Radio frequency driven plasma jets are frequently employed as efficient plasma sources for surface modification and other processes at atmospheric pressure. The radio-frequency driven micro atmospheric pressure plasma jet (µAPPJ) is a particular variant of that concept whose geometry allows direct optical access. In this work, the characteristics of the µAPPJ operated with a heliumoxygen mixture and its interaction with a helium environment are studied by numerical simulation.The density and temperature of the electrons, as well as the concentration of all reactive species are studied both in the jet itself and in its effluent. It is found that the effluent is essentially free of charge carriers but contains a substantial amount of activated oxygen (O, O 3 and O 2 ( 1 ∆)).The simulation results are verified by comparison with experimental data.

We study dilute magnetic impurities and vacancies in two-dimensional frustrated magnets with noncollinear order. Taking the triangular-lattice Heisenberg model as an example, we use quasiclassical methods to determine the impurity contributions to the magnetization and susceptibility. Most importantly, each impurity moment is not quantized but receives nonuniversal screening corrections due to local relief of frustration. At finite temperatures, where bulk long-range order is absent, this implies an impurity-induced magnetic response of Curie form, with a prefactor corresponding to a fractional moment per impurity. We also discuss the behavior in an applied magnetic field, where we find a singular linear-response limit for overcompensated impurities.

Microcavity plasma arrays of inverse pyramidal cavities have been fabricated in p-Si wafers. Each cavity acts as a microscopic dielectric barrier discharge. Operated at atmospheric pressure in argon and excited with high voltage at about 10 kHz, each cavity develops a localized microplasma. Experiments have shown a strong interaction of individual cavities, leading to the propagation of wave-like optical emission structures along the surface of the array. This phenomenon is numerically investigated using computer simulation. The observed ionization wave propagates with a speed of about 5 km/s, which agrees well the experimental findings. It is found that the wave propagation is due to sequential contributions of a drift of electrons followed by drift of ions between cavities seeded by photoemission of electrons by the plasma in adjacent cavities

For isolated vacancies in ordered local-moment antiferromagnets we show that the magnetic-field linear-response limit is generically singular: The magnetic moment associated with a vacancy in zero field is different from that in a finite field h in the limit h → 0 + . The origin is a universal and singular screening cloud, which moreover leads to perfect screening as h → 0 + for magnets which display spin-flop bulk states in the weak-field limit.Defects are ubiquitous in solids. In magnets with localized spin moments, typical classes of defects are missing or extra spins, arising, e.g., from substitutional disorder. Very often, even small concentrations of such defects produce a large magnetic response at low temperatures: Quasi-free spins cause a Curie tail in the magnetic susceptibility, which then is routinely subtracted from raw experimental data. Assuming independent defects, the amplitude of the Curie tail can be utilized to estimate the defect concentration, provided that the behavior of a single defect is known.Here we discuss the physics of isolated vacancies in antiferromagnets (AF) which display semiclassical longrange order (LRO) in the ground state [1]. In zero magnetic field, the state with a single vacancy has a finite uniform magnetic moment, m 0 , because the vacancy breaks the balance between the sublattices. For collinear magnets, m 0 is quantized to the bulk spin value, m 0 = S [2, 3], while in the non-collinear case fractional values of m 0 occur due to the local relief of frustration [4]. These vacancy moments are expected to show up in magnetization measurements, and they produce a low-temperature Curie contribution to the uniform susceptibility in the two-dimensional (2d) case where bulk order is prohibited by the Mermin-Wagner theorem [3][4][5][6][7].In this paper we show that, in an applied field h, nontrivial screening of the vacancy moment occurs, such that the linear-response limit h → 0 + is singular for a magnet with a single vacancy, Fig. 1: The vacancy-induced magnetization jumps discontinuously from its zero-field value m 0 to a different value m(h → 0 + ) upon applying an infinitesimal field h. Thus, measurements of the vacancy-induced moment m(h) in a finite field h cannot detect the zero-field value m 0 even for small h [8], which is of obvious relevance for any experiment trying to quantify the defect contribution to a sample's magnetization or susceptibility. Furthermore, the spin texture around the vacancy at finite h has a piece [9] which is singular as h → 0 + -in a sense made precise belowwhich screens the vacancy-induced moment perpendicular to h. For magnets which feature spin-flop states (with all spins perpendicular to h as h → 0 + ) in the absence of the vacancy, this leads to a semiclassical version of perfect screening of the vacancy moment, m(h → 0 + ) = 0.In the body of paper, we present general arguments and microscopic calculations supporting these claims. Explicit results will be given in a 1/S expansion for spin-S AFs on 2d lattices, with(1) but our results...