Wavenumber dependence of structural α relaxation in a molecular liquid

(received ; accepted) PACS. 61.20Lc – Relaxations, time dependent properties. PACS. 61.25Em – Molecular liquids. PACS. 64.70Pf – Liquid glass transition. Abstract. – Structural α relaxation in liquid orthoterphenyl is studied by means of coherent neutron time-of-flight and backscattering spectroscopy over a large temperature range.Not only amplitude and relaxation time but also the spectral line shape show a significant variation with wavenumber. As expected from mode coupling theory, these variations are correlated with the static structure factor. Even far above the melting point, α relaxation remains non-exponential. The most outstanding features of structural α relaxation in glass-forming liquids are a strong temperature dependence of the relaxation time and the stretching phenomenon of the response function [1]. Very often, the correlation function Φ X (t) of some observable X obeys time temperature superposition (TTS): by a shift of the characteristic relaxation time τ X (T) the response functions at different temperatures can be superimposed Φ X (t; T) = ˆ Φ X (ˆ t), withˆt = t/τ X (T). While the master functionˆΦ X (ˆ t) varies weakly with temperature, the scale τ X (T) increases rapidly upon cooling. The other remarkable feature is the stretching phenomenon: the decay of Φ X extends over a wider time range than expected for exponential relaxation; empirically, it is generally well described by the Kohlrausch function (KWW) Φ X (t) = A X exp[−(t/τ X) βX ], β X < 1. (1) Different observables X may have not only different relaxation times τ X but also different stretching exponents β X. In light scattering, for instance, β can be changed by just switching between polarized and depolarized geometry. Uncountable experiments have confirmed both observations for times varying from nanoseconds to hours, days, or longer. A microscopic explanation of stretched relaxation has been given by mode coupling theory (MCT) [2]. Within MCT, time correlations decay in a two-step process which becomes