Chasing exotic magnetism by lattice vibrations
Distorting a crystal can affect its magnetic properties; vice versa, magnetic interactions can affect the lattice and its vibrations. By probing optical phonons – quantized sound waves excited by infrared radiation – we identify changes to the crystal symmetry related to the magnetic ground state of the widely studied material Herbertsmithite, ZnCu3(OH)6Cl2. This insulating compound is considered to exhibit an exotic magnetic phase where spin excitations can proliferate freely, termed ‘quantum spin liquid’.
Since electrons in a periodic lattice are coupled primarily to their nearest neighbors, a local change in distance alters their mutual interactions. This can happen also in a dynamic process, provided the time scale of distortions is long compared to the magnetic interaction. If the deformation is energetically favorable, the system tends to stay longer in this arrangement effectively slowing down the vibration. At low temperatures such a reduction of resonance frequency occurs also for the low-energy phonons of Herbertsmithite. Here we determine the atomic motions of all 54 phonon modes and identify the symmetry changes associated with the slowest lattice vibrations, providing insight into the coupling of magnetic and structural properties . Our findings nourish the idea of striped valence-bond-solid phases constituting the ground state of Herbertsmithite.