Abstract
Second harmonic light scattering due to molecular correlation in the paraelectric phase in sodium nitrite has been studied in the critical region. Measurements have been performed using a repetitive Q-switched YAG laser and single-crystal samples of good quality. The weak harmonic power, P2ω, scattered in the forward direction was detected by a photon-counting technique. A drastic enhancement of P2ω was observed as the Neel temperature was approached. The value of the critical exponent, v, obtained is 0.65 ± 0.05, which agrees well with the recent 3-dimensional Ising-model theory rather than the molecular-field one.

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