SYNCHROTRON X-RAY-SCATTERING STUDY OF THE NORMAL-INCOMMENSURATE PHASE-TRANSITION IN RB2ZNCL4

The results of high-resolution synchrotron x-ray-scattering experiment s on the normal-incommensurate phase transition in Rb2ZnCl4 are report ed. Measured critical exponents for the intensities of the first three harmonics of the incommensurate modulation wave below T-c agree well with theoretical predictions for the n=2, D=3 (SD-XY) universality cla ss. The scattering observed above T-c corrresponds to critical fluctua tions on two distinct length scales: the critical exponents for the sh ort length scale component of the scattering agree with those expected for the 3D-XY universality class. The value found for the exponent of the inverse correlation length of the long length scale component is nu(s)=0.7+/-0.2. A comparison of data taken in Laue and Bragg geometri es at different photon energies suggests that the second length scale scattering originates in the near-surface region. Possible explanation s of the two length scale behavior are discussed.