arXiv subjects

Dmytro Khomenko

Publications and source records attributed to Dmytro Khomenko.

11 recordsLinked to original sources

arXiv2026

On non-phononic modes and glassy dynamics

The relationship between vibrational modes and glassy dynamics has been an important and controversial topic of study for many decades. We introduce a family of glass-forming model potentials that have identical molecular and swap dynamics but different low-frequency vibrational modes. In sufficiently small systems low frequency modes do not hybridize with phonons, and this enables us to study their statistical properties independently. We investigate i) the role of the optimization algorithm used to find a minimum; ii) that of the finite-size effects; iii) that of the truncation of the interaction potential; and, iv) that of the parent temperature. We find that models with identical supercooled liquid structure and dynamics can have dramatically different non-phononic vibrational modes at low frequency, based on how the potential is truncated. These findings challenge ideas relating the low-frequency non-phononic vibrational modes with the slow supercooled liquid relaxation.

cond-mat.dis-nn↗

arXiv2026

Further testing the validity of generalized heterogeneous-elasticity theory for low-frequency excitations in structural glasses

We summarize the salient features of our theory of non-phononic vibrational excitations in glasses [W. Schirmacher et al., Nature Comm. 15, 3107 (2024)]. Next, we provide further evidence of the non-universality of the $ω^4$ scaling of the non-phononic vibrational density of states (DoS), and the existence of an important class of non-phononic excitations in glasses, which we call defect states. These modes are induced by frozen-in stresses and can be classified as quasi-localized. Our results suggest that the commonly observed low-frequency $ω^4$ scaling of the non-phononic vibrational density of states is highly dependent on technical aspects of the molecular dynamics simulations employed to compute the DoS.

cond-mat.dis-nn↗

arXiv2025

Not-so-glass-like Caging and Fluctuations of an Active Matter Model

Simple active models of matter recapitulate complex biological phenomena. The out-of-equilibrium nature of these models, however, often makes them beyond the reach of first-principle descriptions. This limitation is particularly perplexing when attempting to distinguish between different glass-forming mechanisms. We here consider a minimal active system in various spatial dimensions to identify the processes underlying their sluggish dynamics. Activity is found to markedly impact cage escape processes and critical fluctuations associated with exploring lower-dimensional caging features.

cond-mat.soft↗

arXiv2023

The nature of non-phononic excitations in disordered systems

Using heterogeneous-elasticity theory (HET) and a generalisation of HET theory (GHET), obtained by applying a newly developed procedure for obtaining the continuum limit of the glass's Hessian, we investigate the nature of vibrational excitations, which are present in small systems, which do not allow for low-frequency phonons. We identify two types of such non-phononic excitations. In marginally stable systems, which can be prepared by quenching from a rather high parental temperature, the low-frequency regime is dominated by random-matrix vibrational wavefunctions (type-I) which in macroscopic samples gives rise to the boson peak. They show a density of states (DOS), which scales as $g(ω)\sim ω^2$. In more stable systems (reached by a somewhat lower parental temperature) a gap appears in the type-I spectrum. This gap is filled with other type-II non-phononic excitations, which are not described by the previous version of HET, and have a DOS, which scales as $g(ω)\simω^s$ with $3<s<5$. Using GHET we demonstrate that the type-II excitations are due to local non-irrotational oscillations associated with the stress field. The frequency scaling exponent $s$ turns out to be non-universal, depending on the details of the interaction potential. Specifically, we demonstrate that $s$ depends on the statistics of the small values of the local frozen-in stresses, which are, in turn, governed by the shape of the pair potential close to values where the potential or its first derivative vanishes. All these findings are verified by extensive numerical simulations of small soft-sphere glasses. Further, using level-distance statistics, we demonstrate that both types of non-phononic excitations obey the Gaussian-Orthogonal-Ensemble random-matrix statistics, which means that they are extended and not localized.

cond-mat.dis-nn↗

arXiv2023

Finding defects in glasses through machine learning

Structural defects control the kinetic, thermodynamic and mechanical properties of glasses. For instance, rare quantum tunneling two-level systems (TLS) govern the physics of glasses at very low temperature. Because of their extremely low density, it is very hard to directly identify them in computer simulations. We introduce a machine learning approach to efficiently explore the potential energy landscape of glass models and identify desired classes of defects. We focus in particular on TLS and we design an algorithm that is able to rapidly predict the quantum splitting between any two amorphous configurations produced by classical simulations. This in turn allows us to shift the computational effort towards the collection and identification of a larger number of TLS, rather than the useless characterization of non-tunneling defects which are much more abundant. Finally, we interpret our machine learning model to understand how TLS are identified and characterized, thus giving direct physical insight into their microscopic nature.

cond-mat.dis-nn↗

arXiv2023

Microscopic observation of two-level systems in a metallic glass model

The low-temperature quasi-universal behavior of amorphous solids has been attributed to the existence of spatially-localized tunneling defects found in the low-energy regions of the potential energy landscape. Computational models of glasses can be studied to elucidate the microscopic nature of these defects. Recent simulation work has demonstrated the means of generating stable glassy configurations for models that mimic metallic glasses using the swap Monte Carlo algorithm. Building on these studies, we present an extensive exploration of the glassy metabasins of the potential energy landscape of a variant of the most widely used model of metallic glasses. We carefully identify tunneling defects and reveal their depletion with increased glass stability. The density of tunneling defects near the experimental glass transition temperature appears to be in good agreement with experimental measurements.

cond-mat.dis-nn↗

arXiv2020

Relationship between two-level systems and quasi-localized normal modes in glasses

Tunnelling Two-Level Systems (TLS) dominate the physics of glasses at low temperatures. Yet TLS are extremely rare and it is extremely difficult to directly observe them $\it{in \, silico}$. It is thus crucial to develop simple structural predictors that can provide markers for determining if a TLS is present in a given glass region. It has been speculated that Quasi-Localized vibrational Modes (QLM) are closely related to TLS, and that one can extract information about TLS from QLM. In this work we address this possibility. In particular, we investigate the degree to which a linear or non-linear vibrational mode analysis can predict the location of TLS independently found by energy landscape exploration. We find that even though there is a notable spatial correlation between QLM and TLS, in general TLS are strongly non-linear and their global properties cannot be predicted by a simple normal mode analysis.

cond-mat.dis-nn↗

arXiv2020

Depletion of two-level systems in ultrastable computer-generated glasses

Amorphous solids exhibit quasi-universal low-temperature anomalies whose origin has been ascribed to localized tunneling defects. Using an advanced Monte Carlo procedure, we create {\it in silico} glasses spanning from hyperquenched to ultrastable glasses. Using a multidimensional path-finding protocol, we locate tunneling defects with energy splittings smaller than $k_{B}T_Q$, with $T_Q$ the temperature below which quantum effects are relevant ($T_Q \approx 1 \,$K in most experiments). We find that as the stability of a glass increases, its energy landscape as well as the manner in which it is probed tend to deplete the density of tunneling defects, as observed in recent experiments. We explore the real-space nature of tunneling defects, and find that they are mostly localized to a few atoms, but are occasionally dramatically delocalized.

cond-mat.dis-nn↗

arXiv2016

Coupled dynamics for superfluid $^4$He in the channel

We study the coupled dynamics of normal and superfluid components of the superfluid $^4$He in the channel considering the counterflow turbulence with laminar normal component. In particular, we calculated profiles of the normal velocity, the mutual friction, the vortex line density and other flow properties and compared them to the case when the dynamic of the normal component is "frozen". We have found that the coupling between the normal and superfluid components leads to flattening of the normal velocity profile, increasingly more pronounced with temperature, as the mutual friction, and therefore coupling, becomes stronger. The commonly measured flow properties also change when the coupling between two components is taken into account.

cond-mat.other↗

arXiv2015

Counter-flow Induced Decoupling in Super-Fluid Turbulence

In mechanically driven superfluid turbulence the mean velocities of the normal- and superfluid components are known to coincide: $\mathbf U_{\text{n}} =\mathbf U_{\text{s}}$. Numerous laboratory, numerical and analytical studies showed that under these conditions the mutual friction between the normal- and superfluid velocity components couples also their fluctuations: $\mathbf u'_{\text{n}}(\mathbf r,t) \approx \mathbf u'_{\text{s}}(\mathbf r,t)$ almost at all scales. In this paper we show that this is not the case in thermally driven superfluid turbulence; here the counterflow velocity $\mathbf U_{\text{ns}}\equiv \mathbf U_{\text{n}} -\mathbf U_{\text{s}}\ne 0$. We suggest a simple analytic model for the cross correlation function $\left\langle \mathbf u'_{\text{n}}(\mathbf r,t) \cdot \mathbf u'_{\text{s}}(\mathbf r',t)\right \rangle$ and its dependence on $U_{\text{ns}}$. We demonstrate that $\mathbf u'_{\text{n}}(\mathbf r,t)$ and $ \mathbf u'_{\text{s}}(\mathbf r,t)$ are decoupled almost in the entire range of separations $|\mathbf r-\mathbf r'|$ between the energy containing scale and intervortex distance.

cond-mat.other↗

arXiv2013

Analytic solution of the dynamics of quantum vortex reconnection

Experimental and simulational studies of the dynamics of vortex reconnections in quantum fluids showedthat the distance $d$ between the reconnecting vortices is close to a universal time dependence $d=D[κ|t_0-t|]^α$ with $α$ fluctuating around 1/2 and $κ=h/m$ is the quantum of circulation. Dimensional analysis, based on the assumption that the quantum of circulation $κ=h/m$ is the only relevant parameter in the problem, predicts $α=1/2$. The theoretical calculation of the dimensionless coefficient $D$ in this formula remained an open problem. In this Letter we present an analytic calculation of $D$ in terms of the given geometry of the reconnecting vortices. We start from the numerically observed generic geometry on the way to vortex reconnection and demonstrate that the dynamics is well described by a self-similar analytic solution which provides the wanted information.

cond-mat.other↗