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A nuclear magnetic resonance study of higher-order correlation functions in supercooled ortho-terphenyl

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By performing incoherent inelastic neutron scattering experiments on the Van der Waals glass o‐terphenyl we tested predictions made by a recent microscopic theory — the mode coupling theory of the glass transition. In agreement with theory we find — besides the structural relaxation or α‐process which follows a Kohlrausch‐Williams‐Watts behaviour — the existence of a secondary relaxation process which shows very unusual neutron scattering features. This secondary or ß‐process leads to an anomalous decrease of the Debye‐Waller factor which can be interpreted in terms of a critical (Tc‐T)‐behaviour as predicted by theory. According to mode coupling theory this behaviour should be connected to a critical slowing down of the density fluctuations in the vicinity of a critical temperature Tc which is situated typically 30 to 150 degrees above the caloric glass transition temperature Tg. We find our neutron scattering data to be compatible with the predictions of theory leading to a value of Tc that is situated well above Tg i.e. in the range 285–295 K as compared to Tg = 243 K. This agrees well with a value of Tc = 290 K derived independently by use of a power law, suggested by theory to hold for the behaviour of the viscosity at temperatures higher than Tc.