First-principles Investigations of Advanced Ferroelectrics and Superconducting Materials

Aim of the present project is to study the structural stability, elastic, lattice dynamical, and optical properties of ferroelectric and superconducting materials by means of first-principles calculations. Systematic investigations of many different compounds will give insight into the common features and differences within these types of materials and thus lead to an understanding of their physical properties. A major part of this work will be the study of phase transitions by linear response theory and the resulting changes of physical properties. The investigations will also comprise the implementation of elastic-constants calculations in the full-potential LAPW code.

The aim of this project was to systematically study superconducting and ferroelectric materials with parameter-free methods based on density functional theory. At first sight, the two classes of materials are very different. While ferroelectrics are characterized by very high dielectric constants, superconductors carry electric currents without loss. Having a closer look, however, they do have something in common, which is the importance of lattice vibrations (phonons) and their vicinity to phase transitions. Ferroelectric materials often exhibit structural phase transitions induced by temperature or pressure, which are in many cases associated with the softening of phonons. This turned out to be the case for SrBi 2 Ta2 O9 were an unstable mode was found in our calculations. The different phases can exhibit very different physical properties. This structure-property relationship also plays an important role for the high temperature superconductors. Only upon doping the materials undergo transitions from antiferromagnetic insulators to metallic phases which become superconducting at low temperatures. Slight modifications in the composition as well as pressure can lead to dramatic effects in the superconducting transition temperature. For these reasons a variety of high Tc compounds have been studied as a function of composition, doping, and pressure. One of the main results was to show the relationship between these three parameters with the creation of the charge carriers, which are responsible for the superconducting current. In particular, for the Hg based superconducting cuprates, a systematic study was performed revealing the actual amount of charge carriers created in the copper-oxygen planes, and also some limiting facts for this process. Moreover, the calculation of phonons and Raman scattering intensities for different representatives of the cuprates could shed light on the role of phonons in these compounds. While this role is, however, not satisfactorily clarified in this class of materials up to now, a new promising family of superconductors has been proposed more recently which are clearly phonon- mediated. These are MgB2 and related materials. For one of them, i.e. hole-doped LiBC, we could calculate the superconducting transition temperature as a function of hole doping to reach 65K. This value is somewhat lower than originally proposed, but is still the highest calculated Tc so far.