Propagation and conversion of non-axisymmetric modes in non-uniform helicon-sustained plasma

An effect of the radial plasma density gradient on the oblique wave propagation in the helicon-frequency range is investigated. It is shown that the dispersion features of electrostatic and electromagnetic modes are essentially changed in strongly non-uniform helicon plasma. In particular, the transition between helicon eigenmode frequency scales ω ～ k _z and ω ～ k _z ² is demonstrated. The process of total conversion of long-wavelength helicon mode into short-wavelength electrostatic wave in near-axis region of a plasma column is analysed. Due to modification of the wave dispersion relations, the densities at which the mode conversion occurs are considerably reduced if the density gradient is steep enough. The estimation of collisionless axial damping rate of helicon mode connected with linear mode conversion is presented. This damping is compared with the usual collision damping.     

Bulk plasma waves in a randomly inhomogeneous conductor

The modification of the spectrum and damping of bulk plasma waves due to three-dimensional random inhomogeneities of the density of a degenerate electron gas in a conductor have been investigated using the averaged Green??s function method. The dependences of the frequency and damping of the averaged plasma waves, as well as the position ?? _m and width ???? of the peak of the imaginary part of the Fourier trans-form of the averaged Green??s function, on the wave vector k have been determined in the self-consistent approximation, which makes it possible to take into account multiple scattering of plasma waves by inhomogeneities. It has been found that, in the long-wavelength region of the spectrum, the decrease revealed in the frequency of the plasma waves is caused by the inhomogeneities, which agrees qualitatively with the behavior of the position of the peak ?? _m . In the range of large values of the correlation length of inhomogeneities and small values of k, the damping of the plasma waves tends to zero, whereas the width of the peak ???? remains finite, which is due to the nonuniform broadening. A comparison with the data of numerical calculations has been performed.     

Propagation of electromagnetic waves in hot magnetoactive plasma

The paper derives the general form of the tensor of dielectric permittivity? _ij(ω,k), Eq. (15), of non-relativistic hot magnetoactive collisionless plasma taking into consideration the influence of spatial dispersion. The general form of the tensor? _ij(ω,k) is used to express the tensorε _ij(ω,k) in the region of weak and strong spatial dispersion and in some special cases. A general dispersion equation (30) is derived and an analysis is made of the waves propagating in hot magnetoactive plasma. The expressions derived are used to investigate the damping of a right-handed circularly polarized wave propagating in hot magnetoactive plasma in the direction of the magnetic field.     

Reduction of stimulated brillouin scattering by the generation of ion acoustic harmonics

Ion wave harmonics generated by the primary ion acoustic wave excited in stimulated Brillouin scattering are shown to introduce heavy nonlinear damping of the process and to considerably reduce the anomalous reflectivity in an underdense plasma (n/n_c<0.5). The generation of ion wave harmonics acts similarly to strong linear damping and provides an alternative explanation to the heavy damping indicated by experiments.     

Momentum dependency of the volume plasmon damping in metals

The wave vector dependency of the damping of volume plasma oscillation has been studied on the metals Al, Ga and Sn by measuring the half-width ΔE _1/2 of the volume plasma loss of 51 kev electrons as a function of their scattering angle ?. We observed an interesting structure in the continuously increasing half width ΔE _1/2(?) which indicates resonance like processes at certain values of the plasmon wave vector.     

Electromagnetic waves in a randomly inhomogeneous Josephson junction

Spectrum modification and damping of Josephson plasma waves induced by random inhomogeneities of the critical current through the superconductor contact and the averaged Green function of such excitations are analyzed. In the self-consistent approximation that makes it possible to take into account multiple wave scattering on the inhomogeneities, the frequency and damping of averaged waves, as well as position ν _m and peak width Δν of the Fourier transform imaginary part of the averaged Green function, are determined as functions of wavevector k. The evolution of such functions with the variation of the correlation radius and the relative r.m.s. fluctuations of inhomogeneities is studied. The inhomogeneity-induced wave frequency decrease observed in the long wavelength spectral region qualitatively agrees with the ν _m behavior. It is established that in the case of “long-range” inhomogeneities, the linear dependence of damping on k changes to the inversely proportional one, and damping tends to zero as k → 0, while Δν at small k attains its maximal values due to nonuniform broadening. In the presence of “short-range” inhomogeneities, the wave damping and Δν are found to be similar functions of k. The results are compared to the numerical calculation data.     

Plasma density measurement in a gas-filled X-band backward wave oscillator with a double conversion heterodyne microwave interferometer

Plasma density was measured with a heterodyne microwave interferometer in both a gas-filled X-band backward wave oscillator (BWO) and in a smooth tube. Plasma is generated by impact ionization of a 650 kV, 2 kA electron beam. For fixed gas pressure we found that the plasma density rise in the operating BWO was much faster than in a smooth tube, indicating that Trivelpiece-Gould modes, or high power microwaves, increase plasma generation. Additional plasma enhanced BWO microwave output power. Measured plasma density at optimum power levels was n_cr ≈ 6 × 10¹²cm⁻³ at onset of emitted microwaves.     

Short-wavelength plasmons in low-dimensional systems

The dispersion of plasma waves in systems of various dimensions is investigated up to the end point of the spectrum. In 2D and 3D systems, the plasmon spectrum still ends (due to Landau damping) within the applicability range of the quasi-classical approximation, i.e., for ?k ? p _F (?k is the plasmon momentum and p _F is the electron Fermi momentum). In 1D systems, the results are qualitatively different, since the Landau damping is concentrated in a region where the quantum effects cannot be ignored. This peculiarity of 1D systems gives rise to undamped branches of acoustic plasmons with a phase velocity lower than the electron Fermi velocity in multicomponent 1D plasmas.     

Observation of extraordinary wave propagation near the lower hybrid resonance frequency

Measurements of k_⊥ for an extraordinary wave near the lower hybrid frequency have been carried out by means of propagation between two dipole antennas into a plasma column. Resonance-like behavior have been observed when plasma parameters and frequency fulfil the condition ω ≈ ω_LH.     

Ferromagnetism in the Hubbard model: instability of the Nagaoka state on the square lattice

The instability of the fully polarized ferromagnetic ground state (Nagaoka state) of the Hubbard model on the square lattice is investigated. We use single spin flip variational wave functions including majority spin correlation effects and calculate spin flip energies in the thermodynamic limit. With very local wave functions and with moderate numbers of variational parameters we reproduce the best known estimate for the critical hole density δ_cr = 0.29 and we obtain an estimate of U_cr = 63 t for the critical coupling which is considerably better than the best estimate of U_cr = 42 t previously known. The simplicity of our wave functions makes the physical origin of the various aspects of the instability particularly transparent.     

Waves in a superlattice with anisotropic inhomogeneities

Dependences of the dispersion laws and damping of waves in an initially sinusoidal superlattice on inhomogeneities with anisotropic correlation properties are studied for the first time. The period of the superlattice is modulated by the random function described by the anisotropic correlation function K_?(r) that has different correlation radii, k _∥ ^?1 and k _⊥ ^?1 , along the axis of the superlattice z and in the plane xy, respectively. The anisotropy of the correlation is characterized by the parameter λ=1?k_⊥/k_∥ that can change from λ=0 to λ=1 when the correlation wave number k⊥ changes from k_⊥=k_∥ (isotropic 3D inhomogeneities) to k_⊥=0 (1D inhomogeneities). The correlation function of the superlattice K(r) is developed. Its decreasing part goes to the asymptote L that divides the correlation volume into two parts, characterized by finite and infinite correlation radii. The dependences of the width of the gap in the spectrum at the boundary of the Brillouin zone δν and the damping of waves ξ on the value of λ are studied. It is shown that decreasing L leads to the decrease of δν, and increase of ξ, with the increase of λ.     

Critical charge in quantum electrodynamics

The critical nuclear charge Z _cr and the critical distance R _cr in the system of two colliding heavy nuclei—they are defined as those at which the ground-state level of the electron spectrum descends to the boundary of the lower continuum, with the result that beyond them (that is, for Z>Z _cr or R<R _cr) spontaneous positron production from a vacuum becomes possible—are important parameters in the quantum electrodynamics of ultrastrong Coulomb fields. Various methods for calculating Z _cr and R _cr are considered, along with the dependence of these quantities on the screening of the Coulomb field of a nucleus by the electron shell of the atom, on an external magnetic field, on the particle mass and spin, and on some other parameters of relevance. The effective-potential method for the Dirac equation and the application of the Wentzel-Kramers-Brillouin method to the Coulomb field for Z>137 and to the two-body Salpeter equation for the quark-antiquark system are discussed. Some technical details in the procedure for calculating the critical distance R _cr in the relativistic problem of two Coulomb centers are described.     

Critical damping of the uniform mode in easy-axis antiferromagnets: MnF2

We report calculations of the critical damping of the uniform spin wave mode in MnF₂. Evaluation of the damping rate using empirical values for the parameters leads to results in reasonable agreement with experiment for 0.1 K < T_N ? T < 1K.     

Ferromagnetism in the Hubbard model: Spin waves and instability of the Nagaoka state

We discuss two single spin flip variational wave functions describing spin wave excitations which were proposed earlier by Shastry, Krishnamurthy and Anderson (SKA) and by Basile and Elser (BE), respectively, in order to investigate the instability of the fully polarized ferromagnetic state (Nagaoka state) in the infinite U Hubbard model. We calculate the energy of these variational states for the square lattice and for multiple chains. At the zone boundary in the vicinity of the point (0, π) the spin wave energy is reduced substantially by the binding of the spin up hole to the flipped down spin. For the square lattice this leads to a critical hole density of δ_cr = 0.407 for the SKA spin wave and of δ_cr = 0.322 for the BE spin wave which implies remarkable improvements in comparison to the corresponding scattering states investigated previously.     

Multiphonon contribution to the structure factor of metals

The structure factor of Na is calculated including two-phonon terms and the Debye-Waller factor. The result is compared with the one-phonon approximation usually employed to evaluate the electronic transport coefficients. This multiphonon contribution can amount to 13 per cent at the melting point and 10 per cent at room temperature in the transport sensitive region of wave vector 1.5 k_F < q < 2 k_F, where k_F is the Fermi wave vector. We conclude that calculations of electronic transport coefficients of metals intended to attain a precision better than 10 per cent above the Debye temperature must take into account the contributions of the Debye-Waller factor and the two phonon terms.     

Spin damping correction to electrostatic modes in kinetic plasma theory

The effect of spin of particles is studied using a semi-classical kinetic theory for a magnetized plasma. No other quantum effects are included. We focus in the simple damping effects for the electrostatic wave modes. Besides Landau damping, we show that spin produces two new different effects of damping or instability which are proportional to ?. These corrections depend on the electromagnetic part of the wave that is coupled with the spin vector.     

High speed in spin‐torque‐based magnetic memory using magnetic nanocontacts

Magnetization switching by a spin‐polarized current in perpendicular anisotropy devices with magnetic nanocontact (NC) is investigated using a micromagnetic formalism. The critical switching current (i_cr) and switching time (τ₀) can be reduced when a soft layer is exchange coupled to the NC. The study reveals that devices with fewer NCs have smaller i_cr compared to those with a large number. Furthermore, τ₀ for nanoconstricted devices is almost constant with anisotropy field (H_k), in contrast to devices without NCs that show an exponential increase with H_k. This suggests that nanoconstricted devices could be used to improve thermal stability, while reducing i_cr and τ₀. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Comparaison de la méthode C.N.D.O. et de modèles analytiques pour la détermination de la polarisabilité de petits amas linéaires

We compare the values of the polarisabilities F(k) of small aggregates obtained by using the C.N.D.O. method to those given by analytical models. The studied clusters are the hypothetical molecules H₇₆ and H₇₈ where the atoms are equally spaced and arranged along a linear closed chain. Our conclusion is that there is a good agreement between C.N.D.O. and RPA results for the absolute value and for the variation with k of F(k). The difference which remains between the 2 curves is mainly due to the neglect of exchange correlations in RPA. This point is checked by taking into account exchange terms in the analytical approach. Moreover we also show that the C.N.D.O. curve presents some specific particularities due to degeneracies in the level spectra. These effects do not alter the forementioned agreement because they are limited to a few values of k (Δk?5) and localized near k = 38 for H₇₆ and k = 0 for H₇₈. In the first case they are due to the spontaneous formation of a charge density wave with wave vector k = 38, in the second case to a depolarization term which would not appear if, for example, the transitions k = 1 and k = ?1 were not exactly degenerated. We extend our analytical study to the n = 1014 case and show how the F(k) values behave when N → ∞.     

Theory for the first-order spin-wave instability threshold in ferromagnetic insulating thin films

A theory for the first-order Suhl and the parallel pumping instability in thin films is presented. Significant differences for the critical microwave field and wave vector to former calculations occur, which discuss the problem in terms of bulk spin-waves neglecting boundary conditions. A coupling matrixC _kk′ is introduced, which describes the couplings between the modes and the driving microwave field. For bulk standing spin-wavesC _kk′ is always diagonal. For the true discrete standing modes of a thin filmC _kk′ changes only in case of 1. Suhl instability and if the wavevector has a non vanishing component perpendicular to the film plane. Here the diagonal bulk couplings have to be replaced in part by off diagonal terms, describing couplings between modes, which perpendicular wave vector componentk _⊥ differs byπ/d (d=film thickness). The decisive quantity, which decides if the finite thickness of the film is of importance or if the film can be treated as a bulk system, is the frequency difference δω _k of the coupled modes. For δω _k much smaller than the spin-wave damping η _k the bulk approximation is correct. For \(\delta \omega _k > > \eta _k \) two experimental situations for 1. Suhl instability are discussed: For a perpendicular to the film plane magnetized film the critical microwave field is by π/2 bigger than in the bulk case. In an in-plane magnetized film the critical spin-waves propagate always in the film plane, as only hereC _kk′ remains identical to the bulk case.