Recent trends of anelastic spectroscopy - how to study the dynamics of most types of defects and any type of phase transitions in any material, and more.

An overview is presented of how the measurement of the complex elastic moduli as a function of temperature and frequency, often called mechanical or anelastic spectroscopy (AS), provides information and insight in many fields of solid state physics and materials science. Rather than focusing on topics where the use of AS is consolidated, like polymers, glasses and metallurgy, topics of timely interest are considered, aligning with application-driven research on functional properties or their degradation. Notable examples are phase transitions, not only ferroelastic, but also ferroelectric, magnetic and electronic and point defects. The research on ultralow damping, already critical in electronics, communication and metrology is gaining renewed relevance for the improvement of gravitational wave detecors and it will be even more so for qubit technology, where decoherence is due to the interaction with low energy excitations and hence damping. In order to fill the communication gap with more familiar spectroscopies, AS is also briefly introduced using the formalism of spectral density. In addition, a few examples are mentioned where the combined use of AS with other spectroscopies is particularly informative.