Domain wall theory of elementary excitations in anisotropic antiferromagneticS=1 chains

We present a theoretical investigation of elementary excitations in anisotropic antiferromagneticS=1 chains using the concept of domain walls in string (hidden) order. Domain walls are classified by the internal spin projectionS _dw ^z . We calculate energies and string correlation functions of low lying excited states of the domain wall type in the Haldane phase and compare the results to those of numerical computations. The boundaries of the Haldane phase are determined from the instability of these excitations with respect to the ground state. The interaction between two domain walls is found to be proportional to the productS _dw ^z , S _dw ^z ₂, it is effectively repulsive for equal spin projections.     

Observation of isomeric states in197Bi

The high spin states of¹¹⁹Te, populated in¹¹⁰Pd(¹³C,4n) and¹¹⁰Pd(¹²C,3n) reactions, have been studied through -ray spectroscopy. The level scheme has been established upto a spin of 55/2. Three-quasiparticle states, based on g² _7/2h_11/2 and g_7/2d_5/2h_11/2 configurations, have been identified. The 35/2 and 39/2 states are suggested to be the fully aligned states constituted by five valence h_11/2 ³, g_7/2, d_5/2 quasiparticles.     

On the interpretation of positron-annihilation data in powders and fine-grained materials

Monte-Carlo simulations of positron diffusion are carried out for powders consisting of spherical and ellipsoidal particles with and without defects. Following Bergersen et al. [1], elastic positron-phonon interaction is considered to be dominant for scattering processes in positron diffusion. The central question is which fraction of the positrons would be able to reach the particle boundaries. Hence, we calculate the Fraction of Positrons reaching particle Surface (FPS). The presence of defects in the particles can drastically reduce FPS depending on the defect concentration and capture rate. We demonstrate that for small-grained materials the grain surface can influence the lifetime signal significantly.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

Luminescence centers in porous silicon

Photoluminescence studies on porous silicon show that there are luminescence centers present in the surface states. By taking photoluminescence spectra of porous silicon with respect to temperature, a distinct peak can be observed in the temperature range 100–150 K. Both linear and nonlinear relationships were observed between excitation laser power and the photoluminescence intensity within this temperature range. In addition, there was a tendency for the photoluminescence peak to red shift at low temperature as well as at low excitation power. This is interpreted as indicating that the lower energy transition becomes dominant at low temperature and excitation power. The presence of these luminescence centers can be explained in terms of porous silicon as a mixture of silicon clusters and wires in which quantum confinement along with surface passivation would cause a mixing of andX band structure between the surface states and the bulk. This mixing would allow the formation of luminescence centers.     

CW diode pumping and FM mode locking of a Nd: KGW laser

We have demonstrated cw diode end pumping of Nd: KGW, a novel solid-state gain medium, with up to 30% conversion efficiency into near-TEM₀₀ (M² < 1.05) output at = 1.067 µm for a pump level of 2.7 W. The slope efficiency was limited by intracavity reflections to 36%; however, direct comparison to a similar Nd:YAG laser indicates the same intrinsic slope efficiency of 60%. FM mode locking of this laser at 200 MHz has produced 12 ps pulses (compared to 16 ps for Nd: YAG), although an intracavity etalon was required. Considerable reduction in pulse width is possible (the line width limit is 0.5 ps) but different techniques may be necessary. Spatial hole burning was evident in both the 120 GHz free-running spectrum and the etalon-limited mode-locked spectrum.     

Fixed-point densities for a quasiperiodic kicked-oscillator map

Suppose that a one-dimensional harmonic oscillator is subjected to instantaneous kicks q times per natural period, with the kick amplitude varying sinusoidally with position. Viewed stroboscopically in phase space, the motion has an infinitely extended periodic or quasiperiodic array of fixed points, as well as an infinite web of chaotic orbits. In the present work (restricted to the quasiperiodic case q=5) the fixed points are classified according to their local linear behavior, which depends essentially on a single variable, the residue R. With the aid of a five-dimensional embedding, a function rho(R) is calculated which for infinitesimal DeltaR gives the average density of fixed points in the plane with residue in the range (R,R+DeltaR). The location and strength of the singularities and discontinuities of rho(R) are extracted from relatively simple transcendental equations, and this makes possible efficient numerical determination of rho(R). An exact equality for the densities of positive-R and negative-R fixed points is proved using decagonal symmetry and the integral representation of rho(R). For parameter values below the period-doubling threshold, there are no unstable fixed points with R greater, similar 0, and so we have equality of the densities of stable centers and unstable saddles. (c) 1995 American Institute of Physics.     

Reaction studies of carbon clusters

Reaction studies of carbon clustersC _n in the rangen=8–37, produced by laser vaporisation in a supersonic nozzle, have been investigated using time-of-flight mass spectrometry. Clear differences in reaction products formed on hydrogenation are detected which show that even and odd clusters behave quite differently and furthermore thatat least two different types of even cluster appear to exist. The reactivity patterns for clusters C _n withn=16, 18 and 22 are in a different class from those withn=20, 24, 26 ..., a behaviour consistent with the existence of closed cage fullerene structures for even clusters with 20 or more carbon atoms (other thann=22).     

The development of a sampler-sensor using a vanadium-oxinate-polymer film for the selective and direct determination of atmospheric ethanol

The development of a sensor for the direct and selective determination of atmospheric ethanol is in the initial stages. The sensor takes advantage of the selective chemical reaction between ethanol and vanadium oxinate. This reaction occurs in an organic medium where a red colored complex is the product. This reaction is determined spectrophotometrically where the absorbance maxima is 475 nm. The focus of this paper is to discuss the parameters necessary to develop a solid sorbent sampling-sensor which can be used to determine atmospheric ethanol.     

Shed plane leaves as a source ofα-tocopherol

A natural source of -tocopherol — shed plane leaves — has been found. The dynamics of the accumulation of a-tocopherol over the vegetation period of the plant has been studied, and a method has been developed for its isolation and quantitative determination.Deceased.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Republic of Uzbekistan, Tashkent, fax (3712) 89 14 75. Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 372–374, May–June, 1995. Original article     

The development of a versatile field program for measuring tritium in “real-time”

Robust sample handling and liquid scintillation counting (LSC) procedures have been developed to routinely monitor tritium in the field relative to the 20,000 pCi/L drinking water standard. This procedure allows tritium to be monitored hourly during 24 hour drilling operations at depths in the saturated zone potentially contaminated by sub-surface nuclear weapons testing at the Nevada Test Site. Using retrofitted, shock hardened, vibration damped counters and strict analytical protocols, tritium may be measured rapidly in the field under hostile conditions. Concentration standards and dead tritium backgrounds are prepared weekly in a central laboratory and delivered to remote drilling locations where they are recounted daily as a check on counter efficiency and calibration. Portable LSC counters are located in trailers and powered off a battery pack and line filter fed by mobile generator. The samples are typically groundwaters mixed with drilling fluids returned after circulation through a drill string. Fluids are aerated and de-foamed, filtered, mixed with scintillation cocktail and dark adapted before counting. Real-time monitoring affords drilling and field personnel early warning against intercepting down-gradient plumes of radioactivity. For routine operations, the tritium activity may not exceed a 10,000 pCi/L threshold.