Synchrotron x-ray-scattering study of the normal-incommensurate phase transition in Rb{sub 2}ZnCl{sub 4}

The results of high-resolution synchrotron x-ray-scattering experiments on the normal-incommensurate phase transition in Rb{sub 2}ZnCl{sub 4} are reported. Measured critical exponents for the intensities of the first three harmonics of the incommensurate modulation wave below {ital T}{sub {ital c}} agree well with theoretical predictions for the {ital n}=2, {ital D}=3 (3D-{ital XY}) universality class. The scattering observed above {ital T}{sub {ital c}} corrresponds to critical fluctuations on two distinct length scales: the critical exponents for the short length scale component of the scattering agree with those expected for the 3D-{ital XY} universality class. The value found for the exponent of the inverse correlation length of the long length scale component is {nu}{sub {ital s}}=0.7{plus_minus}0.2. A comparison of data taken in Laue and Bragg geometries at different photon energies suggests that the second length scale scattering originates in the near-surface region. Possible explanations of the two length scale behavior are discussed. {copyright} {ital 1996 The American Physical Society.}