Light-induced anelastic change in a-Si(H)

The thermal desorption spectra between 400 and 1100 K and the internal friction spectra between 80 and 423 K were studied for a-Si(H). The thermal desorption of hydrogen was observed around 650 K (TDH650 K) and around 900 K (TDH900 K,L and TDH900 K,H). Both TDH900 K,L and TDH900 K,H with the activation energy of 1.6 eV were attributed to the desorption of bonded hydrogen. TDH650 K was not a diffusion controlled process with the activation energy of 1.0 eV, where one part of TDH650 K was attributed to the desorption of isolated hydrogen molecules. The hydrogen-induced internal friction, View the MathML source, was observed between 80 and 423 K. Hydrogen responsible for View the MathML source showed the thermal desorption around 650 K (TDH650 K), indicating that isolated hydrogen molecules in the amorphous structure may be responsible for View the MathML source. Light soaking caused changes in View the MathML source in the temperature ranges between 80 and 200 K and between 200 and 300 K, indicating that light soaking modified the local amorphous structures responsible for these changes in View the MathML source

Noise processes in nanomechanical resonators

Nanomechanical resonators can be fabricated to achieve high natural resonance frequencies, approaching 1 GHz, with quality factors in excess of 10^(4). These resonators are candidates for use as highly selective rf filters and as precision on-chip clocks. Some fundamental and some nonfundamental noise processes will present limits to the performance of such resonators. These include thermomechanical noise, Nyquist-Johnson noise, and adsorption-desorption noise; other important noise sources include those due to thermal fluctuations and defect motion-induced noise. In this article, we develop a self-contained formalism for treating these noise sources, and use it to estimate the impact that these noise processes will have on the noise of a model nanoscale resonator, consisting of a doubly clamped beam of single-crystal Si with a natural resonance frequency of 1 GHz

Control of Material Damping in High-Q Membrane Microresonators

We study the mechanical quality factors of bilayer aluminum/silicon-nitride membranes. By coating ultrahigh-Q Si3N4 membranes with a more lossy metal, we can precisely measure the effect of material loss on Q's of tensioned resonator modes over a large range of frequencies. We develop a theoretical model that interprets our results and predicts the damping can be reduced significantly by patterning the metal film. Using such patterning, we fabricate Al-Si3N4 membranes with ultrahigh Q at room temperature. Our work elucidates the role of material loss in the Q of membrane resonators and informs the design of hybrid mechanical oscillators for optical-electrical-mechanical quantum interfaces

Interface-controlled creep in metallic glass composites

In this work we present molecular dynamics simulations on the creep behavior of $\rm Cu_{64}Zr_{36}$ metallic glass composites. Surprisingly, all composites exhibit much higher creep rates than the homogeneous glass. The glass-crystal interface can be viewed as a weak interphase, where the activation barrier of shear transformation zones is lower than in the surrounding glass. We observe that the creep behavior of the composites does not only depend on the interface area but also on the orientation of the interface with respect to the loading axis. We propose an explanation in terms of different mean Schmid factors of the interfaces, with the amorphous interface regions acting as preferential slip sites.Comment: 11 pages, 13 figure

Conductivity and Dissociation in Metallic Hydrogen: Implications for Planetary Interiors

Liquid metallic hydrogen (LMH) was recently produced under static compression and high temperatures in bench-top experiments. Here, we report a study of the optical reflectance of LMH in the pressure region of 1.4-1.7 Mbar and use the Drude free-electron model to determine its optical conductivity. We find static electrical conductivity of metallic hydrogen to be 11,000-15,000 S/cm. A substantial dissociation fraction is required to best fit the energy dependence of the observed reflectance. LMH at our experimental conditions is largely atomic and degenerate, not primarily molecular. We determine a plasma frequency and the optical conductivity. Properties are used to analyze planetary structure of hydrogen rich planets such as Jupiter

Stability of Cubic FAPbI3_3 from X-ray Diffraction, Anelastic, and Dielectric Measurements

Among the hybrid metal-organic perovskites for photovoltaic applications FAPbI_3 (FAPI) has the best performance regarding efficiency and the worst regarding stability, even though the reports on its stability are highly contradictory. In particular, since at room temperature the cubic alpha phase, black and with high photovoltaic efficiency, is metastable against the yellow hexagonal delta phase, it is believed that alpha-FAPI spontaneously transform into delta-FAPI within a relatively short time. We performed X-ray diffraction and thermogravimetric measurements on loose powder of FAPI, and present the first complete dielectric and anelastic spectra of compacted FAPI samples under various conditions. We found that alpha-FAPI is perfectly stable for at least 100 days, the duration of the experiments, unless extrinsic factors induce its degradation. In our tests, degradation was detected after exposure to humidity, strongly accelerated by grain boundaries and the presence of delta phase, but it was not noticeable on the loose powder kept in air under normal laboratory illumination. These findings have strong implications on the strategies for improving the stability of FAPI without diminishing its photovoltaic efficiency through modifications of its composition

Frequency Dependent Rheology of Vesicular Rhyolite

Frequency dependent rheology of magmas may result from the presence of inclusions (bubbles, crystals) in the melt and/or from viscoelastic behavior of the melt itself. With the addition of deformable inclusions to a melt possessing viscoelastic properties one might expect changes in the relaxation spectrum of the shear stresses of the material (e.g., broadening of the relaxation spectrum) resulting from the viscously deformable geometry of the second phase. We have begun to investigate the effect of bubbles on the frequency dependent rheology of rhyolite melt. The present study deals with the rheology of bubble-free and vesicular rhyolite melts containing spherical voids of 10 and 30 vol %. We used a sinusoidal torsion deformation device. Vesicular rhyolite melts were generated by the melting (at 1 bar) of an Armenian obsidian (Dry Fountain, Erevan, Armenia) and Little Glass Mountain obsidian (California). The real and imaginary parts of shear viscosity and shear modulus have been determined in a frequency range of 0.005–10 Hz and temperature range of 600°–900°C. The relaxed shear viscosities of samples obtained at low frequencies and high temperatures compare well with data previously obtained by parallel plate viscometry. The relaxed shear viscosity of vesicular rhyolites decreases progressively with increasing bubble content. The relaxation spectrum for rhyolite melt without bubbles has an asymmetric form and fits an extended exponent relaxation. The presence of deformable bubbles results in an imaginary component of the shear modulus that becomes more symmetrical and extends into the low-frequency/high-temperature range. The internal friction Q −1 is unaffected in the high-frequency/low-temperature range by the presence of bubbles and depends on the bubble content in the high-temperature/low-frequency range. The present work, in combination with the previous study of Stein and Spera (1992), illustrates that magma viscosity can either increase or decrease with bubble content, depending upon the rate of style of strain during magmatic flow

Towards Realistic Progenitors of Core-Collapse Supernovae

Two-dimensional (2D) hydrodynamical simulations of progenitor evolution of a 23 solar mass star, close to core collapse (about 1 hour, in 1D), with simultaneously active C, Ne, O, and Si burning shells, are presented and contrasted to existing 1D models (which are forced to be quasi-static). Pronounced asymmetries, and strong dynamical interactions between shells are seen in 2D. Although instigated by turbulence, the dynamic behavior proceeds to sufficiently large amplitudes that it couples to the nuclear burning. Dramatic growth of low order modes is seen, as well as large deviations from spherical symmetry in the burning shells. The vigorous dynamics is more violent than that seen in earlier burning stages in the 3D simulations of a single cell in the oxygen burning shell, or in 2D simulations not including an active Si shell. Linear perturbative analysis does not capture the chaotic behavior of turbulence (e.g., strange attractors such as that discovered by Lorenz), and therefore badly underestimates the vigor of the instability. The limitations of 1D and 2D models are discussed in detail. The 2D models, although flawed geometrically, represent a more realistic treatment of the relevant dynamics than existing 1D models, and present a dramatically different view of the stages of evolution prior to collapse. Implications for interpretation of SN1987A, abundances in young supernova remnants, pre-collapse outbursts, progenitor structure, neutron star kicks, and fallback are outlined. While 2D simulations provide new qualitative insight, fully 3D simulations are needed for a quantitative understanding of this stage of stellar evolution. The necessary properties of such simulations are delineated.Comment: 26 pages, 1 table, 4 figure

Competition between Polar and Antiferrodistortive Modes and Correlated Dynamics of the Methylammonium Molecules in MAPbI3_3 from Anelastic and Dielectric Measurements

The mechanisms behind the exceptional photovoltaic properties of the metallorganic perovskites are still debated, and include a ferroelectric (FE) state from the ordering of the electric dipoles of the organic molecules. We present the first anelastic (complex Young's modulus) and new dielectric measurements on CH$_{3}$NH$_{3}$PbI$_3$, which provide new insight on the reorientation dynamics of the organic molecules, and the reason why they do not form a FE state. The permittivity is fitted within the tetragonal phase with an expression that includes the coupling between FE and octahedral tilt modes, indicating that the coupling is competitive and prevents FE ordering. The onset of the orthorhombic phase is accompanied by sharp stiffening, analogous to the drop of permittivity, due to the hindered molecular dynamics. On further cooling, an intense anelastic relaxation process without a dielectric counterpart suggests the reorientation of clusters of molecules with strong antiferroelectric correlations.Comment: accepted in J. Phys. Chem. Let

Zonal flow regimes in rotating anelastic spherical shells: an application to giant planets

The surface zonal winds observed in the giant planets form a complex jet pattern with alternating prograde and retrograde direction. While the main equatorial band is prograde on the gas giants, both ice giants have a pronounced retrograde equatorial jet. We use three-dimensional numerical models of compressible convection in rotating spherical shells to explore the properties of zonal flows in different regimes where either rotation or buoyancy dominates the force balance. We conduct a systematic parameter study to quantify the dependence of zonal flows on the background density stratification and the driving of convection. We find that the direction of the equatorial zonal wind is controlled by the ratio of buoyancy and Coriolis force. The prograde equatorial band maintained by Reynolds stresses is found in the rotation-dominated regime. In cases where buoyancy dominates Coriolis force, the angular momentum per unit mass is homogenised and the equatorial band is retrograde, reminiscent to those observed in the ice giants. In this regime, the amplitude of the zonal jets depends on the background density contrast with strongly stratified models producing stronger jets than comparable weakly stratified cases. Furthermore, our results can help to explain the transition between solar-like and "anti-solar" differential rotations found in anelastic models of stellar convection zones. In the strongly stratified cases, we find that the leading order force balance can significantly vary with depth (rotation-dominated inside and buoyancy-dominated in a thin surface layer). This so-called "transitional regime" has a visible signature in the main equatorial jet which shows a pronounced dimple where flow amplitudes notably decay towards the equator. A similar dimple is observed on Jupiter, which suggests that convection in the planet interior could possibly operate in this regime.Comment: 20 pages, 15 figures, 4 tables, accepted for publication in Icaru