Photodetachment microscopy of

Photodetachment of ions is performed in the presence of a static electric field. A high spatial resolution electron detector is used to map out the ejected electron's wave function. Propagation over half a metre does not alter the stationary structure the electron wave possesses in the direction orthogonal to the detachment motion. Nodes and antinodes of electron current density are observed, that can be interpreted as direct imaging of the internal atomic wave function. Because detachment of a negative ion can be represented accurately in the free-electron approximation, the obtained images can also be viewed as direct experimental representations of the Green function of the uniform acceleration problem. Agreement of the measured distributions with the exact expression of this wave-function appears quite satisfactory. Received: 5 June 1998 / Accepted: 16 October 1998     

Thermomechanical properties of amorphous hydrogenated carbon–germanium alloys

Thermomechanical properties of amorphous hydrogenated carbon-germanium alloys prepared by the rf sputtering technique were determined for films in the 0 at. % to 100 at. % carbon content range. The stress, thermal expansion coefficient, and elastic modulus were obtained using the thermally induced bending technique. The stress was related to the concentration of hydrogen and argon, to the difference in the Ge-Ge and Ge-C bond length, and to the carbon hybridization. The thermal expansion coefficients of pure amorphous germanium and amorphous carbon are higher than that of their corresponding crystalline counterparts, which was attributed to the compressive stress of the films. The biaxial modulus, on the other hand, are always smaller than that of their crystalline counterparts, but increases as the concentration of carbon increases due to the substitution of Ge-Ge bonds by energetically stronger Ge-C and C-C bonds. Received: 9 May 2000 / Accepted: 10 May 2000 / Published online: 13 July 2000     

Modeling transverse dunes with vegetation

In the present work, we use dune modeling in order to investigate the evolution of transverse dunes in the presence of vegetation. The vegetation is allowed to grow up to a maximum height with a growth rate R that oscillates in time. We find that the presence of the vegetation establishes a maximum height for the transverse dunes. If the transverse dune is larger than this maximum size, then the vegetation traps a considerable amount of sand, leading to the formation of vegetation marks at the upwind side of the dune. We also investigate the formation of the transverse dune fields from a flat sand beach under saturated sand flux and vegetation growth. We find that the behavior of the field is determined by the maximum height, , of the vegetation cover.     

Degradation analysis of thin film photovoltaic modules

Five thin film photovoltaic modules were deployed outdoors under open circuit conditions after a thorough indoor evaluation. Two technology types were investigated: amorphous silicon (a-Si:H) and copper indium gallium diselenide (CIGS). Two 14 W a-Si:H modules, labelled Si-1 and Si-2, were investigated. Both exhibited degradation, initially due to the well-known light-induced degradation described by Staebler and Wronski [Applied Physics Letters 31 (4) (1977) 292], and thereafter due to other degradation modes such as cell degradation. The various degradation modes contributing to the degradation of the a-Si:H modules will be discussed. The initial maximum power output (P_MAX) of Si-1 was 9.92 W, with the initial light-induced degradation for Si-1 ∼30% and a total degradation of ∼42%. For Si-2 the initial P_MAX was 7.93 W, with initial light-induced degradation of ∼10% and a total degradation of ∼17%. Three CIGS modules were investigated: two 20 W modules labelled CIGS-1 and CIGS-2, and a 40 W module labelled CIGS-3. CIGS-2 exhibited stable performance while CIGS-1 and CIGS-3 exhibited degradation. CIGS is known to be stable over long periods of time, and thus the possible reasons for the degradation of the two modules are discussed.     

On the temperature dependence of quarkonium correlators

I discuss the temperature dependence of quarkonium correlators calculated in lattice QCD. I show that the dominant source of the temperature dependence comes from the zero-mode contribution, while the temperature dependence associated with the melting of bound states is quite small. I study the zero-mode contribution quantitatively for various quark masses and show that it is well described by a quasi-particle model with temperature-dependent heavy quark mass. As a byproduct, an estimate of the medium dependence of the heavy-quark mass is obtained.     

Lattice QCD thermodynamic results with improved staggered fermions

We present results on the QCD equation of state, obtained with two different improved dynamical staggered fermion actions and almost physical quark masses. Lattice cut-off effects are discussed in detail as results for three different lattice spacings are available now, i.e. results have been obtained on lattices with temporal extent of N _τ =4,6 and 8. Furthermore we discuss the Taylor expansion approach to non-zero baryon chemical potential and present the isentropic equation of state on lines of constant entropy per baryon number.     

Direct evidence for the onset of intruder configurations in neutron-rich Ne isotopes

We report on direct experimental evidence of the population of the 3/²⁻$3/2^{-}$ intruder state in ²⁷Ne in the knockout of a single neutron from the ground state of ²⁸Ne. This low-lying negative parity state is consistent with a narrower shell gap for exotic nuclei with Z?N$Z?N$ and N≈20$N\approx 20$. Monte Carlo shell-model calculations with the modern SDPF-M interaction successfully describe neutron-rich nuclei in the vicinity of N=20$N=20$ where normal and intruder configurations coexist at low excitation energy. This observation demonstrates the importance of direct reactions for the study of exotic nuclei and the predictive power of these large-scale shell-model calculations.     

Search for the Θ1540) in lattice QCD

We report on a study of the pentaquark Θ⁺(1540), using a variety of different interpolating fields. We use chirally improved fermions in combination with Jacobi-smeared quark sources to improve the signal and get reliable results even for small quark masses. The results of our quenched calculations, which have been done on a 12³×24 lattice with a lattice spacing of a = 0.148fm, do not provide any evidence for the existence of a Θ⁺ with positive parity. We do observe, however, a signal compatible with nucleon-kaon scattering state. For the negative parity the results are inconclusive, due to the potential mixture with nucleon-kaon and N^*-kaon scattering states.