The Institute for Solid State Physics (IFP) focuses its research on electronic properties of condensed matter with emphasis on systems where quantum correlations and electronic interactions play a key role. The research is conducted within the HGF Programme Science and Technology of Nanosystems in the topic Condensed Matter and Molecular Building Blocks.
Quantum effects and electronic correlations in condensed-matter systems are not only manifest on the microscopic length scale, but govern the properties of materials on macroscopic length scales as well, thus forming a strong link to the study of quantum effects in nanostructures. Magnetism and superconductivity are such bulk-scale quantum phenomena. Superconductivity is governed by the properties of a macroscopic wave function. The Kondo effect, a hallmark of correlations in metals, is a many-body quantum effect entangling localized magnetic moments and conduction electrons. Again, by the laws of quantum mechanics, these local many-body quantum systems when arranged in a stoichiometric crystal, conspire to form a Fermi liquid, i.e., a quantum state of interacting electrons dubbed heavy fermions. Quantum phase transitions, i.e., transitions at absolute zero temperature, are governed by coherent quantum fluctuations essentially extending over the whole bulk specimen, provide yet another level of quantum effects. Hence research on quantum matter provides unique opportunity to identify new materials, and study and understand their properties.
We concentrate on projects that take advantage of two major research facilities. Our soft x-ray analytics beamline WERA at the Karlsruhe synchrotron ANKA gives insight into element-specific electronic and magnetic structure by performing electron spectroscopies like near-edge x-ray absorption (NEXAFS), photoelectron spectroscopy (PES), x-ray magnetic circular dichroism (XMCD), and photoemission electron microscopy (PEEM). Our thermal triple-axis neutron spectrometer 1T at the neutron reactor ORPHEE at the Laboratoire Léon Brillouin, CEA Saclay yields information on crystallographic and magnetic structures, lattice dynamics and magnetic excitations. Both facilities are operated by IFP as user facilities open for projects of researchers from around the world after peer-review approval of proposals.
Besides photon, photoelectron and neutron spectroscopies we conduct experiments probing thermodynamic properties such as specific heat and thermal expansion that yield information about low-energy excitations and collective phenomena at phase transitions as well as magnetic and electronic transport properties. The range of accessible parameter space encompasses temperature down to 20 mK, magnetic field up to 14 T, and hydrostatic pressures up to 6 GPa. A prerequisite for many experiments are our facilities to prepare single crystals and thin films of various material classes and to characterize them (x-ray diffractometry, electron microscopy, and Rutherford backscattering).
IFP focuses on quantum materials where often superconductivity and magnetism are in close proximity, possibly separated by a quantum phase transition. For superconducting materials, iron pnictides and chalcogenides are studied with various substitutions to map out superconducting and magnetic phase diagrams, and to understand and ultimately control the interplay between structural, magnetic, and superconducting properties. Charge-density-wave systems and the competition of charge-density waves with superconductivity are studied as well.
Magnetism of transition-metal oxides is being studied with focus on cobaltates and manganites to clarify the role of different interactions (Hund's rule coupling, crystal-field interactions, superexchange and double exchange, Jahn-Teller distortions, and electron-phonon coupling) and their competition, which lead to or affect different types of magnetic order. Experimentally, interactions can be tuned by applying magnetic fields, substitution and/or charge carrier doping as well as biaxial or hydrostatic pressure.
Our studies of thin films currently concentrate on iron-based superconductors, doped and un-doped cobaltates with perovskite structure, and interfaces between oxide insulators. Key questions concern the superconductor-insulator quantum phase transition in disordered FeSe thin films, the magnetic exchange in epitaxially strained cobaltate films, and the effect of strain and pressure on the two-dimensional electron gas generated at the interface of oxide heterostructures.
An important activity of IFP is in the field of quantum phase transitions, i.e., phase transitions that can be driven to zero temperature by variation of an external tuning parameter such as pressure, magnetic field or chemical composition. The proximity to a quantum phase transition facilitates the appearance of novel electronic or magnetic phases, forming a strong link to the IFP activities mentioned above because they are investigated in several material classes above. In addition, rare-earth intermetallic compounds and alloys, notably with Ce or Yb systems with their competition between magnetically ordered (RKKY) and paramagnetic (Kondo) ground states, are studied.
The activities of IFP's theory group are aimed at explaining and modelling phenomena investigated by our experimental studies outlined above, along with the prediction of novel behavior and novel states of matter in collective quantum systems.