Crossover of Microscopic Dynamics in Metallic Supercooled Liquid Observed by NMR

Abstract

Nuclear magnetic resonance is used to characterize local atomic motions in the glassy and supercooled liquid states of the bulk metallic glass system Pd 43 Ni 10 Cu 27 P 20 . The temperature dependence of the Knight shift reveals that certain local atomic motion decreases rapidly below a crossover temperature T c down to the glass transition temperature T g . Above T c as well as below T g the meansquared amplitude of local motions depends linearly on the temperature. The observed gradual transition below T c is inconsistent with heterogeneity effects. It reveals that qualitative changes of microscopic properties in the supercooled liquid take place at temperatures significantly above T g . DOI: 10.1103/PhysRevLett.91.265502 PACS numbers: 63.50.+x, 61.43.-j, 76.60.-k An important issue of the nature of glass transition is whether it is a manifestation of qualitative changes of the microscopic property above T g or a hidden phase transition below T g [1,2]. Among various theories, the modecoupling theory (MCT) predicts a qualitative change in the microscopic dynamics in the supercooled liquid at a crossover temperature T c significantly above. In a liquid of densely packed atoms, an atom is temporarily trapped inside the cage formed by neighboring atoms and undergoes vibrations and rattling before escaping the cage [3,4]. Here, the time dependence of the position vectorr r i t of a given atom i can be described byr r i t R R i t ũ u i t where the rapidly changingũ u i t describes local motions with time average hũ u i ti 0 and the slowly varyingR R i t describes the transition of the atom from one cage to another [5]. Consistent with this picture, MCT predicts that q; t, the correlation function of density fluctuations with wave number q, undergoes a two-step relaxation process, called fast relaxation and relaxation [3,4]. An important prediction of MCT is the temperature (T) dependence ofũ u i associated with the fast relaxation [3,4]. The effect ofũ u i contributes to an effective Debye-Waller factor f q expÿ2W. MCT predicts that f q increases critically from f q f c q above a crossover temperature T c > T g to f q f c q h q " p below T c where " T c ÿ T=T c and h q is an amplitude. Experimentally, the separation between the time scales of and fast relaxations in the temperature range of interest is often insufficient for model-independent determination of f q by neutron scattering [6]. So far, measurements of the effects ofũ u i have not revealed the critical behavior of f q in metallic supercooled liquids [7][8][9]. A recent diffusion measurement [10], which measures the effect ofR R i t, showed changes of diffusion mechanisms in the supercooled liquid region of Pd 43 Ni 10 Cu 27 P 20 , one of the best metallic glass formers [11,12]. Here, we introduce a nuclear magnetic resonance (NMR) method for probingũ u i in Pd 43 Ni 10 Cu 27 P 20 using the average Knight shift. The result reveals clearly a crossover behavior of the microscopic property in the supercooled liquid atThe preparation of high purity Pd 43...

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