Nonlinear Magnetic Response of a Putative Quantum Spin Liquid
In a paramagnet such as common salt or a piece of aluminum the number of electrons that align with an external magnetic field, H, increases linearly initially with field. But as all spins line up the magnetization, M, saturates. Thus, the leading order correction to the linear susceptibility, M/H, is always negative and diverges as the temperature T is lowered towards absolute zero. This divergence is destroyed when spins correlate and the nonlinear response thus can provide unique insights into magnetic order. In particular, preponderance of quantum effects can imprint their signatures in the nonlinear magnetic response and assist in revealing the mysteries of the magnetic order. Spotting enhanced nonlinear terms experimentally is unique and as such becomes even more valuable when combined with theoretical expectations. This is precisely what Profs. Shivaram and Gia-Wei Chern working with their students have demonstrated in a recent a publication in the Nature Journal on Quantum Materials (https://www.nature.com/articles/s41535-021-00364-z). Focusing on α-RuCl3, a leading physical realization of a theoretical construct by Kitaev, they have discovered an unexpected quadratic field dependence of the magnetization, which arises only when the field is in the ab-plane of the α-RuCl3crystal, below a certain temperature, Tc. They also find unusually large and positive higher order cubic terms, both below and above Tc crossing zero only at a very large temperature T > 50 K. These findings shed new light on the physics of Kitaev materials which are strong candidates for a new state of matter called quantum spin liquid. These novel magnetic states have been proposed as platforms to realize topological quantum computing. In addition to the UVa researchers the work presented in the publication also involved an International collaboration with Germany and the US Naval Research Labs.