24 research outputs found
Harmonic behavior of metallic glasses up to the metastable melt
In two amorphous alloys ZrTiCuNiBe and ZrAlNiCu coherent neutron scattering has been measured over five decades in energy, including measurements in the metastable melt of a metallic alloy more than 80 K above Tg. In the vibrational spectra a pronounced "boson" peak is found: Even in crystallized samples the density of states exceeds the Debye Ï2 model, and in the amorphous state low-frequency vibrations are further enhanced. The peak position shows no dispersion in q, while intensities are strongly correlated with the static structure factor. Over the full energy range the temperature dependence is strictly harmonic. From high-energy resolution measurements we establish lower bounds for the temperatures at which structural α and fast ÎČ relaxation become observable
Fractal geometry of rocks
The analysis of small- and ultra-small-angle neutron scattering data for sedimentary rocks shows that the pore-rock fabric interface is a surface fractal (Ds = 2.82) over 3 orders of magnitude of the length scale and 10 orders of magnitude in intensity. The fractal dimension and scatterer size obtained from scanning electron microscopy image processing are consistent with neutron scattering data
Fast relaxation in a fragile liquid under pressure
The incoherent dynamic structure factor of ortho-terphenyl has been measured
by neutron time-of-flight and backscattering technique in the pressure range
from 0.1 MPa to 240 MPa for temperatures between 301 K and 335 K.
Tagged-particle correlations in the compressed liquid decay in two steps. The
alpha-relaxation lineshape is independent of pressure, and the relaxation time
proportional to viscosity. A kink in the amplitude f_Q(P) reveals the onset of
beta relaxation. The beta-relaxation regime can be described by the
mode-coupling scaling function; amplitudes and time scales allow a consistent
determination of the critical pressure P_c(T). alpha and beta relaxation depend
in the same way on the thermodynamic state; close to the mode-coupling
cross-over, this dependence can be parametrised by an effective coupling Gamma
~ n*T**{-1/4}.Comment: 4 Pages of RevTeX, 4 figures (submitted to Physical Review Letters
Enantioselective Synthesis of (-)-Roccellaric Acid
A new strategy for the synthesis of anti-4,5-disubstituted -butyrolactones starting from inexpensive furan-2-carboxylic methyl ester was developed. By applying this methodology, the enantioselective synthesis of (-)-roccellaric acid ((-)-17) was accomplished using a copper(I)-catalyzed asymmetric cyclopropanation, a tin(IV)-catalyzed retroaldol/lactonization sequence of cyclopropanols, and a ruthenium-catalyzed intermolecular metathesis reaction as key steps