatures in ARTEMIS data from the lunar wake are discussed.

Modulational instability of electron-acoustic waves in a plasma with Cairns–Tsallis distributed electrons

The problem of the modulational instability (MI) of electron-acoustic waves (EAWs) in a plasma with Cairns–Tsallis distributed electrons is addressed. Using the standard multiple scale method, we derive a nonlinear Schrödinger-like equation. Electron nonextensivity and nonthermality are found to significantly influence the region stability of the EAWs. In particular, it is found that the critical value kc, beyond which the instability sets in, is slightly lowered as the electrons evolve far away from their Maxwellian thermodynamic equilibrium. Electron nonthermality renders more effective and more important the influence and role of nonextensivity. Moreover, the effect of the unperturbed hot electron and cold electron number density imbalance μ on the onset of the MI is analyzed. Although both dark and bright excitations are obtained, the trend is in contrast to our earlier observations. Our results should be of relevance in wave propagation stability. Nonthermal nonextensive models may play an increasingly important role in predicting complex plasma behavior, and understanding the underlying physical processes.

Effective recombination coefficient and solar zenith angle effects on low-latitude D-region ionosphere evaluated from VLF signal amplitude and its time delay during X-ray solar flares

Excess solar X-ray radiation during solar flares causes an enhancement of ionization in the ionospheric D-region and hence affects sub-ionospherically propagating VLF signal amplitude and phase. In first part of the work, using the well known LWPC technique, we simulated the flare induced excess lower ionospheric electron density by amplitude perturbation method. Unperturbed D-region electron density is also obtained from simulation and compared with IRI-model results. Using these simulation results and time delay as key parameters, we calculate the effective electron recombination coefficient ($\alpha_{eff}$) at solar flare peak region. Our results match with the same obtained by other established models. In the second part, we dealt with the solar zenith angle effect on D-region during flares. We relate this VLF data with the solar X-ray data. We find that the peak of the VLF amplitude occurs later than the time of the X-ray peak for each flare. We investigate this so-called time delay ($\bigtriangleup t$). For the C-class flares we find that there is a direct correspondence between $\bigtriangleup t$ of a solar flare and the average solar zenith angle $Z$ over the signal propagation path at flare occurrence time. Now for deeper analysis, we compute the $\bigtriangleup t$ for different local diurnal time slots $DT$. We find that while the time delay is anti-correlated with the flare peak energy flux $\phi_{max}$ independent of these time slots, the goodness of fit, as measured by $reduced$-$\chi^2$, actually worsens as the day progresses. The variation of the $Z$ dependence of $reduced$-$\chi^2$ seems to follow the variation of standard deviation of $Z$ along the $T_x$-$R_x$ propagation path. In other words, for the flares having almost constant $Z$ over the path a tighter anti-correlation between $\bigtriangleup t$ and $\phi_{max}$ was observed.

Electrons and Phonons in High Temperature Superconductors

The defect-induced anharmonic phonon-electron problem in high-temperature superconductors has been investigated with the help of double time thermodynamic electron and phonon Green’s function theory using a comprehensive Hamiltonian which includes the contribution due to unperturbed electrons and phonons, anharmonic phonons, impurities, and interactions of electrons and phonons. This formulation enables one to resolve the problem of electronic heat transport and equilibrium phenomenon in high-temperature superconductors in an amicable way. The problem of electronic heat capacity and electron-phonon problem has been taken up with special reference to the anharmonicity, defect concentration electron-phonon coupling, and temperature dependence.

Is it possible to derive quantitative information on polarization of electron density from the multipolar model?

The accuracy of electrostatic properties estimated from the Hansen-Coppens multipolar model was verified. Tests were carried out to determine whether the multipolar model is accurate enough to study changes of electrostatic properties under the influence of a crystal field. Perturbed and unperturbed electron densities of individual molecules of amino acids and dipeptides were obtained from cluster and perturbation theory calculations. This enabled the changes in electrostatic properties values caused by polarization of the electron density to be characterized. Multipolar models were then fitted to the subsequent theoretical electron densities. The study revealed that electrostatic properties obtained from the multipolar models are significantly different from those obtained directly from the theoretical densities. The electrostatic properties of isolated molecules are reproduced better by multipolar models than the electrostatic properties of molecules in a crystal. Changes of electrostatic properties caused by perturbation of electron density due to the crystal environment are barely described by the multipolar model. As a consequence, the electrostatic properties obtained from multipolar models fitted to the perturbed theoretical densities derived either from cluster or periodic calculations do not differ much from those estimated from multipolar models fitted to densities of isolated molecules. The main reason for this seems to be related to an inadequate description of electron-density polarization in the vicinity of the nuclei by the multipolar model.

Theory of Electron Density of States of High Temperature Impurity Induced Anharmonic Superconductors

The expression for the electron density of states (EDOS) of high temperature superconductors (HTS) has been derived taking the disorder and anharmonicity effects as a central problem. This has been dealt with the help of double time thermodynamic Green’s function theory for electrons via a generalized Hamiltonian which consists of the contribution due to 1) unperturbed electrons; 2) unperturbed phonons; 3) isotopic impurities; 4) anharmonicities (no BCS type Hamiltonian has been taken up in the formulation); and 5) electron-phonon interactions. The renormalization effects and emergence of pairons appears as a unique feature of the theory and dependence of EDOS on impurity concentration and temperature has been discussed in details with special reference to the HTS.

The parametric decay of dust ion acoustic waves in non-uniform quantum dusty magnetoplasmas

The parametric decay instability of a dust ion acoustic wave into low-frequency electrostatic dust-lower-hybrid and electromagnetic shear Alfvén waves has been investigated in detail in an inhomogeneous cold quantum dusty plasma in the presence of external/ambient uniform magnetic field. The quantum magnetohydrodynamic model of plasmas with quantum effect arising through the Bohm potential and Fermi degenerate pressure has been employed in order to find the linear and nonlinear responses of the plasma particles for three-wave nonlinear coupling in a dusty magnetoplasma. A relatively high frequency electrostatic dust ion acoustic wave has been taken as the pump wave. It couples with two other low-frequency internal possible modes of the dusty magnetoplasma, viz., the dust-lower-hybrid and shear Alfvén waves. The nonlinear dispersion relation of the dust-lower-hybrid wave has been solved to obtain the growth rate of the parametric decay instability. The growth rate is at a maximum for a small value of the external magnetic field B0. It is noted that the growth rate is proportional to the unperturbed electron number density noe and is independent of inhomogeneity beyond Le=2 cm. An extraordinary growth rate is observed with the quantum effect.

The parametric decay of Alfvén waves into shear Alfvén waves and dust lower hybrid waves

The parametric decay instability of Alfven wave into low-frequency electrostatic dust-lower-hybrid and electromagnetic shear Alfven waves has been investigated in detail in a dusty plasma in the presence of external/ambient uniform magnetic field. Magnetohydrodynamic fluid equations of plasmas have been employed to find the linear and nonlinear response of the plasma particles for this three-wave nonlinear coupling in a dusty magnetoplasma. Here, relatively high frequency electromagnetic Alfven wave has been taken as the pump wave. It couples with other two low-frequency internal possible modes of the dusty magnetoplasma, viz., the dust-lower-hybrid and shear Alfven waves. The nonlinear dispersion relation of the dust-lower-hybrid wave has been solved to obtain the growth rate of the parametric decay instability. The growth rate is maximum for small value of external magnetic field Bs. It is noticed that the growth rate is proportional to the unperturbed electron number density noe.

The plasma sheath around large discs and ion collection by planar Langmuir probes

The structure of the plasma sheath around a thin, circular disc biased below the plasma potential is calculated assuming cold, collisionless, unmagnetized ions and Boltzmann electrons. The sheath area As (i.e. the effective ion collecting area of the disc) is computed for disc radii from 25 to 200 times the unperturbed electron Debye length and biases from −5 to −50 times the electron temperature (in eV). The normalized edge effect area, (As − Ap)/Ap, where Ap is the disc's area, is found to have a power law dependence on bias with an exponent of ≈0.6. The edge effect area scales inversely with the disc radius for large radii. An empirical analytical expression with only four parameters is given that fits the computed sheath areas with an error <2.2%. This work is directly applicable to the problem of ion collection by planar Langmuir probes.

INVESTIGATION OF SPIN TRANSPORT IN SEMICONDUCTORS BY SPATIALLY RESOLVED TWO-COLOR HANLE-MOKE MEASUREMENTS

We report on a considerable impact of light on spin transport in unstrained n -type GaAs . Increasing the intensity of optical spin pumping results in a significant expansion of spin diffusion. Furthermore, in the regime of strong spin pumping the spin gradient, which can be interpreted in terms of a spin outflow from the injection point is enhanced by application of a weak magnetic field. In addition we report on optical spin pumping of modulation doped CdTe -based quantum wells. Using a two-color Hanle-MOKE technique, we find a spin relaxation time of 34 ns in the nearly unperturbed electron gas. Independent variation of pump and probe energies demonstrates the presence of additional electrons in the quantum well, whose spin relaxation time is substantially shorter.