The Vibrational Modes of Model Bulk Metallic Glasses

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The Vibrational Modes of Model Bulk Metallic Glasses P. M. Derlet1* , R. Maaß2 and J. F. Löffler2 1 Condensed Matter Theory Group, Paul Scherrer Institut, 5232 PSI-Villigen, Switzerland 2 Laboratory of Metal Physics and Technology, Department of Materials, ETH Zurich, 8093 Zurich, Switzerland ABSTRACT Bulk metallic glasses exhibit confined low- and high-frequency vibrational properties resulting from the significant bond and topological disorder occurring at the atomic scale. The precise nature of the low-frequency modes and how they are influenced by local atomic structure remains unclear. Using standard harmonic analysis, this study investigates various aspects of the problem by diagonalizing the Hessian of atomistic samples derived from molecular dynamics simulations via a model binary Lennard Jones pair potential. INTRODUCTION & METHODOLOGY Bulk metallic glasses (BMGs) exhibit unique vibrational properties. At low frequencies there exists an excess of modes relative to the Debye solid – the Bose peak [1] – which in a three dimensional system may be simply revealed by plotting the low-frequency part of the vibrational density of states (VDOS) divided by the square of the vibrational frequency. The Bose peak regime is considered a fundamentally important material property because its frequency range correlates strongly with the breakdown of transverse linear dispersion. At much higher phonon frequencies there also exists a critical frequency at which there is a transition from extended modes to strongly localized modes – the so-called mobility edge for phonons [2]. Here we investigate some of these aspects within the framework of the Harmonic approximation using atomistic samples that contain both spring-constant and topological disorder. The BMG samples were generated by quenching from the melt, a 1:1 model A-B binary Lennard-Jones (LJ) [3,4] mixture via constant atom number/pressure/temperature molecular dynamics. We note that the well equilibrated initial liquid state was prepared at a hydrostatic pressure of 14.5 GPa and therefore the quenching procedure involved the simultaneous reduction of both temperature and hydrostatic pressure. Two samples are presented here: sample0 containing 1728 atoms and sample1 containing 13824 atoms. To obtain the final 0K configurations a combination of molecular statics and conjugant gradient relaxation algorithms were used. Fig. 1 displays the volume (per atom), total instantaneous pressure and energy (per atom) as a function of instantaneous temperature during the quench. Inspection of this figure shows that with decreasing quenching rate and/or increasing sample size, the transition from liquid to glass becomes more abrupt with respect to temperature, indicating the underlying firstorder phase transition. This trend is confirmed when using quenching rates several orders of magnitude slower, and larger samples sizes – indicating that in smaller samples inherent finite size effects play a role in smoothening out the phase transition. When considering the high que

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