arXiv subjects

Alexander V. Savin

Publications and source records attributed to Alexander V. Savin.

At least 19 recordsLinked to original sources

arXiv2026

Thermal Stability of Encapsulated Molecular Structures with Extended OH-Hydrogen-Bond Chains

Using a coarse-grained model, we performed numerical simulations of the dynamics of linear molecular chains adsorbed on a flat substrate (on the surface of an h-BN crystal). It is shown that molecules containing benzene rings and hydroxyl groups in their structure can form stable hydrogen-bond chains OH$\cdots$OH$\cdots$OH. Such chains can be formed by phenol C$_6$H$_5$OH, 4-phenylphenol C$_6$H$_5$--C$_6$H$_4$OH, paracetamol CH$_3$C(O)NHC$_6$H$_4$OH, and 4-hydroxybenzanilide C$_6$H$_5$C(O)NHC$_6$H$_4$OH molecules. The dissociation of these chains occurs at temperatures above $T_1=190$, 240, 300, and 400K, respectively. Coating such molecular systems with a hexagonal boron nitride sheet (their van der Waals encapsulation) significantly enhances their thermal stability. Such encapsulated molecular structures retain hydrogen-bond chains up to temperatures of $T_2=470$, 800, 880, and 1140K, respectively. The simulations allow us to conclude that h-BN-encapsulated chains of these molecules can be used to create anhydrous proton-exchange membranes capable of operating at high temperatures. The most promising are encapsulated chains of paracetamol and 4-hydroxybenzanilide molecules.

cond-mat.mes-hall↗

arXiv2026

Thermal Stability of Two-Dimensional Crystals with Extended OH Hydrogen-Bonded Chains

Numerical simulations of the dynamics of monolayer structures of molecules deposited on a sheet of hexagonal boron nitride (h-BN) have been performed. It is shown that molecules containing benzene rings and hydroxyl groups in their structure can form stable two-dimensional crystals with linear chains of hydrogen bonds OH$\cdots$OH$\cdots$OH$\cdots$ Such structures are formed by the following molecules: phenol (C$_6$H$_5$OH), hydroquinone (C$_6$H$_4$(OH)$_2$), 4-phenylphenol (C$_6$H$_5$--C$_6$H$_4$OH), 4-(4-phenylphenyl)phenol (C$_6$H$_5$--C$_6$H$_4$--C$_6$H$_4$OH), paracetamol (CH$_3$C(O)NHC$_6$H$_4$OH), 4-hydroxybenzanilide (C$_6$H$_5$C(O)NHC$_6$H$_4$OH) and 4,4-dihydroxybenzanilide (C$_6$H$_4$OHC(O)NHC$_6$H$_4$OH). On the one hand, the benzene rings in these molecules ensure their strong interaction with the flat substrate; on the other hand, they do not hinder the formation of extended hydrogen-bonded chains. The monolayer structures of these molecules exhibit high thermal stability: the onset melting temperatures of their 2D crystals are 47, 187, 127, 247, 167, 307, and 377 $^\circ$C, respectively. The simulations allow us to conclude that multilayer structures composed of h-BN sheets and molecules of hydroquinone, paracetamol, and 4-hydroxybenzanilide can be used for the development of novel proton-exchange membranes capable of operating at elevated temperatures.

cond-mat.mes-hall↗

arXiv2026

Influence of a graphene substrate on the stabilization of molecular systems with hydrogen bonds

Numerical simulation of the dynamics of planar two- and three-layer molecular structures formed by $β$-sheets of polyglycine peptide chains and systems of parallel Kevlar (para-aramid) molecules placed on a graphene sheet has been performed. It is shown that in these structures the $β$-sheets retain their shape, due to the presence of parallel chains of hydrogen bonds, up to a temperature of $T=800$K. An even higher stability is exhibited by the system of parallel Kevlar molecules. Here, the parallel chains of hydrogen bonds between peptide groups of neighboring molecules are preserved even at higher temperatures. The performed modeling allows us to conclude that the addition of graphene to Kevlar fibers can significantly increase their thermal stability.

cond-mat.mes-hall↗

arXiv2026

Multistability of graphene nanobubbles

Using He, Ne, Ar, Kr, and Xe atoms as a model system, it is demonstrated that graphene nanobubbles on flat substrates are multistable systems. A nanobubble can adopt multiple stable stationary states, each characterized by the number of layers $l$ within the cluster of encapsulated atoms. The layers are circular, concentrically stacked, and form an $l$-stepped pyramid with a flat top. Encapsulation of this pyramid by the graphene sheet is achieved through local stretching of the membrane: the valence bonds elongate only directly above the confined atoms. Outside this coverage zone, the sheet remains undeformed and lies flush against the substrate. The maximum number of possible layers, $l_m$, increases monotonically with the number of encapsulated atoms $N$, reaching $l_m=6$ for $N=4000$. The graphene membrane, through van der Waals interaction with the substrate, compresses the internal atomic cluster, generating pressures on the order of $P\sim 1$~GPa. Numerical simulations of thermal vibrations reveal that among all $l$-layer configurations, one ground state always exist. Upon heating, this state smoothly transitions into a layerless liquid configuration. All other stationary states transform into this ground state once a characteristic temperature $T_l$ is reached. For $N=4000$, the ground state corresponds to the four-layer packing ($l=4$). The coexistence of multiple stable states with distinct layer numbers at low temperatures leads to the absence of a universal shape for the nanobubbles. In this scenario, the height-to-radius ratio, $H/R$ is not constant and can vary from 0 to 0.28, depending on the number of layers.

cond-mat.mes-hall↗

arXiv2025

Stability of Planar Slits in Multilayer Graphite Crystals

Using a two-dimensional coarse-grained chain model, planar slits in multilayer graphite crystals are simulated. It is shown that when covering a linear cavity on the flat surface of a graphite crystal with a multilayer graphene sheet, an open (unfilled slit) can form only if the cavity width does not exceed a critical value L_o (for width L>L_o, only a closed state of the slit is formed, with the cavity space filled by the covering sheet). The critical width of the open slit L_o increases monotonically with the number of layers K in the covering sheet. For a single-layer cavity, there is a finite critical value of its width L_o<3nm, while for two- and three-layer cavities, the maximum width of the open slit increases infinitely with increasing K as a power function K^αwith exponent 0<α<1. Inside the crystal, two- and three-layer slits can have stable open states at any width. For a slit with width L>7.6nm, a stationary closed state is also possible, in which its lower and upper surfaces adhere to each other. Simulation of thermal oscillations showed that open states of two-layer slits with width L<15nm are always stable against thermal oscillations, while wider slits at T>400K transition from the open to the closed state. Open states of three-layer slits are always stable against thermal oscillations.

cond-mat.mes-hall↗

arXiv2025

Structural and helix reversal defects of carbon nanosprings

Due to their chiral structure, carbon nanosprings possess unique properties that are promising for nanotechnology applications. The structural transformations of carbon nanosprings in the form of spiral macromolecules derived from planar coronene and kekulene molecules (graphene helicoids and spiral nanoribbons) are analyzed using molecular dynamics simulations. While the tension/compression of such nanosprings has been analyzed in the literature, this study investigates other modes of deformation, including bending and twisting. Depending on the geometric characteristics of the carbon nanosprings, the formation of structural and helix reversal defects is described. It is found that nanosprings demonstrate a significantly higher coefficient of axial thermal expansion than many metals and alloys. These results are useful for designing nanosensors that operate over a wide temperature range.

cond-mat.mes-hall↗

arXiv2025

Shock absorption by multilayer carbon nanotube packings

The propagation of transverse impact energy in a multilayer packing (in an array) of parallel single-walled carbon nanotubes has been simulated. It has been shown that such nanotube arrays are effective shock absorbers. The depreciation effect is most pronounced for packings of nanotubes with a diameter of 2.7-3.9 nm. Here, part of the impact energy is absorbed due to the transfer of the packing to a higher energy stationary state, in which part of the nanotubes is in a collapsed state. The impact impulse reaches the other edge of the packing most weakened and distributed over time. For nanotubes with a smaller diameter, the compression of the array occurs elastically without energy accumulation, and for nanotubes with a larger diameter - with energy release.

cond-mat.mes-hall↗

arXiv2024

Wrinkles in graphene suspended on flat substrates: structure and collapse under hydrostatic pressure

The method of molecular dynamics and molecular mechanics has been used to numerically simulate the formation of wrinkle systems during compression of a graphene sheet lying on a flat solid substrate. It is shown that under uniaxial compression the nanosheet can transition into several stable wrinkled states: the most energetically favorable one is a linear wrinkle of infinite length. Higher energy states include wrinkles of finite length aligned along the same line where their ends partially overlap. Under biaxial compression, the graphene nanosheet can contain one linear wrinkle or two linear non-intersecting or intersecting wrinkles corresponding to weak, medium and strong compression, respectively. There are several nanosheet states with intersecting wrinkles that differ in structure of intersection area. The effect of external hydrostatic pressure on the shape of wrinkles has been studied. It is shown that there is a critical pressure value at which the wrinkle either completely flattens (disappears) or collapses into a vertical two-layer fold (the first scenario is possible only with weak compression of the sheet).

cond-mat.mes-hall↗

arXiv2024

Wrinkle formation during uniaxial compression of a graphene sheet lying on a soft (polymer) substrate

Modeling of wrinkles and folds formation in single and multilayer graphene sheets lying on flat deformable (polymer) substrates has been carried out. It is shown that the deformability of the substrate leads to the appearance of significant features. In contrast to the flat surfaces of rigid crystals molecules of soft polymer substrates can penetrate into wrinkles and folds of the graphene sheet, completely filling the voids beneath the sheet. Moreover, the vertical folds of the sheet can be directed not only upward from the substrate, but also down into the substrate, penetrating it. By modeling the uniaxial compression of the two-component graphene/polymer system, the external pressure and thermal vibrations of the substrate molecules have been taken into account. High external pressure $p\ge 150$~bar leads to a noticeable additional stabilization of the initial ground state of the system. At uniaxial compression above the critical value, a system of localized wrinkles whose interior is filled with molecules of the substrate appears in the graphene sheet. Increasing temperature leads to an increase in the size of wrinkles and to a decrease in their number. The largest wrinkles form before the substrate begins to melt. Melting leads to the complete disappearance of all wrinkles and small folds. Cooling of the melted substrate leads to its crystallization, but the system of wrinkles in the graphene sheet on the surface is not restored. Therefore, melting of the polymer substrate and its subsequent cooling can serve as a method getting rid of localized wrinkles and folds of the graphene sheet.

cond-mat.mes-hall↗

arXiv2024

Stabilization of molecular hydrogen-bonded chains by carbon nanotubes

We study numerically nonlinear dynamics of several types of molecular systems composed of hydrogen-bonded chains placed inside carbon nanotubes with open edges. We demonstrate that carbon nanotubes provide a stabilization mechanism for quasi-one-dimensional molecular chains via the formation of their secondary structures. In particular, a polypeptide chain (Gly)$_N$ placed inside a carbon nanotube can form of a stable helical chain ($3_{10}$, $α$, $π$ and $β$-helix) with parallel chains of hydrogen-bonded peptide groups. A chain of hydrogen fluoride molecules can form hydrogen-bonded zigzag chain. We reveal that in such geometries the hydrogen-bonded chains may remain stable even at $T=500$~K. Thus, our results suggest that the use of carbon nanotubes with encapsulated hydrogen fluoride molecules may support high proton conductivity operating at high temperatures.

cond-mat.mes-hall↗

arXiv2023

Temperature-induced reversal effects of kink dynamics in carbon nanotube on flat substrate

Carbon nanotubes are nano-objects with quite anisotropic properties, for example the mechanical properties in longitudinal and radial directions differ significantly. This feature of the carbon nanotubes yields many interesting phenomena investigated in last decades. One of them is the ability to form both hollow and collapsed states if the radius of the nanotube is large enough. The transitions between the two states have been also reported. In our study we present single-walled carbon nanotube interacting with a plane substrate and characterize the energy of interaction with the substrate using effective Lennard-Jones-type potential. We show energy of the homogeneous open and collapsed states depending on the radius of the carbon nanotube and report on the bi-stability in some range of the nanotube diameters. Using the molecular-dynamical simulations we look at the evolution of the initial half-opened, half-collapsed state and demonstrate that the transition area from one state to another is spatially localized having features of topological soliton (kink or anti-kink). We show that the value and the direction of the kink propagation speed depend significantly on the nanotube diameter as well as on the temperature of the system. We also discuss the mechanism of the process using a simplified model with asymmetric double-well potential and show the entropic nature of the transition.

cond-mat.mes-hall↗

arXiv2023

Chiral organic molecular structures supported by multilayer surfaces

We study numerically the dynamics of acetanilide (ACN) molecules placed on a flat surface of a multilayer hexagonal boron nitride structure. We demonstrate that the ACN molecules, being achiral in three dimensions, become chiral after being placed on the substrate. Homochirality of the ACN molecules leads to stable secondary structures stabilized by hydrogen bonds between peptide groups of the molecules. Numerical simulations of systems of such molecules reveal that the structure of the resulting hydrogen-bond chains depends on the isomeric composition of the molecules. If all molecules are homochiral (i.e. only one isomer is present), they form secondary structures (chains of hydrogen bonds in the shapes of arcs, circles, and spirals). If the molecules at the substrate form a racemic mixture, then no regular secondary structures appear, and only curvilinear chains of hydrogen bonds of random shapes can emerge. A hydrogen-bond chain can form a straight zigzag only if it has an alternation of isomers. Such chains can create two-dimensional (2D) regular lattices, or 2D crystals. The melting scenarios of such 2D crystals depend on density of its coverage of the substrate. At 25% coverage, melting occurs continuously in a certain temperature interval. For a complete coverage, melting occurs at 415-470 K due to a shift of 11% of all molecules into the second layer of the substrate.

cond-mat.mes-hall↗

arXiv2023

Localized nonlinear excitations of a columnar chain of coronene molecules

The nonlinear dynamics of a one-dimensional molecular crystal in the form of a chain of planar coronene molecules is analyzed. Using molecular dynamics, it is shown that a chain of coronene molecules supports acoustic solitons, rotobreathers, and discrete breathers. An increase in the size of planar molecules in a chain leads to an increase in the number of internal degrees of freedom. This results in an increase in the rate of emission of phonons from spatially localized nonlinear excitations and a decrease in their lifetime. Presented results contribute to the understanding of the effect of the rotational and internal vibrational modes of molecules on the nonlinear dynamics of molecular crystals.

cond-mat.mes-hall↗

arXiv2022

Modeling of second sound in carbon nanostructures

The study of thermal transport in low-dimensional materials has attracted a lot of attention recently after discovery of high thermal conductivity of graphene. Here we study numerically phonon transport in low-dimensional carbon structures being interested in the hydrodynamic regime revealed through the observation of second sound. We demonstrate that correct numerical modeling of such two-dimensional systems requires semi-classical molecular dynamics simulations of temperature waves that take into account quantum statistics of thermalized phonons. We reveal that second sound can be attributed to the maximum group velocity of bending optical oscillations of carbon structures, and the hydrodynamic effects disappear for $T>200$K, being replaced by diffusive dynamics of thermal waves. Our numerical results suggest that the velocity of second sound in such low-dimensional structures is about 6 km/s, and the hydrodynamic effects are manifested stronger in carbon nanotubes rather than in carbon nanoribbons.

cond-mat.mes-hall↗

arXiv2022

Friction and mobility of carbon nanoparticles on a graphene sheet

It is shown using the method of molecular dynamics that the motion of carbon nanoparticles (rectangular graphene flakes, spherical fullerenes of size $L<10$ nm) on the surface of a thermalized graphene sheet lying on a flat substrate can be described as the motion of particles in a viscous medium with a constant coefficient of friction, the value of which depends on the temperature and particle size. It has been shown that there are two types of effective friction: diffusion and ballistic. In ballistic regime of motion (at velocities $v>100$ m/s), deceleration occurs due to the interaction of moving nanoparticles with thermal out-of-plane bending vibrations of a graphene sheet. Because of this, with the increasing temperature, the coefficient of friction monotonically increases. In the diffusion regime of motion (at $v<10$ m/s), friction arises due to the need for the particle to overcome local energy barriers, therefore it decreases with increasing emperature. The difference between ballistic and diffusion friction is most pronounced at low temperatures, since the mobility of nanoparticles in the ballistic regime of motion decreases with increasing temperature, while in the diffusion regime it monotonously increases. It is shown that the presence of a normal force pressing the nanoparticle to the substrate leads to an increases in its friction with the substrate.

cond-mat.mes-hall↗

arXiv2021

Multistability of carbon nanotube packings on flat substrate

It is shown by the method of molecular dynamics using a chain model that a multilayer packaging of identical single-walled carbon nanotubes with a diameter of D>2.5 nm located on a flat substrate is a multistable system. The system has many stationary states, which are characterized by the portion of collapsed nanotubes. The thickness of the package monotonically decreases with an increase in the portion of such nanotubes. For nanotubes with a chirality index (60,0), depending on the portion of collapsed nanotubes, the thickness of the 11-layer package can vary from 12 to 36 nm. All stationary states of the package are stable to thermal fluctuations at T=300K. The transverse compression of the package is not elastic; due to the collapse of a part of the nanotubes, it only transfers the package from one stationary state to another with a smaller thickness.

cond-mat.mes-hall↗

arXiv2021

Rotobreathers in a chain of coupled elastic rotators

Rotobreathers in the chain of coupled linearly elastic rotators are analyzed. Each rotator is a particle connected by a massless elastic rod with a frictionless pivot; it has two degrees of freedom, length and angle of rotation. The rods of the rotators and the elastic bonds between the nearest rotators are linearly elastic, and the nonlinearity of the system is of a purely geometric nature. It is shown that long-lived rotobreathers can exist if the stiffness of the rods is high enough to create a relatively wide gap in the phonon spectrum of the chain. The frequency of angular rotation of the rotobreather cannot be above the optical band of the phonon spectrum and is in the spectrum gap. Generally speaking, the rotation of the rotobreather is accompanied by radial oscillations, however, one can choose such initial conditions so that the radial oscillations are minimal. Some parameters of rotobreathers with minimal radial vibrations are presented on the basis of numerical simulations. The results obtained qualitatively describe the behavior of physical systems with coupled rotators.

nlin.PS↗

arXiv2021

Eigenmodes and resonance vibrations of 2D nanomembranes -- Graphene and hexagonal boron-nitride

Natural and resonant oscillations of suspended circular graphene and hexagonal boron nitride (h-BN) membranes (single-layer sheets lying on a flat substrate having a circular hole of radius $R$) have been simulated using full-atomic models. Substrates formed by flat surfaces of graphite and h-BN crystal, hexagonal ice, silicon carbide 6H-SiC and nickel surface (111) have been used. The presence of the substrate leads to the forming of a gap at the bottom of the frequency spectrum of transversal vibrations of the sheet. The frequencies of natural oscillations of the membrane (oscillations localized on the suspended section of the sheet) always lie in this gap, and the frequencies of oscillations decrease by increasing radius of the membrane as $(R+R_i)^{-2}$ with nonezero effective increase of radius $R_i>0$. The modeling of the sheet dynamics has shown that small periodic transversal displacements of the substrate lead to resonant vibrations of the membranes at frequencies close to eigenfrequencies of nodeless vibrations of membranes with a circular symmetry. The energy distribution of resonant vibrations of the membrane has a circular symmetry and several nodal circles, whose number $i$ coincides with the number of the resonant frequency. The frequencies of the resonances decrease by increasing the radius of the membrane as $(R+R_i)^{α_i}$ with exponent $α_i<2$. The lower rate of resonance frequency decrease is caused by the anharmonicity of membrane vibrations.

cond-mat.mes-hall↗