Measurement of a Magnon Chemical Potential
For a system in equilibrium, the chemical potential of particles whose numbers are not conserved (e.g. phonons, magnons) is zero. However, for a system out-of-equilibrium, such as one where a temperature gradient is applied, there are length and time scales over which the phonon and magnon numbers are approximately conserved. For these length and time scales one may define a chemical potential for the excitations.
In this work, we used a heating laser in conjunction with a spatially separated probe laser to measure the independent local temperatures of the phonons and magnons in yittrium iron garnet (YIG). (See figure on right.) From the length dependence of these differences one can extract the magnon chemical potential, the first time this quantity has been directly measured.