Correlations of fourth order in polarization fluctuations, the quantity whose ordering is frustrated, are observed in a structural glass via strictive coupling to sound. The results were obtained on a Rbi_ x (NH 4 ) x H 2 P0 4 crystal with Brillouin scattering. The existence of an Edwards-Anderson order parameter much above Vogel-Fulcher freezing is revealed. This suggests that the observed susceptibility slowing-down might, in many glasses, be controlled by a much slower hidden competing network.PACS numbers: 64.70. Ew, 62.65. +k, 78.35.+c Mixed crystals of Rb 1 _ x (NH 4 ) x H 2 P0 4 (RADP) form an excellent model system for the investigation of structural glasses. 1 The dynamics of polarization fluctuations (8P) was studied in detail for x = 0.35, a concentration near the ferroelectric (FE) side, but sufficiently high to prevent the paraelectric (PE) to FE transition. 2,3 It is described by a broad distribution of relaxation times r, with a temperature (T) dependent cutoff r c , that follows a Vogel-Fulcher (VF) law over a surprisingly broad region of 7 1 and r c ? The longwavelength linear susceptibility X(q, w) of interest in those measurements is not of course the response to a force producing the glass state, but is instead the susceptibility related to the frustrated FE state. This situation parallels exactly that of spin-glasses. Thus it is of considerable interest to investigate quantities of higher order in 8P, as these relate more directly to glass fluctuations. We have used Brillouin scattering to observe strains coupling electrostatically to 8P8P, but whose linear coupling to 8P is symmetry forbidden by the crystal point group, in contrast to measurements in Ref. 3. In this way, we have been able, apparently for the first time in a structural glass, to exhibit the dynamics of a correlation function of fourth order in the frustrated quantity, here 8P. Although the coupling of 8P8P fluctuations to the refractive index is too weak for direct observation, the contributions of those fluctuations to the elastic stiffness tensor C and to sound attenuation are easily seen thanks to the large electrostriction of this system.Brillouin scattering results for the longitudinal sound wave propagating along [100] are shown in Fig. 1. These were obtained in backward geometry (scattering angle -177°, collection angle -1°) on a good-quality single crystal with x = 0.35. The spectra were excited at 5145 A, and analyzed with a tandem spectrometer consisting of a tuned triple-pass planar Fabry-Perot interferometer (thickness -3.2 mm) followed by a scanned spherical inteferometer of thick-ness 25, 50, or 100 mm, depending on the resolution needed. This tandem system, and its locking by microwave modulation, has been previously described. 4 The sample, of size 10x5x5 mm 3 , had polished faces perpendicular to its long direction [100]. It was temperature stabilized in a He-flow cryostat. The laser was 33000 N X 5 32000 FREQUENCY v c 00 o o o 30000 « 7 V V "• cPo -• • i V • 1 • ^s,.
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