Strain field of soft modes in glasses

Abstract

The strain field surrounding the center of low frequency vibrational modes in a glass is analyzed for numerically created binary glasses with a 1/r 10 repulsive interatomic potential. Outside the unstable inner core of twenty to thirty atoms, one finds a mixture of a motion similar to the string motion in the core with the strain field of three oscillating elastic dipoles in the center. The additional outside string motion contributes more to the stabilization of the core than the strain field. The average creation energy of a soft mode in this binary glass is about 2.5 times the thermal energy at the freezing temperature. The elastic dipole field predicts a damping for the longitudinal sound waves at the boson peak in quantitative agreement with x-ray Brillouin data in a metallic glass.

Sound absorption in glasses

“…A very recent numerical study [22] has determined the soft potential parameters for a binary glass with a repulsive 1/r 10 -potential and scaled them via the shear modulus, the atomic mass and the atomic volume to PdSiCu. The obtained value W/kB of 3.2 K is in excellent agreement with the two experimental values reported here.…”

“…Very recently, the application of the elastic Greens function to describe the long range elastic distortion around a numerical soft mode has been implemented [21]. With this technique, the soft potential parameters of a binary numerical glass have been calculated and successfully scaled to metallic glasses [22].…”

A note on Kauzmann's paradox

“…The unstable core of the soft mode is a local saddle point configuration between two minima, lower than the saddle point in energy by the creation energy E s of the soft mode. E s was found to be about 2.5 k B T g in a binary glass former [25]. Since the modes couple strongly to an external shear [25,26], the core is a local shear instability, and the shear modulus (measured at times shorter than the shear relaxation time) decreases with an increasing number of soft modes.…”

“…E s was found to be about 2.5 k B T g in a binary glass former [25]. Since the modes couple strongly to an external shear [25,26], the core is a local shear instability, and the shear modulus (measured at times shorter than the shear relaxation time) decreases with an increasing number of soft modes. This explains the strong decrease of the shear modulus with increasing temperature [10,11].…”

“…Another crucial numerical finding [23][24][25] is the unstable core of the soft modes ("soft spot"), stabilized by the surroundings. The unstable core of the soft mode is a local saddle point configuration between two minima, lower than the saddle point in energy by the creation energy E s of the soft mode.…”