Dielectric Relaxation by Quantum Critical Magnons*

Quantum magnetism research aims to discover exotic phases of spin matter and quantum phase transitions between them. Low dimensionality and geometric frustration boost quantum fluctuations, stabilizing highly correlated states at low temperatures. Conversely, conventional multiferroics research focuses on practical applications of the interplay between ferroelectricity and magnetism, exploring typically classical effects. As a result, little overlap exists between these fields. 

In this talk, the potential of merging methods from both areas will be demonstrated. Investigating the magnetoelectric coupling in a quantum magnetic material leads not only to a deeper understanding of its magnetism and quantum criticality but also to uncover qualitatively new phenomena. In particular, the latest results on the quantum spin systems Rb₂Cu₂Mo₃O₁₂ and Cs₂Cu₂Mo₃O₁₂ will be reviewed. These are geometrically frustrated one-dimensional ferro-antiferromagnets that show rich magnetic phase diagrams with a variety of field-induced magnetic phases. A critical divergent electric susceptibility has been found in both systems at a magnetic quantum phase transition, providing a new way to explore criticality. In addition, we find novel quantum multiferroic behavior demonstrating for the first time the relaxation of electric dipoles mediated by magnons, which become soft at a quantum critical point.