Counting discrete emission steps from intrinsic localized modes in a quasi-1D antiferromagnetic lattice

Intrinsic localized modes (ILMs) in a quasi-1D antiferromagnetic material (C2H5NH3)2CuCl4 are counted by using a novel nonlinear energy magnetometer. The ILMs are produced by driving the uniform spin wave mode unstable with an intense microwave pulse. Subsequently a subset of these ILMs become captured by and locked to a cw driver so that their properties can be examined at a later time with a tunable cw low power probe source. Four wave mixing is used to enhance the emission signal from the few large amplitude ILMs over that associated with the many small amplitude plane wave modes. A discrete step structure observed in the emission signal is identified with individual ILMs becoming unlocked from the driver. At most driver power and frequency settings the resulting emission step structure appears uniformly distributed; however, sometimes, nearby in parameter space, families of emission steps are evident as the driver frequency or power is varied. Two different experimental methods give consistent results for counting individual ILMs. Because of the discreteness in the emission both the size of an ILM and its energy can be estimated from these experiments. For the uniformly distributed case each ILM extends over ~42 antiferromagnetic unit cells and has an energy value of 1.3x10^-12 [J] while for the case with families the ILM length becomes ~54 antiferromagnetic unit cells with an energy of 1.5x10^-12 [J]. An unresolved puzzle is that the emission step height does not depend on experimental parameters the way classical numerical simulations suggest.