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Dynamical Materials: From Atomic Scale Modeling via Machine Learning to Experiments

Gemeinsames Kolloquium des Fachbereichs Physik und des SFB 1083

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Abstract:
The dynamical behavior of materials at the atomic scale, i.e., the motion of individual atoms, is crucial not only for their thermodynamic stability but directly impacts their electronic, optical, and transport properties. Detailed insight into these dynamics is therefore fundamental for understanding and designing materials. By the very nature of the problem the length and time scales involved are extremely short. As a result, atomic scale modeling plays an important role in guiding and interpreting experimental studies, and discovering novel phenomena and mechanisms.
Traditional approaches of condensed matter physics are built on perturbation theory and typically assume a regular (crystalline) reference lattice. As the complexity of materials increases, for example, through dimensional engineering as in the case of layered materials or by integrating organic and inorganic components such as in hybrid perovskites, these approaches reach their applicability limit, both due to the explosion of the degrees of freedom and a failure of the underlying assumptions. Here, the combination of atomic scale simulations, correlation function based analysis, and machine learned potentials is emerging as a tool set that can lead to a paradigm shift in how we approach these questions.
In this presentation, I will show some recent work from my group that showcases the approach and illustrates its potential. I will focus on atomic dynamics in perovskites, a very large class of materials with wide-ranging applications in, for example, actuators, sensors, energy harvesting, and optical devices. I will show recent work concerned with the systematic construction of transferable and accurate models for these materials, including both inorganic and hybrid inorganic-organic materials. These models enable one to quantitatively analyze the dynamics associated with the phase transitions that are pivotal for the unique properties of perovskites. In particular, one can show that the so-called soft modes associated with these transitions exhibit overdamped behavior already hundreds of Kelvin above the actual transition temperature. This gives rise to a pronounced feature in the vibrational density of states in the zero-frequency limit, which is confirmed by quasi-elastic neutron scattering experiments. These results have implications for our understanding of the local structure in these materials, which is important for the electronic and optical properties.

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