Yangyu Guo

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Verified

Yangyu verified their affiliation via an institutional email.

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• PhD
• Professor (Full) at Harbin Institute of Technology

The recent experimental observation of thermal rectification in a graphite tesla valve indicates the existence of nonlinear effects in the phonon hydrodynamic regime, which remains yet an open puzzle currently. In this work, we explore the effects of nonlinearity on phonon hydrodynamics in graphite based on a semianalytical solution of the nonlinea...

Graphitic materials are widely used in thermal management owing to the high basal-plane thermal conductivity. However, the modeling and physics of basal-plane heat transport along finite-sized graphite ribbons remain to be fully established. In this work, we present a computational approach by directly solving the phonon Boltzmann transport equatio...

The understanding of extreme near-field heat transport across vacuum nanogaps between polar dielectric materials remains an open question. In this work, we present an investigation of heat transport across vacuum nanogaps between magnesium oxide (MgO) surfaces by nonequilibrium molecular dynamics (NEMD) simulation , which naturally accounts for the...

The understanding and modeling of inelastic scattering of thermal phonons at a solid/solid interface remain an open question. We present a fully quantum theoretical scheme to quantify the effect of anharmonic phonon-phonon scattering at an interface via nonequilibrium Green's function (NEGF) formalism. Based on the real-space scattering rate matrix...

The coherent quantum effect has become increasingly important in the heat dissipation bottleneck of semiconductor nanoelectronics with the characteristic size recently shrinking down to a few nanometers scale. However, the quantum mechanical model remains elusive for anharmonic phonon-phonon scattering in extremely small nanostructures with broken...

Thermal management at silicon-diamond interface is critical for advancing high-performance electronic and optoelectronic devices. In this study, we calculate the interfacial thermal conductance between silicon and diamond using a computationally efficient machine learning (ML) interatomic potential trained on density functional theory (DFT) data. U...

Accurate prediction of infrared dielectric functions in polar materials is fundamental for thermal and photonic applications, yet it remains unexplored whether the two main methods, Green-Kubo formula and Lorentz model, can give unified predictions. In this work, we present a detailed comparison of these two approaches using MgO and LiH as prototyp...

Understanding the mechanism of interfacial thermal transport is crucial for thermal management of electronics. Recent experiments have shown the strong impact of interfacial roughness on inelastic phonon scattering and interfacial thermal conductance (ITC), while the theoretical modeling and underlying physics remain missing. Through non-equilibriu...

The Tesla valve benefits the rectification of fluid flow in microfluidic systems1–6 and inspires researchers to design modern solid-state electronic and thermal rectifiers referring to fluid-rectification mechanisms in a liquid-state context. In contrast to the rectification of fluids in microfluidic channels, the rectification of thermal phonons i...

Recently, the application of isotope engineering to design and optimize near-field radiative thermal diodes based on polar materials has emerged as an appealing and promising technique. Besides the isotope effect, the lattice thermal expansion in those polar materials with light atoms impacts appreciably their phonon and optical properties. In this...

The decisive experimental evidence of enhanced heat conduction driven by surface phonon polaritons (SPhPs) has been recently demonstrated along polar nanofilms. However, a proper quantitative interpretation remains to be fully established. In this work, we provide a consistent theoretical explanation of the measured thermal conductivities of polar...

Thermal management at silicon-diamond interface is critical for advancing high-performance electronic and optoelectronic devices. In this study, we calculate the interfacial thermal conductance between silicon and diamond using machine learning (ML) interatomic potentials trained on density functional theory (DFT) data. Using non-equilibrium molecu...

Despite recent experiments exhibiting an impressive enhancement in radiative heat flux between parallel planar silica surfaces with gap sizes of about 10 nm, the exploration of sub-nanometric gap distances remains unexplored. In this work, by employing non-equilibrium molecular dynamics (NEMD) simulations, we study the heat transfer between two SiO...

We experimentally demonstrate the enhancement of the far-field thermal radiation between two nonabsorbent Si microplates coated with energy-absorbent silicon dioxide (SiO2) nanolayers supporting the propagation of surface phonon polaritons. By measuring the radiative thermal conductance between two coated Si plates, we find that its values are twic...

Despite recent experiments exhibiting an impressive enhancement in radiative heat flux between parallel planar silica surfaces with gap sizes of about 10 nm, the exploration of sub-nanometric gap distances remains unexplored. In this work, by employing non-equilibrium molecular dynamics (NEMD) simulations, we study the heat transfer between two SiO...

The modeling and understanding of micro- and nano-scale transport processes have raised increasing attention and extensive investigation during the past decades. In this mini-review, we aim to summarize our recent progress on the non-equilibrium thermodynamics of micro- and nano-scale flow and heat transfer. Special emphasis is put on the entropy g...

The hydrodynamic behavior of phonons is of particular interest and importance owing to the strong demand for highly thermal conductive materials. Thermal transport in hydrodynamic regime becomes essentially nonlocal, which can give rise to a number of new and counterintuitive phenomena. In this work, we present a direct numerical study of nonlocal...

When two solids are separated by a vacuum gap of thickness smaller than the wavelength of acoustic phonons, the latter can tunnel across the gap thanks to van der Waals forces or electrostatic interactions. Here we show that these mechanical vibration modes can also contribute significantly, at the atomic scale, to the nonlocal radiative response o...

Heat transport plays a crucial role in various applications such as waste energy harvesting, thermal barrier coatings, and heat dissipation in microelectronics. In this paper, the frequency-dependent heat transport characteristics in mass disordered Si1−xGex nanowires (NWs) are investigated based on the nonequilibrium Green's function method. The r...

The understanding of extreme near-field heat transport across vacuum nanogaps between polar dielectric materials remains an open question. In this work, we present a molecular dynamic simulation of heat transport across MgO-MgO nanogaps, together with a consistent comparison with the continuum fluctuational-electrodynamics theory using local dielec...

While the phonon hydrodynamic regime has recently been highlighted experimentally in graphite films, the understanding and modeling of heat transport along their basal plane remain elusive. From first-principles-based modeling, we predict a significant influence of the surface roughness on basal-plane thermal conductivity due to the collective phon...

As a fundamental physical quantity of thermal phonons, temporal coherence participates in a broad range of thermal and phononic processes, while a clear methodology for the measurement of phonon coherence is still lacking. In this paper, we derive a theoretical model for the experimental exploration of phonon coherence based on spectroscopy, which...

In recent times, the unique collective transport physics of phonon hydrodynamics motivates theoreticians and experimentalists to explore it in micro- and nanoscale and at elevated temperatures. Graphitic materials have been predicted to facilitate hydrodynamic heat transport with their intrinsically strong normal scattering. However, owing to the e...

We analyze the heat transfer between two metals separated by a vacuum gap in the extreme near-field regime. In this crossover regime between conduction and radiation, heat exchanges are mediated by photon, phonon, and electron tunneling. We quantify the relative contribution of these carriers with respect to both the separation distance between the...

When two solids are separated by a vacuum gap of smaller thickness than the wavelength of acoustic phonons, the latter can tunnel across the gap thanks to van der Waals forces or electrostatic interactions. Here we show that the acoustic vibrational modes in polar crystals can also significantly contribute, at atomic scale, to the non-local optical...

Classical Planck's theory of thermal radiation predicts an upper limit of the heat transfer between two bodies separated by a distance longer than the dominant radiation wavelength (far-field regime). This limit can be overcome when the dimensions of the absorbent bodies are smaller than the dominant wavelength due to hybrid electromagnetic waves,...

We analyze the heat transfer between two metals separated by a vacuum gap in the extreme near-field regime. In this cross-over regime between conduction and radiation heat exchanges are mediated by photon, phonon and electron tunneling. We quantify the relative contribution of these carriers with respect to both the separation distance between the...

With the peculiar collective transport behaviors and potential applications in thermal management, phonon hydrodynamics at elevated temperatures draws increasing attention in host materials, such as graphite. We map the strength of steady-state phonon hydrodynamic flow in ¹² C purified graphite micro-structures with finite length and width in a bro...

The understanding and modeling of heat transport across nanometer and subnanometer gaps, where the distinction between thermal radiation and conduction becomes blurred, remains an open question. In this work, we present a three-dimensional atomistic simulation framework by combining the molecular dynamics (MD) and phonon nonequilibrium Green's func...

In recent times, the unique collective transport physics of phonon hydrodynamics motivates theoreticians and experimentalists to explore it in micro- and nanoscale and at elevated temperatures. Graphitic materials have been predicted to facilitate hydrodynamic heat transport with their intrinsically strong normal scattering. However, owing to the e...

We report a theoretical investigation of coherent-to-incoherent heat conduction in multilayer nanostructures. In the coherent regime where the phonon motion is quasi-harmonic, the elastic continuum model gives accurate cross-plane thermal conductivity predictions of upper limits and demonstrates that the coherent transport is the result of the inte...

As a fundamental physical quantity of thermal phonons, temporal coherence participates in a broad range of thermal and phononic processes, while a clear methodology for the measurement of phonon coherence is still lacking. In this Lettter, we derive a theoretical model for the experimental exploration of phonon coherence based on spectroscopy, whic...

The understanding and modeling of the heat transport across nanometer and sub-nanometer gaps where the distinction between thermal radiation and conduction become blurred remains an open question. In this work, we present a three-dimensional atomistic simulation framework by combining the molecular dynamics (MD) and phonon non-equilibrium Green's f...

Thermal transport in amorphous materials has remained one of the fundamental questions in solid state physics while involving a very large field of applications. Using a heat conduction theory incorporating coherence, we demonstrate that the strong phase correlation between local and non-propagating modes, commonly named diffusons in the terminolog...

Thermal transport at the nanoscale level is attracting attention not only because of its physically interesting features such as the peculiar behavior of phonons due to their pronounced ballistic and wave-like properties but also because of its potential applications in alleviating heat dissipation problems in electronic and optical devices and the...

Understanding and quantifying the fundamental physical property of coherence of thermal excitations is a long-standing and general problem in physics. The conventional theory, i.e., the phonon gas model, fails to describe coherence and its impact on thermal transport. In this Letter, we propose a general heat conduction formalism supported by theor...

Heat dissipation through interfaces becomes challenging in nanodevices which impedes the dissipation of waste heat. Accordingly, effective approaches are needed to optimize interfacial thermal transport. In this work, by combining the molecular dynamics simulations and machine learning technique, we systematically study the optimization of interfac...

Based on the Boltzmann transport equation, we demonstrate that the thermal conductance per unit width of a sufficiently thin polar nanofilm supporting the propagation of surface-phonon polaritons along its surfaces is independent of the material properties and is given by 12z(3)k_B^3T^2/ch^2 , where k_B and h are the respective Boltzmann and Planck...

Engineering low-frequency phonon transport in nanostructures with the phonon resonant mechanism has become an important research direction. On the basis of non-equilibrium molecular dynamics simulations, the thermal transport in pristine and resonant Si-membranes bounded with {100}, {110} and {111} facets is investigated. It is found that the creat...

Based on the spatiotemporal modulation of thermal conductivity and volumetric heat capacity, we propose a thermal-wave diode characterized by the rectification of the heat currents carried by thermal waves. By transforming Fourier’s law for the heat flux and the diffusion equation for the temperature into equations with constant coefficients, it is...

Understanding the thermal vibrations and thermal transport in amorphous materials is an important but long-standing issue in several theoretical and practical fields. Using direct molecular dynamic simulations, we demonstrate that the strong phase correlation between local and non-propagating modes, commonly named diffusons in the terminology of am...

Understanding and quantifying the fundamental physical property of coherence of thermal excitations is a long-standing and general problem in physics. The conventional theory, i.e. the phonon gas model, fails to describe coherence and its impact on thermal transport. In this letter, we propose a general heat conduction formalism supported by theore...

Based on the spatiotemporal modulation of the thermal conductivity and volumetric heat capacity, we propose a thermal diode characterized by the rectification of conductive heat currents. By transforming the Fourier's law for the heat flux and the diffusion equation for the temperature into equations with constant coefficients, it is shown that: (i...

Anderson localization of thermal phonons has been shown only in few nanostructures with strong random disorder by the exponential decay of transmission to zero and a thermal conductivity maximum when increasing the system length. In this work, we present a path to demonstrate the phonon localization with distinctive features in graded superlattices...

The understanding of hydrodynamic heat transport in finite-sized graphitic materials remains elusive due to the lack of an efficient methodology. In this paper, we develop a computational framework enabling an accurate description of heat transport in anisotropic graphite ribbons by a kinetic theory approach with full quantum mechanical first-princ...

Self-synchronization is a ubiquitous phenomenon in nature, in which oscillators are collectively locked in frequency and phase through mutual interactions. While self-synchronization requires the forced excitation of at least one of the oscillators, we demonstrate that this mechanism spontaneously appears due to activation from thermal fluctuations...

Heat transport guided by the combined dynamics of surface phonon-polaritons (SPhPs) and phonons propagating in a polar nanowire is theoretically modeled and analyzed. This is achieved by solving numerically and analytically the Boltzmann transport equation for SPhPs and the Fourier’s heat diffusion equation for phonons. An explicit expression for t...

Nano-phononic crystals have attracted a great deal of research interest in the field of nanoscale thermal transport due to their unique coherent thermal transport behavior. So far, there have been many advances in the theory and simulation studies of coherent thermal transport in nano-phononic crystals. In this paper, we summarize the state-of-the-...

In this work, a lattice Boltzmann scheme is developed for numerical solution of the phonon Boltzmann equation under Callaway's dual relaxation model in the hydrodynamic limit. Through a Chapman-Enskog expansion to the lattice Boltzmann equation with the resistive scattering term as an equivalent source term, we recover a phonon hydrodynamic equatio...

Our direct atomic simulations reveal that a thermally activated phonon mode involves a large population of elastic wave packets. These excitations are characterized by a wide distribution of lifetimes and coherence times expressing particlelike and wavelike natures. In agreement with direct simulations, our theoretical derivation yields a generaliz...

Nanophononic metamaterials have broad applications in fields such as heat management, thermoelectric energy conversion, and nanoelectronics. Phonon resonance in pillared low-dimensional structures has been suggested to be a feasible approach to reduce thermal conductivity (TC). In this work, we study the effects of imperfections in pillared nanostr...

The Anderson localization of thermal phonons has been shown only in few nano-structures with strong random disorder by the exponential decay of transmission to zero and a thermal conductivity maximum when increasing system length. In this work, we present a path to demonstrate the phonon localization with distinctive features in graded superlattice...

The understanding and modeling of inelastic scattering of thermal phonons at a solid/solid interface remain an open question. We present a fully quantum theoretical scheme to quantify the effect of anharmonic phonon-phonon scattering at an interface via non-equilibrium Green's function (NEGF) formalism. Based on the real-space scattering rate matri...

In this work, we study thermal phonon vortex in graphene ribbon by a discrete-ordinate solution of phonon Boltzmann equation under Callaway's dual relaxation model. The phonon scattering rates of normal and resistive processes are acquired from ab initio calculation without need of any empirical input parameters. The temperature, size and isotope e...

Hydrodynamic phonon transport in solids exhibits unique thermal transport behaviors, such as second sound, the Poiseuille flow, and ultrahigh thermal conductivity. However, those have been limited up to the cryogenic temperature (∼1 K) for a few materials. In this work, by employing the phonon Boltzmann transport equation , we demonstrate hydrodyna...

In this work, a lattice Boltzmann scheme is developed for numerical solution of the phonon Boltzmann equation under Callaway's dual relaxation model in the hydrodynamic limit. Through a Chapman-Enskog expansion to the lattice Boltzmann equation with the resistive scattering term as an equivalent source term, we recover a phonon hydrodynamic equatio...

The coherent quantum effect becomes increasingly important in the heat dissipation bottleneck of semiconductor nanoelectronics with the characteristic size shrinking down to few nanometers scale nowadays. However, the quantum mechanical model remains elusive for anharmonic phonon-phonon scattering in extremely small nanostructures with broken trans...

Self-synchronization is a ubiquitous phenomenon in nature, in which oscillators are collectively locked in frequency and phase through mutual interactions. While self-synchronization requires the forced excitation of at least one of the oscillators, we demonstrate that this mechanism spontaneously appears due to the activation from thermal fluctuat...

Our direct atomic simulations reveal that a thermally activated phonon mode involves a large population of elastic wavepackets. These excitations are characterized by a wide distribution of lifetimes and coherence times expressing particle- and wave-like natures. In agreement with direct simulations, our theoretical derivation yields a generalized...

The direct simulation of the dynamics of second sound in graphitic materials remains a challenging task due to lack of methodology for solving the phonon Boltzmann equation in such a stiff hydrodynamic regime. In this work we aim to tackle this challenge by developing a multiscale numerical scheme for the transient phonon Boltzmann equation under C...

The direct simulation of the dynamics of second sound in graphitic materials remains a challenging task due to lack of methodology for solving the phonon Boltzmann equation in such a stiff hydrodynamic regime. In this work, we aim to tackle this challenge by developing a multiscale numerical scheme for the transient phonon Boltzmann equation under...

The electron is the dominant heat and charge carrier in metal, yet the Monte Carlo method for thermal and electrical transport remains not fully established due to the high density and high degeneracy of electrons. In this work, we develop a deviational Monte Carlo scheme to directly solve the Boltzmann transport equation for electron transport thr...

In this work, we develop a numerical framework for gas diffusion in nanoporous materials including a random generation-growth algorithm for microstructure reconstruction and a multiple-relaxation-time lattice Boltzmann method for solution of diffusion equation with Knudsen effects carefully considered. The Knudsen diffusion is accurately captured b...

A phonon hydrodynamic equation has been recently derived from the kinetic theory of phonons for nanoscale heat transport at ordinary temperatures. The classical irreversible thermodynamics is no longer valid due to the failure of the local equilibrium hypothesis from temporal and spatial strong nonequilibrium effects. In the present paper, we inves...

The knowledge of interfacial phonon transport accounting for detailed phonon spectral properties is desired because of its importance for design of nanoscale energy systems. In this work, we investigate the interfacial phonon transport through Si/Ge multilayer films using an efficient Monte Carlo scheme with spectral transmissivity, which is valida...

The classical Fourier’s law fails in extremely small and ultrafast heat conduction even at ordinary temperatures due to strong thermodynamic nonequilibrium effects. In thiswork, a macroscopic phonon hydrodynamic equation beyond Fourier’s law with a relaxation term and nonlocal terms is derived through a perturbation expansion to the phonon Boltzman...

The single mode relaxation time approximation has been demonstrated to greatly underestimate the lattice thermal conductivity of two-dimensional materials due to the collective effect of phonon normal scattering. Callaway's dual relaxation model represents a good approximation to the otherwise ab initio solution of the phonon Boltzmann equation. In...

This paper presents a method to establish a relationship between internal microstructure and the effective gas diffusion coefficient in fibrous materials via a mesoscopic modeling approach and, when possible and based on the analysis, to propose user-friendly formulas as functions of structural parameters for practical engineering applications. The...

Minimum entropy production principle (MEPP) is an important variational principle for the evolution of systems to nonequilibrium stationary state. However, its restricted validity in the domain of Onsager’s linear theory requires an inverse temperature square dependent thermal conductivity for heat conduction problems. Previous derivative principle...

Energy loss in a flow field occurs due to the dissipation of mechanical energy, and an accurate estimation of the loss coefficient is important for both energy conservation and engineering design. In the current study, the concept of the head loss coefficient K of a local structure for turbulent flows was extended to laminar flow. A thermodynamic d...

In this paper, a generalized heat transport equation including relaxational, nonlocal and nonlinear effects is provided, which contains diverse previous phenomenological models as particular cases. The aim of the present work is to establish an extended irreversible thermodynamic framework, with generalized expressions of entropy and entropy flux....

In this paper, an approach based on the entropy production theory is applied to investigate the accuracy and stability of numerical simulation of the heat transfer and fluid flow problems, con-stituting important parts of the simulation procedure. Using the formula of the entropy produc-tion of the fluid flow and heat transfer process, the accuracy...