Phonon Heat Research Articles

Phonon transport mechanism of HfO2 ultrathin film with temperature-correction full-band Monte Carlo simulation

Hafnium dioxide (HfO2) has been widely used in microelectronics nowadays and commonly withstands extremely high temperatures, so the investigation of its thermodynamic properties is particularly essential. This paper develops a temperature-correction full-band Monte Carlo (TFMC) method to investigate the HfO2 ultrathin film. The phonon dynamics parameters of HfO2 are calculated based on the first-principles method. TFMC can better describe the phonon cumulative distribution function in different temperatures by modifying the phonon relaxation time and heat capacity. The thermal conductivity of HfO2 ultrathin film is calculated based on the above method and is in good agreement with the literature. It is observed that the optical phonons in HfO2 ultrathin film are prominent in the phonon heat transport, which is quite different from the mechanism in common semiconductor materials. Combined with the full-band diffuse mismatch model, the Si-based HfO2 ultrathin film is studied. It is found that the existence of the interface with substrates improves the thermodynamic properties of the ultrathin film, which provides a reference for the selection of substrate materials.

Dynamic and giant bandgap renormalization dictates the transient optical response in perovskite quantum dots

Transient optical response in perovskite quantum dots (QDs) has remained elusive until now, which hinders their full utilization in optoelectronics. Herein, we reveal that the bandgap renormalization (BGR) dictates the main spectral and dynamical features of transient response in CsPbBr3 QDs. By monitoring the absorption spectral evolution of the monodispersed QDs, the representative BGR is explicitly observed, giving rise to the photoinduced absorption at the higher energy side of the lowest exciton peak in transient absorption spectroscopy. The BGR gradually increases upon photoexcitation as a result of the carrier distribution dependent screening effect. We further demonstrate that the BGR arises from both Coulomb screening and phonon heating under high pump intensities in CsPbBr3 QDs. The synergistic effect leads to the giant BGR energy (ΔE > 60 meV) and the unconventional relationship of ΔE ∝ n1/2, where n is the carrier density. These findings are important for the fundamental understanding and potential applications of the emerging halide perovskite semiconductors.

Intrinsic thermal conductivities of BC3-C3N superlattice nanoribbons: A molecular dynamics study

Superlattice nanostructures enable controllable thermal conductivity minimization in nanodevices for thermoelectric applications. This is especially true regarding recently developed carbon nitride (C 3 N) and boron carbide (BC 3 ) nanostructures. In this study, we explored phonon heat transport in a superlattice nanoribbon with C 3 N and BC 3 domains using non-equilibrium molecular dynamics (NEMD) simulation. Specifically, we investigated the impacts of changing the unit cell length, nanoribbon length, average temperature, and temperature difference between the hot source and cold sink on the thermal conductivity of the nanoribbon. Based on our results, the value of the intrinsic thermal conductivity ( k ∞ ) reaches a minimum of 206 W m -1 K -1 at room temperature for a superlattice nanoribbon unit cell length of 8 . 5 nm. At infinite total length, the minimum thermal conductivity obtained for the BC 3 -C 3 N superlattice nanoribbon is about 40% and 25% of the thermal conductivities of pristine BC 3 and C 3 N nanoribbons, respectively. The observed minimum thermal conductivity in the nanoribbons is attributed to the phonons transitioning from coherent to incoherent transport, where the unit cell length is comparable to the phonon coherence length. The results of this work provide atomistic insights into the development of low-thermal-conductivity nanomaterials for thermal insulation applications.

Transient Three-Dimensional Thermal Simulation of a Fin Field-Effect Transistor With Electron–Phonon Heat Generation, Three Phonon Scattering, and Drift With Periodic Switching

Abstract The International Roadmap for Devices and Systems shows Fin field-effect transistor (FET) array transistors as the mainstream structure for logic devices. Traditional methods of Fourier heat transfer analysis that are typical of Technology computer-aided design (tcad) software are invalid at the length and time scales associated with these devices. Traditional models for phonon transport modeling have not demonstrated the ability to accurately model three dimensional (3-d), transient transistor thermal responses. An engineering design tool is needed to accurately predict the thermal response of FinFET transistor arrays. The statistical phonon transport model (SPTM) was applied in a 3-d, transient manner to predict nonequilibrium phonon transport in an silicon-on-insulator (SOI)-FinFET array transistor with a 60 nm long fin and a 20 nm channel length. A heat generation profile from electron–phonon scattering was applied in a transient manner to model switching. Simulation results indicated an excess build-up of up to 17% optical phonons giving rise to transient local temperature hot spots of 37 K in the drain region. The local build-up of excess optical phonons in the drain region has implications on performance and reliability. The SPTM is a valid engineering design tool for evaluating the thermal performance of emergent proposed FinFET transistor designs. The phonon fidelity of the SPTM is greater than Monte Carlo and the Boltzmann Transport Equation and the length scale and time scale fidelity of the SPTM is better than direct atomic simulation.

Sympathetic electromagnetically induced transparency ground state cooling of a 40Ca+–27Al+ pair in an 27Al+ clock

We report on electromagnetically induced transparency cooling of 40Ca+ to sympathetically cool the three-dimensional secular modes of motion in a 40Ca+–27Al+ two-ion pair near the ground state. We observe simultaneous ground state cooling across all radial modes and axial modes of a 40Ca+–27Al+ ion pair, occupying a broader cooling range in frequency space over 3 MHz. The cooling time is observed to be less than 1 ms. The mean phonon number and heating rates of all motional modes are measured. This study is not only an important step for reducing the secular motion time-dilation shift uncertainty and uptime ratio of 27Al+ optical clock, but also essential for high-fidelity quantum simulations and quantum information processors using trapped ions.

Polarization-dependent subpicosecond demagnetization in iron garnets

Controlling the magnetization dynamics at the fastest speed is a major issue of fundamental condensed matter physics and its applications for data storage and processing technologies. It requires a deep understanding of the interactions between the degrees of freedom in solids, such as spin, electron, and lattice as well as their responses to external stimuli. In this paper, we systematically investigate the fluence dependence of ultrafast magnetization dynamics induced by below-bandgap ultrashort laser pulses in the ferrimagnetic insulators ${\mathrm{Bi}}_{x}{\mathrm{Y}}_{3\ensuremath{-}x}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ with $1\ensuremath{\le}{x}_{\mathrm{Bi}}\ensuremath{\le}3$. We demonstrate subpicosecond demagnetization dynamics in this material followed by a very slow remagnetization process. We prove that this demagnetization results from an ultrafast heating of iron garnets by two-photon absorption (TPA), suggesting a phonon-magnon thermalization time of $\ensuremath{\le}0.6$ ps. We explain the slow remagnetization timescale by the low phonon heat conductivity in garnets. Additionally, we show that the amplitudes of the demagnetization, optical change, and lattice strain can be manipulated by changing the ellipticity of the pump pulses. We explain this phenomenon considering the TPA circular dichroism. These findings open exciting prospects for ultrafast manipulation of spin, charge, and lattice dynamics in magnetic insulators by ultrafast nonlinear optics.

The role of phonon anharmonicity on the structural stability and phonon heat transport of CrFeCoNiCux high-entropy alloys at finite temperatures

Chemical disorder and lattice distortion make high-entropy alloys (HEAs) promising to become a new generation of thermoelectric materials. However, the large parameter space of HEAs makes it still a great challenge to study their anharmonic unfolded phonon properties, and the role of anharmonicity in their structural stability and phonon heat transport is still unclear. Here we provide a new scheme to achieve a breakthrough in obtaining the unfolded anharmonic phonon properties from the dynamic trajectories. Through careful analysis of the unfolded phonon dispersions, we found that CrFeCoNiCu HEA has a special anharmonic phonon scattering distribution, which makes the contribution of short-wavelength and long-wavelength phonons to lattice thermal conductivity significantly different. In addition, we have developed a model for estimating the formation energy of HEAs based on the formation energy of binary alloy from first-principles calculation. Our results also show that anharmonic phonon scattering plays an important role in the stability of CrFeCoNiCux HEAs solid-solution phase, which also complements the dynamic mechanism of the formation and stability of dual-FCC phase. CrFeCoNiCu HEA has very low lattice thermal conductivity even at extremely low temperatures. The contribution of long-wavelength phonons to lattice thermal conductivity is more than ten times that of short-wavelength phonons, and they are the main contributors of phonon heat transport of CrFeCoNiCu HEA. This study makes it possible to study the unfolded phonon properties of HEAs at finite temperature and provides important theoretical support and information for the stability of solid-solution structure and phonon engineering for HEAs.

Simulations of heat transport in single-molecule junctions: Investigations of the thermal diode effect.

With the objective of understanding microscopic principles governing thermal energy flow in nanojunctions, we study phononic heat transport through metal-molecule-metal junctions using classical molecular dynamics (MD) simulations. Considering a single-molecule gold-alkanedithiol-gold junction, we first focus on aspects of method development and compare two techniques for calculating thermal conductance: (i) The Reverse Nonequilibrium MD (RNEMD) method, where heat is inputted and extracted at a constant rate from opposite metals. In this case, the thermal conductance is calculated from the nonequilibrium temperature profile that is created at the junction. (ii) The Approach-to-Equilibrium MD (AEMD) method, with the thermal conductance of the junction obtained from the equilibration dynamics of the metals. In both methods, simulations of alkane chains of a growing size display an approximate length-independence of the thermal conductance, with calculated values matching computational and experimental studies. The RNEMD and AEMD methods offer different insights, and we discuss their benefits and shortcomings. Assessing the potential application of molecular junctions as thermal diodes, alkane junctions are made spatially asymmetric by modifying their contact regions with the bulk, either by using distinct endgroups or by replacing one of the Au contacts with Ag. Anharmonicity is built into the system within the molecular force-field. We find that, while the temperature profile strongly varies (compared with the gold-alkanedithiol-gold junctions) due to these structural modifications, the thermal diode effect is inconsequential in these systems-unless one goes to very large thermal biases. This finding suggests that one should seek molecules with considerable internal anharmonic effects for developing nonlinear thermal devices.

Order of magnitude reduction in Joule heating of single molecular junctions between graphene electrodes.

Maintaining stability of single-molecular junctions (SMJs) in the presence of current flow is a prerequisite for their potential device applications. However, theoretical understanding of nonequilibrium heat transport in current-carrying SMJs is a challenging problem due to the different kinds of nonlinear interactions involved, including electron-vibration and anharmonic vibrational coupling. Here, we overcome this challenge by accelerating Langevin-type current-induced molecular dynamics using machine-learning potential derived from density functional theory. We show that SMJs with graphene electrodes generate an order of magnitude less heating than those with gold electrodes. This is rooted in the better phonon spectral overlap of graphene with molecular vibrations, rendering harmonic phonon heat transport being dominant. In contrast, in a spectrally mismatched junction with gold electrodes, anharmonic coupling becomes important to transport heat away from the molecule to surrounding electrodes. Our work paves the way for studying current-induced heat transport and energy redistribution in realistic SMJs.

Phononic heat conductance of gold atomic contacts: Coherent versus incoherent transport

We present here a theoretical method to determine the phononic contribution to the thermal conductance of nanoscale systems in the phase-coherent regime. Our approach makes use of classical molecular dynamics (MD) simulations to calculate the temperature-dependent dynamical matrix, and the phononic heat conductance is subsequently computed within the Landauer-B\"uttiker formalism with the help of nonequilibrium Green's function techniques. Tailored to nanostructures, crucial steps of force constant and heat transport calculations are performed directly in real space. As compared to conventional density functional theory (DFT) approaches, the advantage of our method is two-fold. First, interatomic interactions can be described with the method of choice. Semiempirical potentials may lead to large computational speedups, enabling the study of much larger systems. Second, the method naturally takes into account the temperature dependence of atomic force constants, an aspect that is ignored in typical static DFT-based calculations. We illustrate our method by analyzing the temperature dependence of the phononic thermal conductance of gold (Au) chains with lengths ranging from 1 to 12 atoms. Moreover, in order to evaluate the importance of anharmonic effects in these atomic-scale wires, we compare the phase-coherent approach with nonequilibrium MD (NEMD) simulations. We find that the predictions of the phase-coherent method and the classical NEMD approach largely agree above the Debye temperature for all studied chain lengths, which shows that heat transport is coherent and that our phase-coherent approach is well suited for such nanostructures.

Lattice Boltzmann method with effective correction of phonon properties for nano/microscale heat transfer

The significant self-heating effect severely restricts the performance and reliability of nano-electronic devices. Accordingly, it is very important to understand the process and mechanism of nano/microscale heat transfer for thermal management and thermal design of devices. In this work, we propose a new Lattice Boltzmann Method (LBM) scheme with effective correction of phonon mean free path (MFP) and relaxation time to study phonon heat transfer in silicon thin films and silicon medium with defects, where the correction factor is dependent on the lattice structure of LBM. The transformation analysis of phonon transfer mechanism at different scales shows that the size effect of cross-plane thermal conductivity is more remarkable than that of in-plane thermal conductivity. And the thermal conductivity of silicon medium with defects decreases exponentially as defect density increases. The proposed new LBM scheme can generate more accurate results than the traditional ones in the heat conduction simulations of different nano/microscale structures.

Diagonal and off-diagonal thermal conduction with resonant phonon scattering in Ni3TeO6

The coupling between phonon and magnon is ubiquitous in magnetic materials and plays a crucial role in many aspects of magnetic properties, most notably in spintronics. Yet, this academically and technologically interesting problem still poses a severe challenge to a general understanding of the issue in certain materials. We report that Ni3TeO6 exhibits clear evidence of significant magnon-phonon coupling in both longitudinal thermal conductivity ($\kappa_{xx}$) and thermal Hall coefficient ($\kappa_{xy}$). The Debye-Callaway model, a phenomenological description for phonon heat conduction, can explain the measured magnetic field dependence of $\kappa_{xx}(H)$: phonon scattering from spin fluctuation in the paramagnetic phase and additional scattering due to magnon-phonon coupling in the collinear antiferromagnetic phase. We further suggest that a similar approach could be applied to understand the finite $\kappa_{xy}$ values in $Ni_3TeO_6$.

Numerical simulation of homojunction p-i-n In0.4Ga0.6N solar cell with different absorber layer configurations

The effects of thickness, carrier densities, and absorber layer (p-In0.4Ga0.6N) configurations were evaluated. Numerical studies were performed to optimize the homojunction p-i-n In0.4Ga0.6N solar cell using SCAPS-1D. With the optimized thicknesses and carrier densities, the p-i-n In0.4Ga0.6N solar cell shows a maximum conversion efficiency of 18.74 %. By changing the fixed indium composition of the absorber layer into step- and linear-graded configurations, the conversion efficiency, open-circuit voltage, and short-circuit current density of the solar cell were further improved. The results showed that the solar cells with step- and linear-graded absorbers attained maximum conversion efficiency of 19.77 % and 19.84 %, respectively. These results implied that the absorber with the graded compositional design helps to reduce the barrier height of electron-hole transport for band-to-band and inter-band absorption. This eventually reduces the energy loss caused by phonon emission and heat thermalization.

Phonon thermal transport and its tunability in GaN for near-junction thermal management of electronics: A review

The heat dissipation issue has now become one of the most important bottlenecks for power electronics due to the rapid increase in power density and working frequency. Towards the wide bandgap semiconductor GaN high electron mobility transistors (HEMTs), near-junction thermal management is the breakthrough that mainly includes the accurate thermal modeling and effective thermal design for GaN and GaN HEMTs. In this review, we first offer a comprehensive understanding of phonon thermal transport in GaN and GaN HEMTs, including non-equilibrium transport of electrons and phonons in the heat generation process, phonon thermal conductivity, heat spreading, and interfacial thermal transport. Then, we review the current tuning mechanisms and methods for thermal transport in GaN and GaN HEMTs which are classified into three categories according to the particle nature, wave nature, and topological nature of phonons. At last, we conclude by providing our perspectives on challenges and opportunities in the research of phonon thermal transport and its tuning mechanisms in GaN HEMTs.

Regulatable thermal conductivity and excellent mass transport of water-filled carbon nanotube as capillary wicks

• The heat and mass transfer of single wall carbon nanotubes as capillary wicks are studied by molecular dynamics simulations. • The regulatable thermal conductivity can be beneficial to both start-up and overheating operation of heat pipe. • The thermal conductivity of carbon nanotube is reduced due to the thermal motion of water molecules and vdW force. Passive liquid cooling of ultrathin heat pipe has received considerable attention due to its excellent performance of removing heat from hot spot of electronic devices. However, achieving controllable thermal conductivity of the capillary wick materials to reduce the start-up time and total thermal resistance remains a challenge. Here we propose the capillary wick of carbon nanotube array to regulate its heat transfer performance via the water filling ratio. Molecular dynamics simulation results found that the thermal conductivity of water-filled carbon nanotubes is reduced by 17.8%-23.8%, which is benefit to start-up operation. At overheating operation, the high thermal conductivity of empty carbon nanotube can reduce the temperature of the endothermic surface. Importantly, the thermophoretic movement speed of water can reach up to 74 m/s and radial mass exchange occurs driven by temperature gradient. Phonon spectral analysis and heat flow decomposition revealed that the decrease of water-filled carbon nanotube thermal conductivity originates from the low-energy coupling of van der Waals force between water molecules and carbon nanotube and the thermal motion of two water molecules layers. The regulatable thermal conductivity and excellent mass transport performance demonstrate the promising potential of carbon nanotube as next-generation capillary wick of ultrathin heat pipe.

Molecular dynamics study on the contribution of anisotropic phonon transmission to thermal conductivity of silicon

The analysis of the contribution of anisotropic phonon transmission to thermal conductivity is helpful to focus on high-energy phonons in heat transport. We calculated a series of anharmonic phonon properties and heat transport properties of Si by Fourier projection method from atomic trajectories. Under this theoretical scheme, we have obtained very consistent results with the experimental data through very low computational cost, especially the anharmonic phonon properties at high temperature. We carefully analyze the contribution of different phonons to thermal conductivity and the anisotropic feature of phonon. It is found that the longitudinal acoustic (LA) phonons have the special thermal broadening near the point L at the boundary of the Brillouin zone. The optical phonons cannot be safely ignored in the study of heat transport, especially the longitudinal optical phonon that shows a large contribution to thermal conductivity at room temperature. The thermal conductivity contribution of different phonons varies with temperature. The anisotropic features of the contribution of different phonons to thermal conductivity are mainly reflected in the short-wavelength phonons. Our work explains the reason why other research works have different opinions on whether LA phonon is the main contributor of thermal conductivity. These investigations also provide insights for further understanding phonon heat transport and distribution of high-energy phonons.

Phonon heat capacity and self-heating normal domains in NbTiN nanostrips

Self-heating normal domains in thin superconducting NbTiN nanostrips with the granular structure were characterized via steady-state hysteretic current–voltage characteristics measured at different substrate temperatures. The temperature dependence and the magnitude of the current, which sustains a domain in equilibrium at different voltages, can only be explained with a phonon heat capacity noticeably less than expected for 3D Debye phonons. This reduced heat capacity coincides with the value obtained earlier from magnetoconductance and photoresponse studies of the same films. The rate of heat flow from electrons at a temperature to phonons in the substrate at a temperature is proportional to with the exponent p ≈ 3, which differs from the exponents for heat flows mediated by the electron–phonon interaction or by escaping of 3D Debye phonons via the film/substrate interface. We attribute both findings to the effect of grains on the phonon spectrum of thin NbTiN films. Our findings are significant for understanding the thermal transport in superconducting devices exploiting thin granular films.

Phonon anharmonicity in multi-layered WS2 explored by first-principles and Raman studies

Anharmonicity is an essential property of two-dimensional materials due to its significant impact on lattice dynamics – involving interactions between phonons, thermal expansion of the structure, and interaction with the substrate. The effects manifest in temperature-dependent redshifts of the phonon frequencies and reduced lifetime. In this work, we investigate phonon anharmonicity in supported 1–5 layered WS2 by performing Raman measurements in the temperature range of 80–500 K and explore the results by ab-initio DFT study. An ab-initio model including three-phonon interaction processes and thermal expansion of all geometrical degrees of freedom (in-plane lattice constant, interlayer distance, and monolayer thickness) well describes the experimental temperature trends of the fundamental E2g1 (in-plane) and A1g (out-of-plane) phonon modes, giving a great promise of using simulation to quantitatively analyze phonon propagation and heat transport in multi-layered two-dimensional materials. The models predict a noticeable increase in the temperature-induced slope of A1g vibrations’ frequency with the number of layers – primarily due to the thermal expansion of interlayer distance. Additionally, we study the impact of the substrate on the phonon properties – concerning cumulated effects of induced strain and electron doping. We show that the effect of biaxial strain is the most significant in the E2g1 phonon mode and independent of the number of layers, while the excess charge is more significant in thin films and affects mainly the A1g mode. The presented studies based on a combination of ab-initio and experimental approaches can be extended to quantitatively analyze phonon evolution with structural modification and external stimuli.

Atomistic simulation of phonon heat transport across metallic vacuum nanogaps

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 function (NEGF) methods. The relaxed atomic configuration and interaction force constants of metallic nanogaps are generated from MD as inputs into harmonic phonon NEGF. Phonon tunneling across gold-gold and copper-copper nanogaps is quantified, and is shown to be a significant heat transport channel below gap size of 1nm. We demonstrate that lattice anharmonicity contributes to within 20-30% of phonon tunneling depending on gap size, whereas electrostatic interactions turn out to have negligible effect for the small bias voltage typically used in experimental measurements. This work provides detailed information of the heat current spectrum and interprets the recent experimental determination of thermal conductance across gold-gold nanogaps. Our study contributes to deeper insight into heat transport in the extremely near-field regime, as well as hints for the future experimental investigation.

Extreme in-plane phonon blocking in simple chainlike crystals

Taking chainlike $\mathrm{Tl}X{\mathrm{Se}}_{2}$ $(X=\mathrm{Tl}, \mathrm{In}, \mathrm{Al})$ as examples, we find that weak in-plane atomic interactions in these crystals intrinsically lead to extreme in-plane phonon blocking and heat flux confinement. We explicitly evaluate the propagative and diffusive lattice thermal transports based on the unified theory on top of the fourth-order lattice anharmonicity, and the results manifest that high-frequency phonons make negligible contributions to two channel heat transfer along the in-plane directions. This phenomenon provides a feasible approach for an effective suppression of the overall lattice thermal conductivity $({\ensuremath{\kappa}}_{\mathrm{L}})$. The findings are also applicable to a series of quasi-one-dimensional solids with different crystal structures. This work provides fundamental insight into the understanding and minimization of low ${\ensuremath{\kappa}}_{\mathrm{L}}$ of quasi-one-dimensional crystals by focusing on their harmonic lattice dynamics.