The superconducting and magnetic properties of the iron-chalcogenides

Abstract

Superconductivity is a remarkable phenomenon that was discovered exactly 100 years ago by H. Kamerlingh-Onnes. Many famous physicists such as N. Bohr, W. Heisenberg, or A. Einstein, to name only a few, tried for more than 50 years to describe the mechanism that leads to superconductivity. Only in 1957 a theory was suggested by J. Bardeen, L. Cooper, and J. Schrieffer, that was widely accepted. 25 years ago the striking finding of high temperature superconductivity in copper based materials, the so-called cuprates, by J. G. Bednorz and K. A. M¨ller revolutionized the field of superconductivity. Whereas the superconductingu transition temperature in the materials known till then reached values of only 23 K, in the cuprates it approaches approximately 140 K. Since then a great effort has been made towards the understanding of the mechanism of high temperature superconductivity and the microscopic pairing mechanism. However, it remains one of the biggest mysteries in physics. Obviously the high temperature superconductors bear still lots of surprises as ten years ago the diborides were discovered to be superconducting and recently, only three years ago, the finding of the iron-based high temperature superconductors attracted again the attention to the field. To find the microscopic mechanism leading to superconductivity in the iron-based high temperature superconductors might help to resolve the mystery of high temperature superconductivity in general. This thesis is focused on the simplest of the iron-based high temperature superconductors, namely the binary FeCh family. Here Ch stands for the chemical elements belonging to the chalcogenide group like Sulfur (S), Selenium (Se), and Tellurium (Te). It is the simplest among this class because of its simple crystallographic structure consisting of a stack of FeCh layers. Furthermore, it is an ideal modeling system for the other iron-based superconductors because of its simplicity and its similarity with their electronic structure. The electronic phase diagrams of the FeCh family contain the appearance of different ground states. Whereas the mother compounds are in general antiferromagnetically ordered, the material becomes superconducting after going through a region where superconductivity and magnetism coexist. In the framework of this thesis, the FeCh system was tuned solely by changing the lattice either by hydrostatic or chemical pressure and without introducing additional charge carriers. The muon spin rotation/relaxation/resonance (µSR) technique in combination with ac and dc magnetization experiments is an ideal tool to investigate the superconducting and magnetic states and the interplay in a sense of competition and/or coexistence between them. It can be seen that the system is extremely sensitive to pressure. FeSe1−x at ambient pressure is superconducting and nonmagnetic. Upon applying hydrostatic pressure the superconducting transition temperature increases and exhibits one of the biggest pressure effects known. Surprisingly, the compound features the appearance of magnetism that coexists on atomic length scales with superconductivity at high pressures. A similar effect is observed if chemical pressure is applied by substituting Se by the isovalent Te