Resonance and optical spectroscopies
Resonance techniques applied to i) the nuclear spin system (nuclear magnetic resonance (NMR) & quadrupole resonance (NQR)) and ii) the electron spin system (electron spin resonance (ESR)) of strongly correlated f- and d- electron systems are ideal tools to study magnetic excitations at a microscopic level. Due to their local character and their high sensitivity these techniques are suitable for studying systems at the verge of magnetism. Here competing interactions such as the (mostly antiferromagnetic) RKKY interaction and the Kondo interaction lead to a magnetic instability and critical scaling of microscopic observables – the signature of a quantum critical point. In contrast to the well established antiferromagnetic quantum critical systems our research is focused on systems with tunable ferromagnetic criticality which is rarer. Since these are local probes we can also provide information about the degree of disorder which is an important feature in alloyed systems.
The variety of NMR & ESR applications is wide and we are not limited to metals. Correlated semimetals (Kondo insulators) as well as low dimensional spin systems (mostly oxides) are intensively studied by NMR and ESR. Recently we have focused our research on systems with strong spin orbit coupling (SOC). Prominent examples are the Kitaev honeycomb lattices where unconventional 4d- and 5d- magnetism can be studied by NMR. Others are the non- centrosymmetric helimagnets like FeGe where the Dzyaloshinsky - Moriya interaction twists the Heisenberg-type coupled spins and a new form of ferromagnetically ordered state, the Skyrmion lattice, could be found. Resonance experiments are conducted over a wide range of temperature (2 – 300 K) and field (up to 14 T). Additionally we have access to very low temperatures (via collaboration with the mK-NMR group of Prof. H. H. Klauss at TU Dresden) and very high fields (at the Dresden High Magnetic Field Laboratory, Rossendorf HZR).
In addition we use optical spectroscopy with a technique specialized for probing electronic charge dynamics and band structure in a broad energy region from the far-infrared up to the ultraviolet, at temperatures 2-300 K, and in magnetic fields up to 9 T. This provides important information on the energy- and temperature dependence of charge excitations in strongly correlated electron systems.