arXiv subjects

Alexander V. Savin

Publications and source records attributed to Alexander V. Savin.

At least 37 records · Page 2Linked to original sources

arXiv2020

Hydroxygraphene: dynamics of hydrogen bond networks

Using the molecular dynamics method, dynamics of hydrogen bond (HB) networks emerging on the surface of a graphene sheet during its functionalization with hydroxyl groups OH are simulated. It is demonstrated that two OH groups form an energetically more advantageous structure when they are covalently attached on one side of the sheet to carbon atoms forming opposite vertices of one hexagon of valence bonds of the sheet. Attaching of OH groups to carbon atoms located at the opposite vertices of hexagons of valence bonds leads to the emergence of hydroxygraphene C$_4$(OH). In such sheet lying on a flat substrate, attached oxygen atoms on its outer surface form a hexagonal lattice, and hydroxyl groups due to their turns can in various ways form chains of hydrogen bonds. The modification of the sheet from two sides results in forming of hydroxygraphene C$_2$(OH) with HB networks on both sides of the graphene sheet. Simulation of the dynamics of these sheets shows that their heat capacity at low temperatures $T T_0$ is explained by the "melting" of the lattice. For one chain of OH groups connected to the outer side of a nanoribbon the melting temperature is $T_0=500$K, while for a graphene sheet C4(OH) modified on one side $T_0=260$K, and for a graphene sheet C$_2$(OH) modified on both sides $T_0=485$K.

cond-mat.mes-hall↗

arXiv2019

One-side surface modifications of graphene

We study the dependence of mechanical conformations of graphene sheets located on flat substrates on the density of unilateral (one-side) attachment of hydrogen, fluorine or chlorine atoms to them. It is shown that chemically modified graphene sheet can take four main forms on a flat substrate: the form of a flat sheet located parallel to the surface of the substrate, the form of convex sheet partially detached from the substrate with bent edges adjacent to the substrate, and the forms of single and double rolls on the substrate. On the surface of crystalline graphite, the flat form of the sheet is lowest in energy for hydrogenation density p <0.21, fluorination density p<0.20 and chlorination density p<0.16. The surface of crystalline nickel has higher adsorption energy for graphene monolayer and the flat form of chemically modified sheet on such substrate is lowest in energy for hydrogenation density p<0.47, fluorination density p<0.30 and chlorination density p<0.21. The flat shape of the graphene sheet remains basic on a substrate also when molecular groups CH3, CH2-CH3 or rings C6H5 are one-side attached to its outer surface. At the attachment density p=1/6 (one group per 6 sheet atoms) the sheet becomes the nanocarpet the basis of which is formed by a sheet of graphene and the pile of which is formed by the attached molecular groups forming a tightly packed regular lattice. The addition of hydroxyl groups OH with attachment density p=1/4 leads to the formation of hexagonal lattices of hydroxyl groups on the outer surface of graphene sheet on a substrate. In this lattice, the groups can form various configurations of hydrogen bonds, which turns the chemically modified sheet into a multistable system.

cond-mat.mes-hall↗

arXiv2019

Thermal rectifier based on asymmetric interaction of molecular chain with thermostats

The model of thermal rectifier based on the asymmetry of interaction of the molecular chain ends with thermostats is proposed in this work. The rectification mechanism is not related to the chain asymmetry, but to the asymmetry of the interaction of chain ends with thermostats, for instance, due to different lengths of the end thermostats. The chain can be homogeneous, it is only important that the thermal conductivity of the chain should depend on temperature strict monotonically. The effect is maximal when convergence of the thermal conductivity with increasing length just begins to manifest itself. In this case, the efficiency of thermal rectification can reach 25%. These conditions are met for carbon nanoribbons and nanotubes. Therefore, they can be ideal objects for the construction of thermal rectifiers based on the asymmetric interaction with thermostats. Numerical simulation of heat transfer shows that the rectification of heat transfer can reach 14% for nanoribbons and 22% for nanotubes.

cond-mat.mes-hall↗

arXiv2018

The COMPASS force field: validation for carbon nanoribbons

The COMPASS force field has been successfully applied in a large number of materials simulations, including the analysis of structural, electrical, thermal, and mechanical properties of carbon nanoparticles. This force field has been parameterized using quantum mechanical data and is based on hundreds of molecules as a training set, but such analysis for graphene sheets was not carried out. The objective of the present study is the verification of how good the COMPASS force field parameters can accurately describe the frequency spectrum of atomic vibrations of graphene, graphane and fluorographene sheets. We showed that the COMPASS force field allows to describe with good accuracy the frequency spectrum of atomic vibrations of graphane and fluorographene sheets, whose honeycomb hexagonal lattice is formed by sp3 hybridization. On the other hand, the force field doesn't describe very well the frequency spectrum of graphene sheet, whose planar hexagonal lattice is formed by sp2 banding. In that case the frequency spectrum of out-of-plane vibrations differs greatly from the experimental data - bending stiffness of a graphene sheet is strongly over estimated. We present the correction of parameters of out-of-plane and torsional potentials of the force field, that allows to achieve the coincidence of vibration frequency with experimental data. After such corrections the COMPASS force field can be used to describe the dynamics of flat graphene sheets and carbon nanotubes.

cond-mat.mes-hall↗

arXiv2017

Discrete breathers assist energy transfer to ac driven nonlinear chains

One-dimensional chain of pointwise particles harmonically coupled with nearest neighbors and placed in six-order polynomial on-site potentials is considered. Power of the energy source in the form of single ac driven particles is calculated numerically for different amplitudes $A$ and frequencies $ω$ within the linear phonon band. The results for the on-site potentials with hard and soft nonlinearity types are compared. For the hard-type nonlinearity, it is shown that when the driving frequency is close to (far from) the {\em upper} edge of the phonon band, the power of the energy source normalized to $A^2$ increases (decreases) with increasing $A$. In contrast, for the soft-type nonlinearity, the normalized power of the energy source increases (decreases) with increasing $A$ when the driving frequency is close to (far from) the {\em lower} edge of the phonon band. Our further demonstrations indicate that, in the case of hard (soft) anharmonicity, the chain can support movable discrete breathers (DBs) with frequencies above (below) the phonon band. It is the energy source quasi-periodically emitting moving DBs in the regime with driving frequency close to the DBs frequency, that induces the increase of the power. Therefore, our results here support the mechanism that the moving DBs can assist energy transfer from the ac driven particle to the chain.

nlin.PS↗

arXiv2015

Confinement and non-universality of anomalous heat transport and superdiffusion of energy in low-dimensional systems

We provide molecular dynamics simulation of heat transport and thermal energy diffusion in one-dimensional molecular chains with different interparticle pair potentials at zero and non-zero temperature. We model the thermal conductivity (TC) and energy diffusion in the coupled rotator chain and in the Lennard-Jones chain either without or with the confining parabolic interatomic potential. The considered chains without the confining potential have normal TC and energy diffusion, while the corresponding chains with the confining potential are characterized by anomalous (diverging with the system length) TC and superdiffusion of energy. We confirm in such a way that, surprisingly, the confinement makes both heat transport and energy diffusion anomalous in low-dimensional phononic systems. We show that the chain, which has a finite TC, is also characterized by the normal energy diffusion in the thermalized chain (at non-zero temperature), while the superdiffusion of thermal energy occurs in the thermalized chains with only anomalous TC. We present the arguments, supported by our simulations, that the scaling relation between the exponents in time dependence of the mean square displacement of thermal energy distribution and in length dependence of the anomalous TC is not universal and can be different, depending on the main mechanism of energy transport: by weakly-scattered waves or by noninteracting colliding particles performing Levy flights.

cond-mat.mes-hall↗

arXiv2015

Heat conduction in diatomic chains with correlated disorder

The paper considers heat transport in diatomic one-dimensional lattices, containing equal amounts of particles with different masses. Ordering of the particles in the chain is governed by single correlation parameter -- the probability for two neighboring particles to have the same mass. As this parameter grows from zero to unity, the structure of the chain varies from regular staggering chain to completely random configuration, and then -- to very long clusters of particles with equal masses. Therefore, this correlation parameter allows a control of typical cluster size in the chain. In order to explore different regimes of the heat transport, two interatomic potentials are considered. The first one is an infinite potential wall, corresponding to instantaneous elastic collisions between the neighboring particles. In homogeneous chains such interaction leads to an anomalous heat transport. The other one is classical Lennard-Jones interatomic potential, which leads to a normal heat transport. The simulations demonstrate that the correlated disorder of the particle arrangement does not change the convergence properties of the heat conduction coefficient, but essentially modifies its value. For the collision potential, one observes essential growth of the coefficient for fixed chain length as the limit of large homogeneous clusters is approached. The thermal transport in these models remains superdiffusive. In the Lennard-Jones chain the effect of correlation appears to be not monotonous in the limit of low temperatures. This behavior stems from the competition between formation of long clusters mentioned above, and Anderson localization close to the staggering ordered state.

cond-mat.stat-mech↗

arXiv2015

Scroll configurations of carbon nanoribbons

Carbon nanoscroll is a unique topologically open configuration of graphene nanoribbon possessing outstanding properties and application perspectives due to its morphology. However molecular dynamics study of nanoscrolls with more than a few coils is limited by computational power. Here, we propose a simple model of the molecular chain moving in the plane, allowing to describe the folded and rolled packaging of long graphene nanoribbons. The model is used to describe a set of possible stationary states and the low-frequency oscillation modes of isolated single-layer nanoribbon scrolls as the function of the nanoribbon length. Possible conformational changes of scrolls due to thermal fluctuations are analyzed and their thermal stability is examined. Using the full-atomic model, frequency spectrum of thermal vibrations is calculated for the scroll and compared to that of the flat nanoribbon. It is shown that the density of phonon states of the scroll differs from the one of the flat nanoribbon only in the low (omega<100 cm-1) and high (omega>1450 cm-1) frequency ranges. Finally, the linear thermal expansion coefficient for the scroll outer radius is calculated from the long-term dynamics with the help of the developed planar chain model. The scrolls demonstrate anomalously high coefficient of thermal expansion and this property can find new applications.

cond-mat.mes-hall↗

arXiv2013

Mechanical control of heat conductivity in microscopic models of dielectrics

We discuss a possibility to control a heat conductivity in simple one-dimensional models of dielectrics by means of external mechanical loads. To illustrate such possibilities we consider first a well-studied chain with degenerate double-well potential of the interparticle interaction. Contrary to previous studies, we consider varying length of the chain with fixed number of particles. Number of possible energetically degenerate ground states strongly depends on the overall length of the chain, or, in other terms, on average length of the link between neighboring particles. These degenerate states correspond to mechanical equilibrium, therefore one can say that the transition between them mimics to some extent a process of plastic deformation. We demonstrate that such modification of the chain length can lead to quite profound (almost five-fold) reduction of the heat conduction coefficient. Even more profound effect is revealed for a model with single-well non-convex potential. It is demonstrated that in certain range of constant external forcing this model becomes "effectively"\ double-well, and has a multitude of possible states of equilibrium for the same value of the external load. Thus, the heat conduction coefficient can be reduced by two orders of magnitude. We suggest a mechanical model of a chain with periodic double-well potential, which allows control over the heat conduction. The models considered may be useful for description of heat transport in biological macromolecules and for control of the heat transport in microsystems.

cond-mat.mes-hall↗

arXiv2013

Localized defect modes in graphene

We study the properties of localized vibrational modes associated with structural defects in a sheet of graphene. For the example of the Stone-Wales defects, one- and two-atom vacancies, many-atom linear vacancies, and adatoms in a honeycomb lattice, we demonstrate that the local defect modes are characterized by stable oscillations with the frequencies lying outside the linear frequency bands of an ideal graphene. In the frequency spectral density of thermal oscillations, such localized defect modes lead to the additional peaks from the right side of the frequency band of the ideal sheet of graphene. Thus, the general structure of the frequency spectral density can provide a fingerprint of its quality and the type of quantity of the structural defect a graphene sheet may contain.

cond-mat.mes-hall↗

arXiv2013

Two-phase stretching of molecular chains

While stretching of most polymer chains leads to rather featureless force-extension diagrams, some, notably DNA, exhibit non-trivial behavior with a distinct plateau region. Here we propose a unified theory that connects force-extension characteristics of the polymer chain with the convexity properties of the extension energy profile of its individual monomer subunits. Namely, if the effective monomer deformation energy as a function of its extension has a non-convex (concave up) region, the stretched polymer chain separates into two phases: the weakly and strongly stretched monomers. Simplified planar and 3D polymer models are used to illustrate the basic principles of the proposed model. Specifically, we show rigorously that when the secondary structure of a polymer is mostly due to weak non-covalent interactions, the stretching is two-phase, and the force-stretching diagram has the characteristic plateau. We then use realistic coarse-grained models to confirm the main findings and make direct connection to the microscopic structure of the monomers. We demostrate in detail how the two-phase scenario is realized in the α-helix, and in DNA double helix. The predicted plateau parameters are consistent with single molecules experiments. Detailed analysis of DNA stretching demonstrates that breaking of Watson-Crick bonds is not necessary for the existence of the plateau, although some of the bonds do break as the double-helix extends at room temperature. The main strengths of the proposed theory are its generality and direct microscopic connection.

cond-mat.mes-hall↗

arXiv2012

Semi-quantum approach to molecular dynamics simulation of thermal properties of low-dimensional nanostructures

We present a detailed description of semi-quantum molecular dynamics simulation of stochastic dynamics of a system of interacting particles. Within this approach, the dynamics of the system is described with the use of classical Newtonian equations of motion in which the effects of phonon quantum statistics are introduced through random Langevin-like forces with a specific power spectral density (the color noise). The color noise describes the interaction of the molecular system with the thermostat. We apply this technique to the simulation of thermal properties and heat transport in different low-dimensional nanostructures. We describe the determination of temperature in quantum lattice systems, to which the equipartition limit is not applied. We show that one can determine the temperature of such system from the measured power spectrum and temperature- and relaxation-rate-independent density of vibrational (phonon) states. We simulate the specific heat and heat transport in carbon nanotubes, as well as the heat transport in molecular nanoribbons with perfect (atomically smooth) and rough (porous) edges, and in nanoribbons with strongly anharmonic periodic interatomic potentials. We show that the effects of quantum statistics of phonons are essential for the carbon nanotube in the whole temperature range T<500K, in which the values of the specific heat and thermal conductivity of the nanotube are considerably less than that obtained within the description based on classical statistics of phonons.

physics.comp-ph↗

arXiv2012

Transport of fullerene molecules along graphene nanoribbons

We study the motion of C60 fullerene molecules (buckyballs) and short-length carbon nanotubes on graphene nanoribbons. We demonstrate that the nanoribbon edge creates an effective potential that keeps the carbon structures on the surface. We reveal that the character of the motion of C60 molecules depends on temperature: for low temperatures (T<150K) the main type of motion is sliding along the surface, but for higher temperatures the sliding is replaced by rocking and rolling. Modeling of the buckyball with an included metal ion, such as Fe@C60, demonstrates that this molecular complex undergoes a rolling motion along the nanoribbon with the constant velocity under the action of a constant electric field. The similar effect is observed in the presence of the heat gradient applied to the nanoribbon, but mobility of carbon structures in this case depends largely on their size and symmetry, such that larger and more asymmetric structures demonstrate much lower mobility. Our results suggest that both electorphoresis and thermophoresis can be employed to control the motion of carbon molecules and fullerenes and, for example, sort them by their size, shape, and possible inclusions.

cond-mat.mes-hall↗

arXiv2011

The simplest model of polymer crystal exhibiting polymorphism

Almost all the polymer crystals have several polymorphic modifications. Their structure and existence conditions, as well as transitions between them are not understood even in the case of the 'model' polymer polyethylene (PE). For analysis of polymorphism in polymer crystals, we consider the simplest possible model of polymer chain: an extended flat zigzag made of 'united' atoms (replacing CH2-groups in PE chain); the united atoms belonging to different zigzags interact via Lennard-Jones potential. Analysis of potential of interaction between such zigzags allowed to predict the structure of five possible equilibrium lattices in polymer crystal built out of such zigzags. Molecular dynamics simulation of the crystal built out of flexible zigzags showed that, depending on model parameters (dimensions of the zigzag and equilibrium distance of Lennard-Jones potential), one to three of these lattices are stable in bulk at low temperatures. We have determined the model parameters at which the existing stable lattices are analogous to the ones observed in real PE and linear alkanes. The triclinic lattice has the lowest potential energy, then follows the monoclinic lattice, and the orthorhombic lattice has the highest energy, exactly as in full atomic (with hydrogens) molecular dynamics models.

cond-mat.mtrl-sci↗

arXiv2011

Heat conductivity of DNA double helix

Thermal conductivity of isolated single molecule DNA fragments is of importance for nanotechnology, but has not yet been measured experimentally. Theoretical estimates based on simplified (1D) models predict anomalously high thermal conductivity. To investigate thermal properties of single molecule DNA we have developed a 3D coarse-grained (CG) model that retains the realism of the full all-atom description, but is significantly more efficient. Within the proposed model each nucleotide is represented by 6 particles or grains; the grains interact via effective potentials inferred from classical molecular dynamics (MD) trajectories based on a well-established all-atom potential function. Comparisons of 10 ns long MD trajectories between the CG and the corresponding all-atom model show similar root-mean-square deviations from the canonical B-form DNA, and similar structural fluctuations. At the same time, the CG model is 10 to 100 times faster depending on the length of the DNA fragment in the simulation. Analysis of dispersion curves derived from the CG model yields longitudinal sound velocity and torsional stiffness in close agreement with existing experiments. The computational efficiency of the CG model makes it possible to calculate thermal conductivity of a single DNA molecule not yet available experimentally. For a uniform (polyG-polyC) DNA, the estimated conductivity coefficient is 0.3 W/mK which is half the value of thermal conductivity for water. This result is in stark contrast with estimates of thermal conductivity for simplified, effectively 1D chains ("beads on a spring") that predict anomalous (infinite) thermal conductivity. Thus, full 3D character of DNA double-helix retained in the proposed model appears to be essential for describing its thermal properties at a single molecule level.

cond-mat.mes-hall↗

arXiv2011

Nonlinear Breathing-like Localized Modes in C60 Nanocrystals

We study the dynamics of nanocrystals composed of C60 fullerene molecules. We demonstrate that such structures can support long-lived strongly localized nonlinear oscillatory modes, which resemble discrete breathers in simple lattices. We reveal that at room temperatures the lifetime of such nonlinear localized modes may exceed tens of picoseconds; this suggests that C60 nanoclusters should demonstrate anomalously slow thermal relaxation when the temperature gradient decays in accord to a power law, thus violating the Cattaneo-Vernotte law of thermal conductivity.

cond-mat.mes-hall↗

arXiv2010

Suppression of thermal conductivity in graphene nanoribbons with rough edges

We analyze numerically the thermal conductivity of carbon nanoribbons with ideal and rough edges. We demonstrate that edge disorder can lead to a suppression of thermal conductivity by several orders of magnitude. This effect is associated with the edge-induced Anderson localization and suppression of the phonon transport, and it becomes more pronounced for longer nanoribbons and low temperatures.

cond-mat.mes-hall↗

arXiv2010

Vibrational Tamm states at the edges of graphene nanoribbons

We study vibrational states localized at the edges of graphene nanoribbons. Such surface oscillations can be considered as a phonon analog of Tamm states well known in the electronic theory. We consider both armchair and zigzag graphene stripes and demonstrate that surface modes correspond to phonons localized at the edges of the graphene nanoribbon, and they can be classified as in-plane and out-of-plane modes. In addition, in armchair nanoribbons anharmonic edge modes can experience longitudinal localization in the form of self-localized nonlinear modes, or surface breather solitons.

cond-mat.mes-hall↗