Variation of nonlinearity parameter at low fundamental amplitudes

Recent harmonic generation measurements of the nonlinearity parameter beta in polycrystalline Cu-Al alloys have shown a transition to lower values at low fundamental amplitude levels. Values for bat high (>10 Angstrom) fundamental levels are in the range predicted by single-crystal second- and third-order elastic constants while lower fundamental levels (<4 Angstrom) 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. (C) 1999 American Institute of Physics. [S0003-6951(99)00517-3].