Variation of nonlinearity parameter at low fundamental amplitudes

Abstract

Recent harmonic generation measurements of the nonlinearity parameter β in polycrystalline Cu–Al alloys have shown a transition to lower values at low fundamental amplitude levels. Values for β at high (>10 Å) fundamental levels are in the range predicted by single-crystal second- and third-order elastic constants while lower fundamental levels (<4 Å) produce values that are a fraction of those predicted. The effect is believed genuine and has been demonstrated recently in a commercial aluminum alloy by others. The source of the effect is unclear but initial results may require a reexamination of current methods for measurement of third-order elastic constants.