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Univ. Paris-Saclay
"Lattice dynamics of amorphous materials"
Alessio Zaccone - University Lecturer, University of Cambridge
Mardi 12/01/2016, 11:00-12:00
LLB - Bât 563 p15 (Grande Salle), CEA-Saclay

The atomic or molecular dynamics in condensed matter systems can

be probed by various ways, like e.g. by applying a deformation or sending

radiation. Under the applied field, every atom tends to be displaced from

its original position, which, in the case of solid states, represents an

equilibrium or quasi-equilibrium position. For small displacements in the

solid state, the harmonic approximation leads straightforward to standard

lattice dynamics and one recovers the vibrational spectrum and elastic

constants of perfect crystals. For disordered materials like glasses,

however, things are not so simple. When the atom reaches the position

prescribed by the applied field, the forces transmitted to it by its

nearest neighbours cancel mutually to zero in all crystals which possess

local center-inversion symmetry. In glasses and other non-centrosymmetric

and disordered systems, this is no longer true: a net force acts on every

atom in its displaced (affine) position because the nearest-neighbour

forces no longer balance. As a result, an additional displacement is

required to satisfy the mechanical equilibrium. In light of this simple

consideration, we have reformulated classical lattice dynamics to make it

applicable to glasses and disordered systems in general. The theoretical

framework is able to predict elastic constants and their softening due to

disorder, as well as rheological flow curves of different materials. For

example stress-strain overshoots in start-up shear experiments can be

reproduced as well as the shear modulus of polymers across the glass

transition. This conceptual framework can also be used to elucidate the

vibrational density of states of glasses and disordered crystals.

Contact : Aurore VERDIER

 

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