Theoretical investigations on the energy relaxation in two-dimensional GaN systems

We theoretically study the energy relaxation of hot electrons via LO-phonon emission in two-dimensional GaN systems. We employ a model in which electrons and the lattice are in equilibrium separately, and the effective electron temperature is much larger than the lattice temperature. We include the dynamical screening of electrons, electron-phonon interactions reduced phonon self-energy correction, the hot-phonon, and the finite-size effects. The power loss to acoustic phonons is also considered. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA

Collective excitations of strongly coupled bilayer charged Bose liquids in the third-frequency-moment sum rule

We calculate the collective excitation modes of strongly coupled bilayer charged Bose systems. We employ the dielectric matrix formulation to study the correlation effects within the random-phase approximation (RPA), the self consistent field approximation Singwi, Tosi, Land, and Sjölander (STLS), and the quasilocalized charge approximation (QLCA), which satisfies the third-frequency-moment (〈ω3〉) sum rule. We find that the QLCA predicts a long-wavelength correlation-induced energy gap in the out-of-phase plasmon mode, similar to the situation in electronic bilayer systems. The energy gap and the plasmon density of states are studied as a function of interlayer separation and coupling parameter rs. The results should be helpful for experimental investigations. © 2008 The American Physical Society

Metastability and the Casimir Effect in Micromechanical Systems

Electrostatic and Casimir interactions limit the range of positional stability of electrostatically-actuated or capacitively-coupled mechanical devices. We investigate this range experimentally for a generic system consisting of a doubly-clamped Au suspended beam, capacitively-coupled to an adjacent stationary electrode. The mechanical properties of the beam, both in the linear and nonlinear regimes, are monitored as the attractive forces are increased to the point of instability. There "pull-in" occurs, resulting in permanent adhesion between the electrodes. We investigate, experimentally and theoretically, the position-dependent lifetimes of the free state (existing prior to pull-in). We find that the data cannot be accounted for by simple theory; the discrepancy may be reflective of internal structural instabilities within the metal electrodes.Comment: RevTex, 4 pages, 4 figure

Fibre Degradation Rate of Perennial Ryegrass Varieties Measured Using Three Techniques: \u3cem\u3eIn Situ\u3c/em\u3e Nylon Bag, \u3cem\u3ein Vivo\u3c/em\u3e Rumen Evacuation and \u3cem\u3ein Vitro\u3c/em\u3e Gas Production

In Western Europe, perennial ryegrass is the most widely used grass species for grazing cattle, because of its high productivity, palatability and nutritive value. However, the low dry matter intake (DMI) of perennial ryegrass pasture has been identified as a major factor limiting milk production of high producing dairy cows. Altering the chemical, physical and mechanical characteristics that contribute to its low DMI through grass breeding and the choice of variety may be a way forward in trying to maximise its DMI. This study aimed to examine whether perennial ryegrass varieties differ in their NDF degradation rates (kdNDF)

Nonequilibrium Electron Interactions in Metal Films

Ultrafast relaxation dynamics of an athermal electron distribution is investigated in silver films using a femtosecond pump-probe technique with 18 fs pulses in off-resonant conditions. The results yield evidence for an increase with time of the electron-gas energy loss rate to the lattice and of the free electron damping during the early stages of the electron-gas thermalization. These effects are attributed to transient alterations of the electron average scattering processes due to the athermal nature of the electron gas, in agreement with numerical simulations

Stiction, Adhesion Energy and the Casimir Effect in Micromechanical Systems

We measure the adhesion energy of gold using a micromachined doubly-clamped beam. The stress and stiffness of the beam are characterized by measuring the spectrum of mechanical vibrations and the deflection due to an external force. To determine the adhesion energy we induce stiction between the beam and a nearby surface by capillary forces. Subsequent analysis yields a value $\gamma =0.06$ J/m$^{2}$ that is a factor of approximately six smaller than predicted by idealized theory. This discrepancy may be resolved with revised models that include surface roughness and the effect of adsorbed monolayers intervening between the contacting surfaces in these mesoscopic structures.Comment: RevTex, 4 pages, 4 eps figure