Melting chiral magnetic crystals close to zero temperature

• Applicant: Professor Dr. Markus Garst
• Subject Area: Theoretical Condensed Matter Physics
• Term: from 2016 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 324327023

Ferromagnets whose atomic lattice is chiral and lacks inversion symmetry like the metal MnSi or the insulator Cu2OSeO3 possess a spatial modulated magnetization with a typical length scale inversely proportional to spin-orbit coupling. These spatially modulated magnetic textures can be interpreted as magnetic crystals. One- as well as two-dimensional crystals exist corresponding to the magnetic helix and the skyrmion lattice, respectively. Skyrmions are topologically non-trivial textures that strongly couple to itinerant electrons with spectacular consequences like an emergent electrodynamics. At the focus of this proposal is the theoretical investigation of the quantum phase transition between a paramagnet and such a one-dimensional magnetic crystal, i.e., the helix which can be considered as a melting or, alternatively, as a crystallization process. Correspondingly, two complementary approaches will be pursued. First, the defects of helimagnetic ordering will be classified and its properties will be investigated. Similarly to cholesteric liquid crystal, dislocations are especially important defects. Experiments show that dislocations play an important role in the relaxation of helimagnetic order, and we will examine their dynamics theoretically. In particular, we will investigate whether dislocations carry a topological charge similar to skyrmions which results in the emergent electrodynamics. The melting of the helix occurs via a proliferation of such dislocation defects. Following Kosterlitz and Thouless, we will develop an effective theory of defect-mediated melting of the helix at zero temperature. In addition, the quantum phase transition can be considered as a weak crystallization process, the theory of which was developed by Brazovskii in the 1970ies. This theory successfully describes the classical transition at a finite critical temperature Tc, and we plan to generalize it to the transition at Tc=0. The proposed research is motivated by the anomalous behavior of MnSi that is experimentally observed at high pressures where helimagnetic order is already suppressed. In this regime its electrical resistivity shows a temperature dependence T^{3/2}, that is at odds with Fermi liquid theory and remains unexplained now for years. In this context, we will check whether the scattering of electrons off dislocation lines may be able to account for this behavior. The aim of this proposed research is thus to elucidate the relation between topology, strong correlations and non-Fermi liquid behavior in metallic chiral magnets.

• DFG Programme: Research Grants
• Cooperation Partner: Professor Dr. Christian Pfleiderer