Matthias Gutmann

Materials are all around us and underpin our lifestyle and technology. Knowing their microscopic properties such as the crystal, magnetic and electronic structures is of key importance, since this leads to new insights into the mechanisms responsible for the macroscopic properties, which is required in order to develop materials for novel functionalities. The structural/magnetic degrees of freedom responsible for the crystal/magnetic structure are usually determined by elastic scattering, while the electronic degrees of freedom responsible for the electronic band structure are determined by inelastic scattering experiments. The coupling of the electronic, structural and magnetic degrees of freedom (called also electronic-structural interplay) is responsible for macroscopic properties such as metallic, semiconducting or insulating states. Many materials found in current technologies, such as ferroelectrics (finite polarization), multiferroics (finite polarization and magnetization) and barocalorics (strong reversible thermic responses to pressure), are a direct consequence of the electronic-structural interplay. Functionalities related to materials containing elements with unfilled d or f atomic shells (called correlated materials), are a consequence of the temperature dependence of the electronic-structural interplay. In this proposal we aim to study the electronic-structural interplay, at finite temperatures, in correlated materials using the state of the art theoretical and experimental methods. We would also like to address the temperature dependence of the electronic-structural interplay in order to get a better understanding of what is responsible for phase transitions, the electronic or the structural degrees of freedom. To achieve our goals, we are using the synergy of state-of-the-art experimental techniques at Rutherford laboratory (in particular diffuse scattering), experimental and theoretical techniques applied to barocalorics (such as DFT and molecular dynamics) through collaborations with the group of Prof. Felix Fernandez-Alonso based at the Materials Physics Center in San Sebastian, Spain and theoretical techniques such as density functional theory plus embedded dynamical mean field theory (DFT+eDMFT) through collaborations with the theory group of Prof. Gheorghe Lucian Pascut at Stefan cel Mare University of Suceava, Romania. Diffuse scattering is a rather novel technique providing detailed insights of the crystal collective vibrations or any type of disorder inside the crystal, thus materials information such as the elastic tensor as well as the sound velocity. DFT+eDMFT is our method of choice to study the electronic and magnetic properties of correlated materials. The recent development of forces for structural relaxations at finite temperatures and the implementation of efficient ways to compute the lattice vibrations, makes it the state-of-the-art method to study the electronic-structural interplay in correlated materials and its temperature dependence. The applicants novel experimental techniques are suitable for studies of many classes of materials, thus by visiting both the group of Prof. Felix Fernandez-Alonso and the group of Prof. Gheorghe Lucian Pascut, is aiming to bring together collaborators from different fields, such as ferroelectrics, multiferroics and barocalorics in order to shed new light on the old subjects (ferroelectrics, multiferroics) and bring new insights on the current subjects (barocalorics). Recently, barocalorics are of great interest in cooling applications with the potential to replace toxic refrigerants in the future. To close the triangle and foster new collaborations, Prof. Gheorghe Lucian Pascut will also visit Prof. Felix Fernandez-Alonso's group at the same time with the applicant. Secondary outcomes of these research visits are to generate new users for our facilities on the Rutherford Appleton Laboratory campus.