Session R20: Understanding Glasses and Disordered Matter Through Computational Models I

Live

Amorphous materials have a disordered structure inducing sound scattering and anomalous density of vibrations in the THz range. These properties affect their thermal properties, but also acoustic attenuation properties. These acoustic attenuation properties can be described in terms of an effective visco-elastic modelling for the constitutive laws charcterizing the material at the continuum scale. In the low frequency regime however, acoustic propagation is well described by plane waves and ballistic phonons. But the occurrence of plastic deformation results in a succession of local irreversible rearrangements at the nanometric scale known as Transformation Zones. These Transformation Zones are shown to be well described within the theory of Eshelby inclusions, and thus act as acoustic scatterers. We will discuss in this talk the signature of plasticity in acoustic scattering in amorphous materials.

[1] T. Albaret et al. Physical Review E 93, 053002-1-12 (2016): Mapping between atomistic simulations and Eshelby Inclusions in the shear deformation of an amorphous silicon model.
[2] Y. Beltukov et al. Physical Review E 98 023005 (2018): Propagative and diffusive regimes of acoustic damping in bulk amorphous material.
[3] H. Luo et al. Nanomaterials 9, 1471 (2019): Role of bi-phasic materials properties on Acoustic attenuation in a 2D nanophononic solid: the microscopic origin of the energy transport regime from propagative to localized.
[4] A. Tlili et al. Nanoscale 11, 21502 (2019): Enhancement and anticipation of the Ioffe-Regel crossover in amorphous/nanocrystalline composites.
[5] H. Luo et al. Physical Review E 102, 033003 (2020): Continuum constitutive laws to describe acoustic attenuation in glasses.   

Live

Structural heterogeneity of amorphous solids is intimately connected to their mechanical behavior. Based on a perturbation analysis of the potential energy landscape, we derive a new structural indicator, termed the atomic nonaffinity, which qualifies the contribution of an individual atom to the total nonaffine modulus. We find that the atomic nonaffinity can efficiently characterize the locations of shear transformation zones (STZ) as well as their protocol-dependent response arising from their orientational nature. We quantitatively show that plastic events tend to happen in the STZs whose softest shear directions are close to the loading shear direction. These results provide new insight towards understanding and characterizing the plastic response of amorphous solids.   

Live

The cluster expansion (CE) method developed to model disorder in alloys has seen active use in computational materials science since it was proposed 30 years ago. Notable extensions to include multi-lattice systems, and the use of new regression techniques have established the CE as a standard method in the computational study of multicomponent crystals. Recent interest in high entropy alloys (HEA) and cation-disordered rocksalt (DRX) cathode materials has opened a prime setting for continued development of the CE. Studying large multi-component systems involves exploration of high-dimensional spaces. Although the current formulation of the CE is applicable in these high-dimensional spaces, issues surrounding specific sampling and regression techniques necessary to obtain manageable and accurate models for treatment of HEA and DRX material systems remain largely unsettled. We re-examine the CE formalism, and explore the relationship of structural relaxations and composition in setting the maximal degree of the CE. Limiting the degree while preserving predictive accuracy is crucial in controlling model complexity and ensuring interpretability needed for successful use of the CE when studying large multi-component systems.   

Live

The random-first order transition (RFOT) theory explains the slowing down of supercooled liquids with an underlying thermodynamic transition to an ideal glass phase below the experimental glass transition T_g. At the mean-field level, this transition is exactly realized and RFOT theory also predicts a first-order transition line ending at a critical point, when the overlap order parameter [1] (quantifying similarity between equilibrium liquid configurations) is linearly coupled to an external field ε. To assess the fate of mean-field results in finite dimensions, we use computer simulations to investigate the extended phase diagram (ε,T) of a 3D model supercooled liquid. Combining umbrella sampling techniques with an extensive finite-size scaling analysis, we demonstrate the existence of a first-order transition line and of a random-field Ising model critical point in the thermodynamic limit [2]. This result represents the only piece of the mean-field theory to survive other than as a crossover the introduction of finite-dimensional fluctuations.

[1] B. Guiselin, G. Tarjus, L. Berthier, arXiv:2007.07625.
[2] B. Guiselin, L. Berthier, G. Tarjus, Phys. Rev. E (in press).   

Live

We study instantaneous quenches from infinite temperature to well below T_c in the two-dimensional (2D) square lattice Ising antiferromagnet in the presence of a longitudinal external magnetic field. Under single-spin-flip Metropolis algorithm Monte Carlo dynamics, this protocol produces a pair of metastable magnetization plateaus that prevent the system from reaching the equilibrium ground state except for some special values of the field. This occurs despite the absence of intrinsic disorder or frustration. We explain the plateaus in terms of local spin configurations that are stable under the dynamics. Although the details of the plateaus depend on the update scheme, the underlying principle governing the breakdown of ergodicity is quite general and provides a broader paradigm for understanding failures of ergodicity in Monte Carlo dynamics. See also: Iaizzi, Phys. Rev. E 102 032112 (2020), doi:10.1103/PhysRevE.102.032112   

Live

The dynamical arrest found upon cooling glass formers to their glass temperature T_g is caused by the unbounded expansion of cooperative regions as T_g is approached or as the system is left to age below T_g. It is extremely difficult to measure quantities that are accessible both to experiment and simulations, which has led to seemingly irreconcilable approaches.
We solved this dilemma in a framework that harmonizes experiments with theory. We conduct a parallel study of spin-glass dynamics both in an experiment in a CuMn single crystal and in simulations of the Ising-Edwards-Anderson (IEA) model carried out on the Janus II custom-built supercomputer. We introduced a scaling law that describes the system's response over its entire natural range of variation.   

Live

Spin glasses exhibit a fragile behavior in response to perturbations such as temperature changes. Specifically, arbitrary small changes in the temperature would lead to a complete reorganization of the equilibrium configuration of the spin glass. This equilibrium phenomenon is named Temperature Chaos [1,2].

I shall argue that an effect that closely mimics Temperature Chaos is, indeed, present in the non-equilibrium dynamics of a spin glass. This new phenomenon has been observed in a large-scale simulation in the dedicated computer Janus II [3]. Therefore, we believe that this work lays the ground for the theoretical analysis of temperature-cycling experiments in glassy systems.

We find that, the key quantity which is ruling the non-equilibrium Temperature Chaos phenomenon is the correlation length ξ. Also, we find a crossover between weak and strong chaos regime controlled by a crossover length ξ*.

[1] A. J. Bray and M. A. Moore, Phys. Rev. Lett. 58, 57 (1987)
[2] J. R. Banavar and A. J. Bray, Phys. Rev. B 35, 8888 (1987)
[3] J. Moreno-Gordo et al Work in preparation   

Live

Thermal noise from amorphous mirror coatings is one of the limiting noise sources for the laser interferometer gravitational-wave observatory (LIGO).[CQG 25.11,114041] In order to better understand the origin of the mechanical loss in amorphous coatings, their energy landscapes have been characterized by the correlated parameter distributions of two-level systems (TLS). By applying the concept of oxygen cage [J. Chem. Phys. 129, 164507 (2008)], the TLSs we find for pure and doped tantala can be separated into two groups named non-cage-breaking transitions and cage-breaking transitions. Non-cage-breaking transitions only have small structure changes, while cage-breaking transitions involve at least one large atom-atom pair distance change between the initial and final atomic structures. The further mechanical loss calculation of ZrO2-doped Ta2O5 models establish a connection between the atomic structure changes and mechanical loss, which shows that the non-cage-breaking transitions have higher averaged mechanical loss and are responsible for the large mechanical loss peak near 40K, while the cage-breaking transitions are the majority of transitions and mainly contributed to the loss peak near 120K.   

On Demand

A supercooled liquid is a disordered aperiodic system of atoms obtained by rapidly freezing a viscous liquid. One distinguishing characteristic of a supercooled liquid is the tendency of atoms to be trapped by their nearest neighbors engendering a long relaxation time. Several dynamic variables exhibit the extended relaxation process. In this work, we employ atomistic simulations on model glass forming liquids to investigate the shear stress correlations and the attendant relaxation behavior. A range of spatial and temporal scales is probed by analyzing the stress correlations at small, medium and large wavevectors; in the long wavelength limit, the integral of the shear stress correlation (SSC) function provides the shear viscosity. At short wavelengths, the SSC function behaves similar to a normal liquid; from our simulations we draw out the caging effect that results in extended relaxation at longer wavelengths. We also compute the shear viscosity at finite wavelengths through the hydrodynamic relationship with the transverse current correlation (TCC) function. The purpose of this presentation is to provide a description of transverse collective modes in supercooled liquids through the SSC and TCC functions at finite wavelengths.