Yuriy A. Kosevich

Functionalization mediates heat transport in graphene nanoflakes.

The high thermal conductivity of graphene and few-layer graphene undergoes severe degradations through contact with the substrate. Here we show experimentally that the thermal management of a micro heater is substantially improved by introducing alternative heat-escaping channels into a graphene-based film bonded to functionalized graphene oxide through amino-silane molecules. Using a resistance temperature probe for in situ monitoring we demonstrate that the hotspot temperature was lowered by ∼28 °C for a chip operating at 1,300 W cm−2. Thermal resistance probed by pulsed photothermal reflectance measurements demonstrated an improved thermal coupling due to functionalization on the graphene–graphene oxide interface. Three functionalization molecules manifest distinct interfacial thermal transport behaviour, corroborating our atomistic calculations in unveiling the role of molecular chain length and functional groups. Molecular dynamics simulations reveal that the functionalization constrains the cross-plane phonon scattering, which in turn enhances in-plane heat conduction of the bonded graphene film by recovering the long flexural phonon lifetime.

Modulational instability and energy localization in anharmonic lattices at finite energy density

The localization of vibrational energy, induced by the modulational instability of the Brillouin-zone-boundary mode in a chain of classical anharmonic oscillators with finite initial energy density, is studied within a continuum theory. We describe the initial localization stage as a gas of envelope solitons and explain their merging, eventually leading to a single localized object containing a macroscopic fraction of the total energy of the lattice. The initial-energy-density dependences of all characteristic time scales of the soliton formation and merging are described analytically. Spatial power spectra are computed and used for the quantitative explanation of the numerical results.

Phonon interference and thermal conductance reduction in atomic-scale metamaterials

We introduce and model a three-dimensional (3D) atomic-scale phononic metamaterial producing two-path phonon interference antiresonances to control the heat flux spectrum. We show that a crystal plane partially embedded with defect-atom arrays can completely reflect phonons at the frequency prescribed by masses and interaction forces. We emphasize the predominant role of the second phonon path and destructive interference in the origin of the total phonon reflection and thermal conductance reduction in comparison with the Fano-resonance concept. The random defect distribution in the plane and the anharmonicity of atom bonds do not deteriorate the antiresonance. The width of the antiresonance dip can provide a measure of the coherence length of the phonon wave packet. All our conclusions are confirmed both by analytical studies of the equivalent quasi-1D lattice models and by numerical molecular dynamics simulations of realistic 3D lattices.

Semiquantum molecular dynamics simulation of thermal properties and heat transport in low-dimensional nanostructures

We present a detailed description of the semi-quantum approach to the 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 quantum effects 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 the thermal properties of different low-dimensional nanostructures. Within this approach, we simulate the specific heat and heat transport in carbon nanotubes, as well as the thermal transport in a molecular nanoribbon with rough edges and in a nanoribbon with a strongly anharmonic periodic interatomic potential. We show that the existence of rough edges and quantum statistics of phonons change drastically the thermal conductivity of the rough-edge nanoribbon in comparison with that of the nanoribbon with ideal (atomically smooth) edges and classical dynamics. We show how the combination of strong nonlinearity of the interatomic potentials with quantum statistics of phonons changes the low-temperature thermal conductivity of the nanoribbon with periodic interatomic potentials. Molecular dynamics is a method of numerical modeling of molecular systems based on classical Newtonian mechanics. It does not allow for the description of pure quantum effects such as freezing out of high-frequency oscillations at low temperatures and the related decrease to zero of heat capacity for T → 0. In classical molecular dynamics, each dynamical degree of freedom possesses the same energy kBT, where kB is Boltzmann constant. Therefore, in classical statistics the specific heat of a solid almost does not depend on temperature when only relatively small changes, caused the anharmonicity of the potential, can be taken into account [1]). On the other hand, because of its complexity, a pure quantum-mechanical description does not allow in general the detailed modeling of the dynamics of many-body systems. To overcome these obstacles, different semiclassical methods, which allow to take into an account quantum effects in the dynamics of molecular systems, have been proposed [2–8]. The most convenient for the numerical modeling is the use of the Langevin equations with color-noise random forces [5, 7]. In this approximation, the dynamics of the system is described with the use of classical Newtonian equations of motion, while the quantum effects are introduced through random Langevin-like forces with a specific power spectral density (color noise), which describe the interaction of the molecular system with the thermostat. Below we give a detailed description of this semi-quantum approach, in application to the simulation of specific heat and heat transport in different low-dimensional nanostructures.

Ultracompact Interference Phonon Nanocapacitor for Storage and Lasing of Coherent Terahertz Lattice Waves

We introduce a novel ultra-compact nanocapacitor of coherent phonons formed by high-finesse interference mirrors based on atomic-scale semiconductor metamaterials. Our molecular dynamics simulations show that the nanocapacitor stores THz monochromatic lattice waves, which can be used for phonon lasing the emission of coherent phonons. Either oneor two-color phonon lasing can be realized depending on the geometry of the nanodevice. The two color regimes of the capacitor originates from the distinct transmittance dependance on the phonon wave packet incident angle for the two phonon polarizations at their respective resonances. Phonon nanocapacitor can be charged by cooling the sample equilibrated at room temperature or by the pump-probe technique. The nanocapacitor can be discharged by applying tunable reversible strain, resulting in the emission of coherent THz acoustic beams.

Effects of quantum statistics of phonons on the thermal conductivity of silicon and germanium nanoribbons

We present molecular dynamics simulation of phonon thermal conductivity of semiconductor nanoribbons with an account for phonon quantum statistics. In our semiquantum molecular dynamics simulation, dynamics of the system is described with the use of classical Newtonian equations of motion where the effect of phonon quantum statistics is introduced through random Langevin-like forces with a specific power spectral density (color noise). The color noise describes interaction of the molecular system with the thermostat. The thermal transport of silicon and germanium nanoribbons with atomically smooth (perfect) and rough (porous) edges are studied. We show that the existence of rough (porous) edges and the quantum statistics of phonon change drastically the low-temperature thermal conductivity of the nanoribbon in comparison with that of the perfect nanoribbon with atomically smooth edges and classical phonon dynamics and statistics. The rough-edge phonon scattering and weak anharmonicity of the considered lattice produce a weakly pronounced maximum of thermal conductivity of the nanoribbon at low temperature.

Classical to Quantum Transition of Heat Transfer between Two Silica Clusters

Heat transfer between two silica clusters is investigated by using the nonequilibrium Greens function method. In the gap range between 4 Å and 3 times the cluster size, the thermal conductance decreases as predicted by the surface charge-charge interaction. Above 5 times the cluster size, the volume dipole-dipole interaction predominates. Finally, when the distance becomes smaller than 4 Å, a quantum interaction where the electrons of both clusters are shared takes place. This quantum interaction leads to the dramatic increase of the thermal coupling between neighbor clusters due to strong interactions. This study finally provides a description of the transition between radiation and heat conduction in gaps smaller than a few nanometers.

Temporal Fourier spectra of stationary and slowly moving breathers in Fermi-Pasta-Ulam anharmonic lattice

Abstract The temporal Fourier spectra of stationary and slowly moving self-localized large-amplitude modes (breathers) in translationally invariant chain of coupled classical anharmonic oscillators are studied. The breathers arise naturally in the anharmonic lattice from modulational instability of short-wavelength extended vibrational modes. The frequencies of the resonant spectral peaks of the stationary breather are measured numerically and compared with an exact analytical solution for the stationary extended nonlinear sinusoidal wave in the anharmonic lattice, which is conveniently adapted. Symmetrical satellite peaks of the fundamental frequency and its higher odd harmonics in the temporal Fourier spectrum of the stationary breather are observed. Small admixture of the slowly moving breather solution to the stationary one is discussed in connection with these peaks. It is observed that the temporal Fourier spectrum of slowly moving breather consists of two main frequencies symmetrically shifted upwards and downwards with respect to the fundamental frequency of the stationary breather with the same energy, in perfect agreement with the earlier theoretical prediction of one of the authors. The relation between the group velocity of slowly moving breather and the fundamental frequency of the stationary breather with the same energy is derived.

New soliton equation and exotic localized modes in anharmonic lattices

Abstract A new kind of nonlinear envelope-function equation, more general than the nonlinear Klien-Gordon or Schrodinger one, for the short wavelength vibrational excitations in anharmonic lattices is obtained. Soliton and soliton-kink exotic localized modes with amplitude-independent small spatial widths are found in the framework of the consistent continuous approach. These modes include self-localized vibrational modes in homogeneous anharmonic lattices, previously revealed in a discrete-lattice theory.

Energy transfer in coupled nonlinear phononic waveguides: transition from wandering breather to nonlinear self-trapping

We consider, both analytically and numerically, the dynamics of stationary and slowly-moving breathers (localized short-wavelength excitations) in two weakly coupled nonlinear oscillator chains (nonlinear phononic waveguides). We show that there are two qualitatively different dynamical regimes of the coupled breathers: the oscillatory exchange of the low-amplitude breather between the phononic waveguides (wandering breather), and one-waveguide-localization (nonlinear self-trapping) of the high-amplitude breather. We also show that phase-coherent dynamics of the coupled breathers in two weakly linked nonlinear phononic waveguides has a profound analogy, and is described by a similar pair of equations, to the tunnelling quantum dynamics of two weakly linked Bose-Einstein condensates in a symmetric double-well potential (single bosonic Josephson junction). The exchange of phonon energy and excitations between the coupled phononic waveguides takes on the role which the exchange of atoms via quantum tunnelling plays in the case of the coupled condensates. On the basis of this analogy, we predict a new tunnelling mode of the coupled Bose-Einstein condensates in a single bosonic Josephson junction in which their relative phase oscillates around π/2. The dynamics of relative phase of two weakly linked Bose-Einstein condensates can be studied by means of interference, while the dynamics of the exchange of lattice excitations in coupled nonlinear phononic waveguides can be observed by means of light scattering.