Dynamic critical behavior in para-azoxyanisole from nuclear relaxation studies

Abstract

Contributions to the spin-lattice relaxation time (T ₁) and the spin-spin relaxation time (T ₂) due to critical fluctuations (CF) have been determined in para-azoxyanisole (PAA) from the corresponding proton relaxation times measured at 60 MHz over a temperature range of about 35 °K above the nematic-isotropic transition temperatureT _C .In determining the CF contributions, the procedure followed here is the same as the present authors used earlier for a similar study in p-methoxybenzylidene p-n butylaniline (MBBA). Like that in MBBA at 14 MHz, a maximum in the CF contribution toT ₁, has been observed. Following a recent theory we identify this contribution toT ₁ arising from the “nonlocal” part of CF, and we call thisT ₁)_CFN. The correlation time associated with this “nonlocal” part of CF,τ _CFN, has been determined at different temperatures fromT ₁)_CFN. The temperature dependence ofτ _CFN is in good agreement with the Landau-de Gennes theory and is consistent with that observed earlier in MBBA. The temperature dependence ofT ₂)_CF is also in reasonable agreement with the above behavior ofτ _CFN. However, ourT ₂)_CF data can also be fitted to a temperature dependence expected from a contribution toT ₂)_CF arising from the “local” part of CF, such a fitting is comparatively poorer than the former. Since ourT ₂)_CF data are neither sufficiently accurate nor elaborate, no attempt has been made here to separate these two contributions, “nonlocal” and “local” parts of CF, which have been predicted from theoretical considerations. A behavior similar to that of ourT ₂)_CF is also shown by the earlier data of Cabane and Clark obtained from¹⁴N resonance when their data are processed properly.