Physics of strongly correlated electron systems

Our department uses neutron and x-ray diffraction and spectroscopy, optical spectroscopy, and Raman scattering, supported by various supplementary methods, to explore the structure and dynamics of materials with strong electron correlations. Topics of particular current interest include the interplay between spin, charge, and orbital degrees of freedom in transition metal oxides; mechanisms of unconventional superconductivity in solids; and quantum many-body physics at oxide interfaces. We strongly believe that close collaboration between experimentalists and theorists is essential for progress in this field. To facilitate this interaction, a small theory group operates within the department. We also have a strong effort in the development of new spectroscopic methods, especially spectral ellipsometry with synchrotron radiation and neutron resonance spin-echo spectroscopy. To this end, we operate experimental facilities at the ANKA synchrotron in Karlsruhe and at the FRM-II research reactor in Garching, in addition to our in-house laboratories. The recently commissioned TRISP spectrometer at the FRM-II allows the determination of the lifetimes of collective excitations in solids with unprecedented accuracy.

Our research projects Unconventional superconductivity The microscopic description of superconductivity in complex materials such as layered cuprates, cobaltates, or the recently discovered iron pnictides, is one of the most important challenges in current solid-state physics. Our group uses high-quality single crystals and state-of-the-art experimental methods to derive accurate spectra of spin and charge excitations in these materials. Such data are essential to motivate and test new theoretical concepts for the correlated electron systems that support unconventional superconductivity.

Low-dimensional magnetism The discovery of high-temperature superconductivity has stimulated a tremendous upsurge of interest in the quantitative understanding of low-dimensional quantum magnets. Experiments performed in our group elucidate the magnetic structure and dynamics of one- and two-dimensional magnets and their influence on charge transport. Novel compounds synthesized by chemists at our institute are of particular importance.

Orbital physics The exceptionally rich phase behavior observed in transition metal oxides originates in a competition between many-body states with different spin, orbital, and charge ordering patterns. Work in our group seeks to unravel the microscopic mechanisms underlying this competition. To this end, spectroscopic data on orbitally degenerate transition metal oxides obtained in our group are analyzed and interpreted in close collaboration with theorists.

Oxide heterostructures Carefully controlled interfaces between two materials can give rise to novel physical phenomena and functionalities not exhibited by either of the constituent materials alone. Modern synthesis methods have yielded high-quality heterostructures and superlattices of oxide materials with competing quantum many-body states. In order to explore new correlation-driven interface phenomena, our group seeks to understand and manipulate the spin and orbital polarization at oxide interfaces (“orbital engineering”).