Local thermal energy as a structural indicator in glasses

Abstract

Identifying heterogeneous structures in glasses-such as localized soft spots-and understanding structure-dynamics relations in these systems remain major scientific challenges. Here, we derive an exact expression for the local thermal energy of interacting particles (the mean local potential energy change caused by thermal fluctuations) in glassy systems by a systematic low-temperature expansion. We show that the local thermal energy can attain anomalously large values, inversely related to the degree of softness of localized structures in a glass, determined by a coupling between internal stresses-an intrinsic signature of glassy frustration-anharmonicity and low-frequency vibrational modes. These anomalously large values follow a fat-tailed distribution, with a universal exponent related to the recently observed universal [Formula: see text] density of states of quasilocalized low-frequency vibrational modes. When the spatial thermal energy field-a "softness field"-is considered, this power law tail manifests itself by highly localized spots, which are significantly softer than their surroundings. These soft spots are shown to be susceptible to plastic rearrangements under external driving forces, having predictive powers that surpass those of the normal modes-based approach. These results offer a general, system/model-independent, physical/observable-based approach to identify structural properties of quiescent glasses and relate them to glassy dynamics.

Nonlinear quasilocalized excitations in glasses: True representatives of soft spots

“…We emphasize that the existence of QLMs -as anomously soft spots in the glass -and their effect on various physical phenomena, is not dependent on whether they can be realized as harmonic modes or not (as shown e.g. in [39]).…”

“…It is precisely this population of soft QLMs, and their interaction with phonons, that is thought to influence various unexplained universal phenomena that are specific to structural glasses [20][21][22]26]. Furthermore, a subset of QLMs were shown to represent the carriers of plasticity in externally-loaded glasses [30,31,39], and might be key in determining relaxation patterns in supercooled liquids [32][33][34]. Therefore, knowledge of their full distribution is crucial for advancing our fundamental understanding of many glassy phenomena.…”

Anisotropic structural predictor in glassy materials

“…We then construct a structural predictor as the product of the local heat capacity and its linear response to external deformation, and show that it offers enhanced predictability of plastic rearrangements under deformation in different directions, compared to the purely scalar predictor.Introduction.-At the heart of resolving the glass mystery resides the need to quantify the disordered structures inherently associated with glasses and to relate them to glass properties and dynamics, most notably spontaneous and driven structural relaxation [1,2]. Numerous attempts to address and meet this grand challenge have been made [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19], aiming at defining structural indicators with predictive powers. Achieving this goal would constitute major progress in understanding glassiness and would provide invaluable insight for developing macroscopic theories of deformation and flow of glasses.Recently accumulated evidence suggests that spatially localized soft spots are the loci of glassy relaxation, and hence are highly relevant for glass dynamics.…”

“…Achieving this goal would constitute major progress in understanding glassiness and would provide invaluable insight for developing macroscopic theories of deformation and flow of glasses.Recently accumulated evidence suggests that spatially localized soft spots are the loci of glassy relaxation, and hence are highly relevant for glass dynamics. These localized soft spots have been related to quasilocalized, nonphononic excitations in glasses [4][5][6], whose universal ω 4 density of states (ω is the vibrational frequency) has been also established recently [20][21][22][23]. Among the structural predictors proposed, most relevant here is the normalized local thermal energy [6], which quantifies the interparticle interaction contribution to the zero-temperature heat capacity, termed hereafter the local heat capacity (LHC) c α (α is the interaction index).The LHC c α is a general (system/model-independent), first principles statistical mechanical quantity that reveals soft spots in glassy materials [6].…”

“…Finally, a metric for quantifying the predictive power of structural predictors is proposed and extensive computer simulations are used to show that c α dc α /dγ offers enhanced predictability of plastic rearrangements under deformation in different directions, compared to the LHC c α alone. Linear response coupling of the LHC to external deformation.-The starting point for our development is the zero-temperature local heat capacity [6,24] c α ≡ 1 1 2 k B…”