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Spatial scale dependence of diffusion process in glass-forming liquids

Kofu, Maiko   ; Faraone, A.*; Tyagi, M.*; Nagao, Michihiro*; Yamamuro, Osamu*

The self-diffusion of molecules in a liquid is basically governed by Fick's law. It is known that the self-part of the van Hove correlation function is written as the Gaussian function with respect to $$r$$ and decays exponentially with time (Debye relaxation). In the case, the relaxation time obeys the relation $$tau propto Q^{-2}$$. In glass-forming materials, however, the relaxation process is often described by the non-Debye KWW function, $$exp [-(t/tau)^{beta}]$$. A natural question arises as to whether the relation $$tau propto Q^{-2}$$ is valid for the non- Debye diffusion process. To address this issue, we have investigated in detail the $$Q$$-dependence of the relaxation times for two different types of liquid, a molecular liquid (3-methylpentane) and an ionic liquid (C4mimTFSI). The measurements were done using neutron spin echo, backscattering and chopper spectrometers at NIST. Two crossover points were clearly found. The first crossover can be due to coupling with structural relaxation and the second one is Gaussian to non-Gaussian transition. This is the first clear experimental evidence for the two crossover.



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