Magnetic resonance and optical spectroscopy on correlated electron systems & spin orbit entangled quantum magnets and metals

We investigate the spin dynamics of materials by applying magnetic resonance techniques to the nuclear spin system (nuclear magnetic resonance (NMR) and quadrupole resonance (NQR)) and to the electron spin system (electron spin resonance (ESR)). These techniques, due to their high sensitivity and local character, are ideal tools to study magnetic excitations locally at a microscopic level.

In addition and as a complementary property we examine the electronic charge dynamics by measuring the optical reflectivity in a broad energy region (far-infrared up to the ultraviolet) in order to characterize the electronic band structure.  

Magnetic resonance methods are particularly suitable to investigate systems at the verge of magnetic order where the spin dynamics is often strongly determined by quantum criticality. Here, ferromagnetic (FM) quantum criticality is of particular interest as it is rarely observed in contrast to systems with antiferromagnetic quantum criticality. In this respect we investigated the evolution of ferromagnetic correlations in alloyed systems like Ce(Ti,V)Ge3, Fe(Ga,Ge)3 and Ta(Fe,V)2 where the proximity to a FM quantum critical point can be tuned by composition.

We furthermore investigated the magnetic resonance in correlated semimetals such as Kondo insulators like Fe(Sb,Te)2 as well as in various low dimensional spin systems.

Recently we were focusing our methods to systems having strong spin orbit coupling (SOC) because the spin orbit entanglement brought a significant progress in the field of quantum magnets and quantum metals. As prominent examples of a quantum magnet we studied the NMR of Kitaev honeycomb lattices (Li2RhO3, RuCl3, and Ae2IrO3 (Ae=Li,Na) ) and non-centrosymmetric heli-magnets like FeGe and MnSi which features a new sort of magnetic texture within the ferromagnetically ordered state, the Skyrmion lattice. Furthermore, we started with extensive investigations of the newly synthesized delafossite NaYbS2 which is a perfect triangular spin ½ system. With that we tackled the field of 4f-containing spin ½ quantum magnets where a strong SOC is believed to yield new exotic ground states (including quantum spin liquids).

Spin orbit entanglement and topology are key ingredients for entire new classes of materials like the „topological insulators“ or the „Dirac- and Weyl semimetals“. Among the latter class we investigated various transition-metal monopnictides (Nb,Ta)Pn (Pn=P,As) by NMR and optical spectroscopy to probe the Weyl fermion excitations in the bulk.

Resonance and optical experiments are conducted over a wide range of temperatures (2-300K, ESR up to 1000K) and at magnetic fields up to 14 T. Recently we established NMR under pressure (up to 2 GPa). Furthermore we have access to temperatures below 2K in collaboration with the group of Prof. H.H. Klauss (TU Dresden) and with H. Kühne (HZDR Rossendorf research center) and to very high magnetic fields (Dresden High Magnetic Field Laboratory at HZDR). 


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