In this seminar, Mr. Sano presented their study on magnon hydrodynamics for ultraclean ferromagnets. A set of coupled hydrodynamic equations for a magnon fluid are derived and the spin and thermal conductivities are studied by focusing on the most dominant time scales.
As a hallmark of the hydrodynamic regime, the speaker and his colleagues reveal that the ratio between the two conductivities shows a large deviation from the so-called magnonic WF law. The drastic breakdown of the magnonic WF law results from the difference in relaxation processes between spin and heat currents, which is unique to the hydrodynamic regime. Therefore, the presented results will become key evidence for an emergent hydrodynamic magnon behavior and lead to the direct observation of magnon fluids. In the seminar, researchers with different backgrounds (ranging from condensed matter physics to nuclear physics and cosmology) asked a lot of questions, and a lively discussion ensued.

Reported by Yuta Sekino