Dynamic elasticity of complex materials

Science and technology in the 21st century will rely heavily on the development of new materials that are expected to respond to the environmental changes and manifest their own functions according to the optimum conditions, i.e. functional materials. Functional materials are distinctly different from structural materials, and their physical and chemical properties are sensitive to changes in the environment such as temperature, pressure or mechanical stress, electric field, magnetic field, optical wavelength, concentration of substituents, etc. Examples of complex functional materials are ferroelectrics, piezoelectrics, ferroelectric relaxors, ferroelastic materials, giant magnetoresistant and other complex electronic materials, etc. It is well known, that the macroscopic properties of complex materials are widely determined by intermediate scale (mesoscopic) structures which are often driven by elastic interactions. The present project is focussed on the influence of homogeneous and inhomogeneous strains and stresses on the thermodynamic and dynamic behaviour of complex materials. We are targeting on the following major aims: (1) Study the influence of strains and microstructures to the physicals properties of materials (2) Explore crossover to criticality and dynamics of phase transitions (3) Investigate domains and domain walls in random media (4) Work on pressure induced phase transitions To reach these goals we are setting up a concerted combination of experimental work, computer simulations and analytical theory. The project will be run in cooperation with scientific Institutes in England, France, Germany, Slovenia, Poland and Austria. Many of the findings of the project are expected to have eminent implications for the development of smart and multifuntional materials. The results concerning the high pressure part will certainly have substantial impact into Earth Science.

Science and technology in the 21st century rely strongly on the development of new functional materials. These materials are sensitive to changes of the environment such as temperature, pressure or mechanical stress, magnetic or electric field, etc. The macroscopic response of the materials - which is important for practical applications - depends crucially on static and dynamic structures on various length scales, starting from few nanometres, over micrometres to macroscopic scales. Within the present project we have explored the role of (inhomogeneous and homogenous) strains and stresses for a wide range of materials including perovskites (forming about 60% of the Earth`s mantle), shape memory alloys, multiferroics and glass-forming liquids. We could achieve a breakthrough in two different subfields: Molecular glass-forming liquids: By measuring the dynamic elastic response of Salol, Toluene, o-TP and other molecular liquids we have for the first time determined the size and divergence of the dynamic correlation length near the glass transition temperature Tg . We have also studied the effect of confinement on the glass properties of the molecular liquids by confining them in nanometre sized porous materials. Domain wall motion in random media: In close cooperation with the group of Prof. E.K.H. Salje (University of Cambridge, U.K.) we have measured the dynamic elastic response in a number of perovskite materials and found superelastic softening due to the motion of ferroelastic domain walls. We have set up a novel theoretical model to describe the observed domains and their motion in response to an external dynamic stress. Although at a first glance ferroic materials are quite different from organic glasses, they can exhibit domains whose motion freezes at low temperatures, very similar to glass freezing. Understanding domain freezing would be a final goal in this field and would certainly help to understand glass freezing in general. The project`s outcome led to a deeper insight into the difficult field of glass freezing by opening a new door for studying dynamic heterogeneities by detailed measurements of the dynamic elastic susceptibility. The results concerning domain wall motion in ferroelastic materials are of big relevance for applications, since the macroscopic response of a material can be drastically enhanced by the presence of domains (see e.g. the giant piezoelectric response in relaxor ferroelectrics). They also have strong impact for Earth`s science, i.e. the seismic properties of our Earth are influenced by domain wall motion of ferroelastic materials which build a large part of our Earth interior. Understanding finally domain freezing in random materials may help to understand the 6000 years old problem of glass freezing.