Dynamic generation of scalar chirality and topological Hall effect in spiral magnets

Igor Mazin, George Mason University

The concept of scalar chirality (SS) was introduced in the 1990s by Kubo
and others, as a measure of noncoplanarity of a spin texture. In a triangle, it is defined simply as the triple product of the three spins.
In a continuous medium, it can be generalized as a triple product of the magnetization and its two spatial derivatives. It has generated considerable interest in the last few years, after it was shown that SS generates a nontrivial contribution to the Hall effect, dubbed
Topological Hall Effect (THE). Intriguingly, a THE was discovered in a few materials where the known crystal and magnetic structure could not afford SS. Hidden phases of unknown nature, not detected by usual means, were hypothesized to explain these observations.
In this talk, I will present a theory that explains how SS can emerge in an external field in a particular class of non-chiral textures, in a manner similar to emergence of nematic order without underlying Magnetic order in Fe-based superconductors. The essence of the theory is extremely basic: we show that while single-spiral states do not support SS, in some cases a single magnon can generates SS, which then couples with the external magnetic field. While both positive and negative SS
are generated, thermodynamically one sign is preferred. The resulting THE amplitude has a simple temperature and magnetic field dependence, confirmed by the experiment. I will discuss three examples, of which two are predicted to have such fluctuation-driven THE (and they do), and
the third is not (and it doesn’t).

PDF file of the talk available here