Stefanie Russ

Acoustical properties of irregular and fractal cavities

Acoustical properties of irregular cavities described by fractal shapes are investigated numerically. Geometrical irregularity has three effects. First, the low-frequency modal density is enhanced. Second, many of the modes are found to be localized at the cavity boundary. Third, the acoustical losses, computed in a boundary layer approximation, are increased proportionally to the perimeter area of the resonator and a mathematical fractal cavity should be infinitely damped. We show that localization contributes to increase the losses. The same considerations should apply to acoustical waveguides with irregular cross section.

Excess modes in the vibrational spectrum of disordered systems and the boson peak

We study a disordered vibrational model system, where the spring constants k are chosen from a distribution

A new interpretation of the dynamic structure model of ion transport in molten and solid glasses

P(k)\ensuremath{\propto}1/k

Experimental study of a fractal acoustical cavity

above a cut-off value

Fractal geometry impact on nuclear relaxation in irregular pores

{k}_{\mathrm{min}}g0.

Ising-like dynamics and frozen states in systems of ultrafine magnetic particles

We can motivate this distribution by the presence of free volume in glassy materials. We show that the model system reproduces several important features of the boson peak in real glasses: (i) a low-frequency excess contribution to the Debye density of states, (ii) the hump of the specific heat

Pore opening effects and transport diffusion in the Knudsen regime in comparison to self- (or tracer-) diffusion

{c}_{V}(T)

Anderson localization in a correlated landscape near the band edge

including the power-law relation between height and position of the hump, and (iii) the transition to localized modes well above the boson peak frequency.

Eigenstates in irregular quantum wells: application to porous silicon

We explore progress in understanding the behaviour of cation conducting glasses, within the context of an evolving “dynamic structure model” (DSM). This behaviour includes: in single cation glasses a strong dependence of ion mobility on concentration, and in mixed cation glasses a range of anomalies known collectively as the mixed alkali effect. We argue that this rich phenomenology arises from the emergence during cooling of a well-defined structure in glass melts resulting from the interplay of chemical interactions and thermally driven ionic motions. The new DSM proposes the existence of a new site relaxation process, involving the shrinkage of empty Ā sites (thus tailored to the needs of A+ ions), and the concurrent emergence of empty C′ sites, which interrupt the conduction pathways. This reduction of Ā sites is responsible in the molten glass for the sharp fall in conductivity as temperature drops towards Tg. The C′ sites play an important role also in the mixed alkali effect, especially in regard to the pronounced asymmetries in diffusion behaviour of dissimilar cations.

Monte Carlo simulations of frozen metastable states in ordered systems of ultrafine magnetic particles

The resonance properties of a prefractal cavity are studied in an acoustical transmission experiment. Resonance frequencies and quality factors are measured and compared to theory. All the delocalized modes are detected, and their measured eigenfrequencies closely fit numerical predictions. Most of the localized modes appear to be missing in the experimental spectra because of their weak coupling with the acoustic excitation and detection. The measurement of the quality factor of the acoustic resonances confirms the existence of increased damping due to the irregular shape of the cavity. This constitutes the first experimental evidence for the damping power of fractal structures.