arXiv subjects

Sven Lohmann

Publications and source records attributed to Sven Lohmann.

3 recordsLinked to original sources

arXiv2026

Internal friction and frequency-dependent conductivity in the bivariate site energy model for the mixed alkali effect in glasses

The mixed alkali effect is fundamental to understanding ion transport in glasses. It manifests itself in strong changes of various ionic transport properties when one type of ion is successively replaced by another type. Recently, we have developed a new theory of thermally activated hopping in bivariate random site energy landscapes that takes into account site occupation statistics according to generalized Fermi distributions and mixture kinetics in accordance with linear irreversible thermodynamics. The theory was successfully applied to predict the behavior of partial conductivities and their activation energies at low frequencies corresponding to the dc limit. Here we show that the theory is capable to capture the salient features of the mixed alkali effect observed in ac ion conductivities and in the peculiar behavior of the internal friction.

cond-mat.mtrl-sci↗

arXiv2026

Theory for the mixed alkali effect in glasses

The mixed alkali or mixed mobile ion effect in glasses manifests itself by strong nonlinear variations of ionic transport properties upon mixing of different types of mobile ions. We develop a theory for this effect based on thermally activated hopping transport in disordered site energy landscapes that consistently incorporates the statistical-mechanical and kinetic aspects of a mobile ion mixture. This includes a consideration of the joint probability density of site energy states, generalized Fermi distributions for mean site occupations, and cross-terms in the current response described by nondiagonal Onsager coefficients. The theory shows that a mixed alkali effect can arise even when the two ion species share identical site energy distributions. It suffices that sites have distinct energies when occupied by ions of different type. Taking into account that a mismatch energy is needed for ions of one type to occupy sites adapted to the other type, the mixed alkali effect becomes stronger. Spatial correlations between site energies are needed for the mobility of the majority ion to decrease stronger than exponential upon replacement by the minority ion. The theory agrees well with kinetic Monte Carlo simulations. Application to mixed alkali phosphate glasses yields good agreement with measured conductivity activation energies.

cond-mat.mtrl-sci↗

arXiv2026

Universal time-temperature scaling of conductivities in random site energy and associated random barrier model

Universal time-temperature scaling of conductivity spectra in disordered solids has been explained by thermally activated hopping of noninteracting particles over random energy barriers. An open problem is whether the random barrier model accounts for site energy disorder in real materials. Through mapping many-particle hopping in a disordered site energy landscape to that of independent particles in a barrier landscape, we show that time-temperature scaling is correctly described by the associated barrier model in the low temperature limit. However, the site energy model displays good scaling behavior at substantially higher temperatures than the barrier model, in agreement with experimental observations. Extending the mapping to different types of mobile charge carriers allows us to understand why time-temperature superposition can be absent in mixed alkali glasses.

cond-mat.dis-nn↗