Resonance splitting of phonons in Al0.965Ag0.035

Resonance vibrations of heavy defects can interact with phonons. For low defect concentrations, this leads to a broadening and distortion of the phonon dispersion, whereas for higher concentrations and low resonance frequencies a splitting should occur. Neutron inelastic scattering measurements in the system Al_0.965Ag_0.035 show the predicted splitting. The observed resonance frequency agrees well with the one predicted for the pure mass defect.     

Polarographie

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Inferring Boolean functions via higher-order correlations

Both the Walsh transform and a modified Pearson correlation coefficient can be used to infer the structure of a Boolean network from time series data. Unlike the correlation coefficient, the Walsh transform is also able to represent higher-order correlations. These correlations of several combined input variables with one output variable give additional information about the dependency between variables, but are also more sensitive to noise. Furthermore computational complexity increases exponentially with the order. We first show that the Walsh transform of order 1 and the modified Pearson correlation coefficient are equivalent for the reconstruction of Boolean functions. Secondly, we also investigate under which conditions (noise, number of samples, function classes) higher-order correlations can contribute to an improvement of the reconstruction process. We present the merits, as well as the limitations, of higher-order correlations for the inference of Boolean networks.     

Phonon-roton excitations in liquid 4He at negative pressures

We present neutron scattering measurements of the phonon-roton excitations of superfluid 4He held at negative pressures from zero to -5 bar. The liquid was stretched to negative pressures by immersing it in the porous medium MCM-41. In the wave vector range 0.35< or =Q< or =1.55 A(-1) and temperature T=0.4 K investigated, the phonon and maxon energies decrease systematically below bulk values as the negative pressure is increased. The energies are consistent with extrapolation of positive pressure values from which the negative internal pressure can be estimated. The maximum negative pressure realized is consistent with surface tension arguments and the MCM-41 pore diameter of 47 A.     

Pressure dependence of diffusion in simple glasses and supercooled liquids

Using molecular dynamics simulation, we have calculated the pressure dependence of the diffusion coefficient in a binary Lennard-Jones glass. We observe four temperature regimes. The apparent activation volume drops from high values in the hot liquid to a plateau value. Near the critical temperature of the mode coupling theory it rises steeply, but in the glassy state we find again small values, similar to the ones in the liquid. The peak of the activation volume at the critical temperature is in agreement with the prediction of mode coupling theory.