Ten questions on glassformers, and a real space `excitations' model with some answers on fragility and phase transitions

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Abstract
We formulate ten questions, covering outstanding aspects of the phenomenology of glassforming liquids, which we believe must be properly answered by any successful theory of structural glassformers. The questions range across thermodynamic, mass transport and vibrational dynamics phenomena. While these questions will only be addressed properly by a collective variables approach (many aspects of which are reported in these proceedings) a number of them can be dealt with by use of simple physical models of appropriate form. Here we discuss one such model in which the existence of elementary configurational excitations of the amorphous quasilattice is proposed. These states, which may range from broken bonds to packing defects, can be excited independently in the majority of cases, or cooperatively in others. We summarize essential results of this model. These suggest that the source of the different fragilities in liquids (and the reason that structural glasses, alone among `glassy' systems, have marked heat capacity jumps at Tg) may lie largely in the way these configurational excitations couple to the vibrational modes of the system. The generation of low frequency modes in the density of vibrational states, as a direct consequence of the excitation of configurational states, explains why the quasi-elastic scattering from fragile liquids is so much stronger near and above Tg than in the case of strong liquids, and why the normal glass transition can be detected in picosecond time scale experiments. Interactions among the `excitations', or `defects', are taken into account using the one component system equivalent of the binary system `regular solution' model (which keeps only the first order term of the free energy of mixing expansion). We show that a liquid-liquid first order transition must occur at sufficiently strong defect-defect interactions. The highly overconstrained amorphous silicon quasilattice is a strong candidate for such a transition. We identify the `first order melting' of amorphous silicon, and the sudden, reproducible, termination of supercooling in experimental liquid silicon and germanium, with the phase transition predicted by the model. Many more cases of this phase transition may be anticipated, and a corresponding range of glasses with low residual entropies - approaching the `perfect' glass state - are predicted.