Low-frequency Phonons Research Articles

Study of the effect of phase state on the interfacial thermal conductance between PCMs and ceramic skeletons

ABSTRACT Composite phase change materials with sugar alcohol as the phase change material and highly thermally conductive ceramics as the porous skeleton are widely used in various thermal storage systems. The interfacial thermal conductance (ITC) between the phase change materials under different phases and the skeleton is an important factor affecting the rate of heat storage (release) in thermal storage systems. The ITC between ceramics(AlN, SiC) and sugar alcohols (mannitol and galactitol) in the solid and liquid states is investigated by means of both time-domain thermoreflectance and molecular dynamics simulations. The results show that the ITC between phase change materials and ceramic is better in liquid state than in solid state, and that the ITC between mannitol and ceramic is better, and that the ITC betweenAlN and sugar alcohol is better. More low-frequency phonons are involved in the thermal transport of the sugar alcohols in the liquid state, with an average overlap energy of about 9.5% higher than that of the solid state and an average phonon participation rate of about 6.8% higher. It was also found that it isthe H atom in the sugar alcohol that is linked to the C atom that governs the ITC.

Enhanced Thermoelectric Performance and Low Thermal Conductivity in Cu2GeTe3 with Identified Localized Symmetry Breakdown

Highly efficient and eco-friendly thermoelectric generators rely on low-cost and nontoxic semiconductors with high symmetry and ultralow lattice thermal conductivity κL. We report the rational synthesis of the novel cubic (Ag, Se)-doped Cu2GeTe3 semiconductors. A localized symmetry breakdown (LSB) was found in the composition of Cu1.9Ag0.1GeTe1.5Se1.5 (i.e., CAGTS15) with an ultralow κL of 0.37 W/mK at 723 K, the lowest value outperforming all Cu2GeCh3 (Ch = S, Se, and Te). A joint investigation of synchrotron X-ray techniques identifies the LSB embedded into the cubic CAGTS15 host matrix. This LSB is an Ångström-scale orthorhombic symmetry unit, characteristic of multiple bond lengths, large anisotropic atomic displacements, and distinct local chemical coordination of anions. Computational results highlight that such an unusual orthorhombic symmetry demonstrates low-frequency phonon modes, which become softer and more predominant with increasing temperatures. This unconventional LSB promotes bond complexity and phonon scattering, highly beneficial for extraordinarily low lattice thermal conductivity.

Light-induced shear phonon splitting and instability in bilayer graphene

Coherent engineering of landscape potential in crystalline materials is a rapidly evolving research field. Ultrafast optical pulses can manipulate low-frequency shear phonons in van der Waals layered materials through the dynamical dressing of electronic structure and photoexcited carrier density. In this work, we provide a diagrammatic formalism for nonlinear Raman force and implement it to shear phonon dynamics in bilayer graphene. We predict a controllable splitting of double degenerate shear phonon modes due to light-induced phonon mixing and renormalization according to a coherent nonlinear Raman force mechanism. Intriguingly, we obtain a light-induced shear phonon softening that facilitates structural instability at a critical field amplitude for which the shear phonon frequency vanishes. The phonon splitting and instability strongly depend on the laser intensity, frequency, chemical potential, and temperature of photoexcited electrons. This study motivates future experimental investigation of the optical fine tuning and regulation of shear phonons and layer stacking order in layered van der Waals materials.

Chebyshev pseudosite matrix product state approach for the spectral functions of electron-phonon coupling systems

The electron-phonon ($e$-ph) coupling system often has a large number of phonon degrees of freedom, whose spectral functions are numerically difficult to compute using matrix product state (MPS) formalisms. To solve this problem, we propose a new and practical method that combines the Chebyshev MPS and the pseudosite density matrix renormalization group (DMRG) algorithm. The Chebyshev vector is represented by a pseudosite MPS with global $U(1)$ fermion symmetry, which maps $2^{N_p}$ bosonic degrees of freedom onto $N_p$ pseudosites, each with two states. This approach can handle arbitrary $e$-ph coupling Hamiltonians where pseudosite DMRG performs efficiently. We use this method to study the spectral functions of the doped extended Hubbard-Holstein model in a regime of strong Coulomb repulsion, which has not been studied extensively before. Key features of the excitation spectra are captured at a modest computational cost. Our results show that weak extended $e$-ph couplings can increase the spectral weight of the holon-folding branch at low phonon frequencies, in agreement with angle-resolved photoemission observations on one-dimensional (1D) cuprates.

Phonon thermal transport in two-dimensional PbTe monolayers via extensive molecular dynamics simulations with a neuroevolution potential

Two-dimensional (2D) PbTe monolayers as newly fabricated thermoelectric materials have sparked great interest due to their excellent physical properties, which are expected to play an essential role in converting waste heat energy into electrical energy. Herein, it is imperative to have a clear and comprehensive understanding of the thermal properties of 2D PbTe monolayers, as this is critical for their practical applications. Molecular dynamics (MD) simulations are widely employed to predict physical properties at the microscopic scale and are particularly suitable for evaluating phonon thermal conductivity. Generally, predicting the thermal conductivity of 2D materials is a routine task through MD simulations when appropriate interatomic potentials exist. However, the existing interatomic potential for PbTe allotropes is not suitable for their 2D derivatives. In this paper, we develop an efficient machine-learned potential (MLP) based on a newly developed MLP model called neuroevolution potential to build a specific potential for 2D PbTe monolayers. Then, by using this potential, we report the thermal conductivity of 2D PbTe monolayers at different temperatures and under different biaxial strains. Surprisingly, we find an abnormal increase of thermal conductivity with the increase of the biaxial strain due to the enhancement of low-frequency phonons. We hope these results can play a guiding role in their practical use once upon experimental validation.

Exploring Spin-Phonon Coupling in Magnetic 2D Metal-Organic Frameworks

Layered magnetic metal-organic frameworks (MOFs) are an emerging class of materials that can combine the advantages of both MOFs and 2D magnetic crystals. The recent discovery of large coercivity and long-range magnetic ordering up to 515 K in a layered MOF of general formula MCl2(pyz)2 (M = transition metal, pyz = pyrazine) offers an exciting versatile platform to achieve high-TC magnetism at the 2D limit. In this work, we investigate the exfoliation feasibility down to the monolayer of VCl2(pyz)2 and CrCl2(pyz)2 by means of first-principles calculations. We explore their structural, electronic, magnetic and vibrational properties, as well as the effect of halide substitution. Then, we provide a full analysis of the spin-phonon coupling (SPC) in both 2D derivatives. Our calculations reveal a low SPC and thermal evolution of the magnetic exchange interactions and single-ion anisotropy mainly governed by low-frequency phonon modes. Finally, we provide chemical insights to improve the performance of these magnetic 2D MOFs based on the effective manipulation of the phonon modes that can present a major impact on their magnetic properties.

The mechanism of enhanced photothermal conversion of low-dimensional plasmonic nanofluids with LFPs resonance

Solar-thermal utilization is one of the important ways of energy utilization at present and in the future. Nanofluids play a key role in photothermal conversion. In this work, the low-dimensional plasmonic TiN/MWCNTs nanofluids were firstly proposed. The photothermal conversion properties of low-dimensional plasmonic nanofluids was systematically investigated. The analytical results indicated that the heat transfer and optical properties of plasmonic nanofluids were significantly enhanced. Specifically, thanks to the expanded light absorption by the LSPR of nano-TiN and the enhanced thermal transport by its low-frequency phonon (LFP) resonance with MWCNTs, the photothermal conversion efficiency can reach 68.1% with 10 ppm TiN/MWCNTs nanofluids, which was 22.9% higher than that of the MWCNTs nanofluids. And the photothermal conversion efficiency of TiN/MWCNTs nanofluids (40 ppm) reached 76.4%. Furthermore, through the simulation and experimental results, this paper explained the mechanism of the enhanced solar-thermal conversion performance of nanofluids from the perspective of phonon heat transfer.

Mismatched atomic bonds and ultralow thermal conductivity in Ag-based ternary chalcopyrites

Diamond like ternary chalcopyrites $MB{X}_{2}$ ($M=\text{Cu},\text{Ag}$; $B=\text{Ga},\text{In},\text{Tl}$; $X=\text{S},\text{Se},\text{Te}$) have attracted tremendous research interest in thermoelectric society due to the competitive performance and a great variety of transport properties. Interestingly, the lattice thermal conductivity of $\text{Ag}B{X}_{2}$ is systematically two to five times lower that of $\text{Cu}B{X}_{2}$, in spite of their similar crystal and band structures. Based on the careful theoretical analysis, we show that the ultralow thermal conductivity of $\text{Ag}B{X}_{2}$ originates in the mismatched atomic bonds between Ag-$X$ and B-$X$ pairs. Owing to the very expanded $4d$ orbital of Ag, the bonding strength of Ag-$X$ is much weaker than that of $B\text{\ensuremath{-}}X$. As a result, the vibrations of Ag-$X$ and B-$X$ are well separated within the low-frequency modes. The phonon density of states of $\text{Ag}B{X}_{2}$ exhibits four divided peaks, compared with the three peaks in $\text{Cu}B{X}_{2}$. The calculated joint phonon density of states reveals that such characteristic in $\text{Ag}B{X}_{2}$ would increase the scattering rate of low-frequency phonons significantly. This study provides essential understandings on the ultralow thermal conductivity of Ag-based ternary chalcopyrites and indicates a general strategy to suppress the thermal conductivity in ternary compounds.

Thermal cloaking phenomenon in the convex structure silicon film

Silicon thin films have important applications in the electronics and nanotechnology industries, and with the miniaturization of electronic devices, the importance of thermal protection for electronic devices is increasing. The researchers have successfully constructed thermal cloaks based on silicon films through amorphization, perforation, and concave to achieve heat flux regulation. The key to designing a thermal cloak is to use appropriate means to construct a functional region with low thermal conductivity. Our recent research has found that the thermal conductivity of periodically convex silicon films is lower than that of perfect silicon films, and no thermal cloaks have been constructed using this method. Therefore, in this paper, a thermal cloak is constructed using a periodically convex structure and compared with a thermal cloak of the same concave depth. By calculating the ratio of thermal cloaking (RTC), we find that cloaking can be produced using the convex structure, but is less effective compared to the concave structure, mainly because the base film is a perfect silicon film, which does not affect the heat flux transmission. Finally, we use phonon localization theory to explore the underlying mechanism. By calculating the mode participation rate (MPR), we find that phonon localization in the functional region is the main reason for cloaking, and the low-frequency phonon modes contribute more to the cloaking efficiency. Our study can provide experience for the design of nanoscale thermal cloaks.

Significantly reinforced thermoelectric performance in the novel 1T-Au6Se2 monolayer

Ultra-low lattice thermal conductivity has long been a requirement for the high thermoelectric properties of materials. In this work, the novel 1T-Au6Se2 monolayer was obtained by introducing Au6 clusters into the selenide monolayer, and its electrical and thermal transport characteristics are investigated using first-principles computations supplemented with semi-classical Boltzmann transport theory. The calculation shows that the 1T-Au6Se2 monolayer exhibits ultra-low lattice thermal conductivity and excellent thermoelectric properties owing to its low phonon frequency, group velocity, and extremely strong anharmonicity. Based on strain engineering from 0% to 2%, the lattice thermal conductivity further reduces by restricting the thermal transport on the premise of maintaining outstanding electrical transport properties in the p-type doped system. Thence, the value of ZT for the p-type system increases nearly by 70% compared with the non-stressed state at 700 K. Our investigation indicates the ultra-low thermal conductivity and high ZT of the 1T-Au6Se2 monolayer that might be prepared in the lab, providing new insights into enhancing the thermoelectric performance of the material in the future.

Anharmonic phonon renormalization and two-channel thermal transport in SrTiO3 using full temperature-dependent interatomic force constant

The microscopic mechanism of lattice thermal conductivity of SrTiO3 is investigated by full temperature-dependent anharmonic phonon renormalization and two-channel thermal transport. The contribution of normal phonon is calculated by the Boltzmann transport equation, while the contribution of diffusion phonon is treated by random-walk diffusion theory and wave-like tunneling of phonon. Based on the anharmonic phonon renormalization, the full temperature-dependent interatomic force constant is utilized to calculate the thermal conductivity of normal phonon and diffusion phonon. With the combined inclusion of normal phonon and diffusion phonon, our work successfully reproduces the experimental thermal conductivity of SrTiO3 on top of full temperature-dependent anharmonic phonon renormalization. Our results show that the anharmonic phonon renormalization (a) strongly hardens the acoustic branch and softens the optical branch, (b) suppresses the weighted phase space of low-frequency phonon modes, reducing the coupling between acoustic and optical phonons, and (c) diminishes the strength of three-phonon scattering and four-phonon scattering, enlarging the lattice thermal conductivity. The anharmonic phonon renormalization plays a significant role in the thermal transport process of SrTiO3, and the diffusion phonon transport also makes an important contribution to the total thermal conductivity. The methodology may be used to resolve the discrepancies between experimental and theoretical thermal conductivity.

Phonon-driven intra-exciton Rabi oscillations in CsPbBr3 halide perovskites

Coupling electromagnetic radiation with matter, e.g., by resonant light fields in external optical cavities, is highly promising for tailoring the optoelectronic properties of functional materials on the nanoscale. Here, we demonstrate that even internal fields induced by coherent lattice motions can be used to control the transient excitonic optical response in CsPbBr3 halide perovskite crystals. Upon resonant photoexcitation, two-dimensional electronic spectroscopy reveals an excitonic peak structure oscillating persistently with a 100-fs period for up to ~2 ps which does not match the frequency of any phonon modes of the crystals. Only at later times, beyond 2 ps, two low-frequency phonons of the lead-bromide lattice dominate the dynamics. We rationalize these findings by an unusual exciton-phonon coupling inducing off-resonant 100-fs Rabi oscillations between 1s and 2p excitons driven by the low-frequency phonons. As such, prevailing models for the electron-phonon coupling in halide perovskites are insufficient to explain these results. We propose the coupling of characteristic low-frequency phonon fields to intra-excitonic transitions in halide perovskites as the key to control the anharmonic response of these materials in order to establish new routes for enhancing their optoelectronic properties.

Conductivity of an electron coupled to anharmonic phonons: Quantum-classical simulations and comparison of approximations

We study the impact of phonon anharmonicity on the electronic dynamics of soft materials using a nonperturbative quantum-classical approach. The method is applied to a one-dimensional model of doped organic semiconductors with low-frequency intermolecular lattice phonons. We find that anharmonicity that leads to phonon hardening increases the mobility and anharmonicity that leads to phonon softening decreases the mobility. With ab initio simulations in mind, we also test various approximations, including the use of adiabatic phonon disorder, an effective harmonic model with temperature-dependent frequencies, and the Boltzmann transport equation with second-order perturbation theory scattering rates. Overall, we find surprisingly good agreement between all methods but that accounting for phonon anharmonicity is important for accurate prediction of electronic transport including both quantitative mobility values and their qualitative temperature dependence. For the model studied, phonon lifetime effects have relatively little impact on carrier transport, but the effective frequency shift due to anharmonicity is essential. In cases with highly asymmetric, non-Gaussian disorder, an effective harmonic model cannot quantitatively reproduce mobilities or finite-frequency conductivity, and this is especially true for acoustic phonons.

Suppressing Nonradiative Recombination by Electron-Donating Substituents in 2D Conjugated Triphenylamine Polymers toward Efficient Perovskite Optoelectronics.

Highly efficient perovskite optoelectronics (POEs) have been limited by nonradiative recombination. We report a strategy to inhibit the nonradiative recombination of 2D triphenylamine polymers in the hole transport layer (HTL) via introducing electron-donating groups to enhance the conjugation effect and electron cloud density. The conjugated systems with electron-donating groups present smaller energy level oscillation compared to the ones with electron-absorbing groups, as confirmed by nonadiabatic molecular dynamics (NAMD) calculation. Further study reveals that the introduction of low-frequency phonons in the electron-donating group systems shortens the nonadiabatic coupling and inhibits the nonradiative recombination. Such electron-donating groups can decrease the valence band maximum of 2D polymers and promote hole transport. Our report provides a new design strategy to suppress nonradiative recombination in HTL for application in efficient POEs.

The impact of pure and mixed self-activated YXO4 phosphor materials (X=V, Nb and Ta) on downshifting and quantum cutting emission behaviours of Ln3+ doped (Ln3+=Ho3+ and Yb3+) ions

In this work, the downconversion (DC) behaviour of Ho3+ ions doped in pure YVO4, YNbO4, YTaO4 hosts and their intermixed (i.e. Ta/V, Ta/Nb and Nb/V) compositions have been investigated and compared. The samples were synthesized by solid-state reaction technique at 1473 K. The UV–Vis absorption and photoluminescence behaviour of 1 mol % Ho3+ doped YVO4, YNbO4 and YTaO4 as well as their intermixed compositions have been studied. The DS green emission of Ho3+ ion in vanadate host (λexc = 326 nm) is 5 times stronger than in niobate host (λexc = 262 nm) due to efficient energy transfer between [VO4]3- and Ho3+ ion. The downshifting (DS) emissions are also investigated on direct excitation of Ho3+ ion in YVO4, YNbO4 and YTaO4 hosts on excitation with 450/456 nm (5I8→5G6 transition of Ho3+ ion) wavelength. It is found that the overall DS emission intensity is dominant in YNbO4:Ho3+ phosphor due to low phonon frequency and larger particle size of YNbO4. The Ho3+ doped in mixed hosts viz. YTa1-xVxO4, YTa1-yNbyO4, YNb1-zVzO4 (where x, y and z = 0, 0.25, 0.50, 0.75 and 1.0) phosphors also emit intense green DS emission along with their violet-blue band emission on ultraviolet (UV) excitation. The mixing of Ta/V and Ta/Nb enhances the blue emission of host matrix on UV excitation. The green emission is found to increase with ‘V’ and ‘Nb’ concentrations in YTa1-xVxO4:0.01Ho3+, YTa1-yNbyO4:0.01Ho3+, respectively. Further, we also investigated quantum cutting (QC) in YNb1-zVzO4:5 mol% Yb3+ (where z = 0, 0.25, 0.50, 0.75, 1.0) phosphors on UV excitation (λexc = 262 and 326 nm). The intensity of QC NIR emission is found to increase with the incorporation of ‘V’ in YNb1-zVzO4 phosphors. The strongest QC emission is observed in pure YVO4 host as compared to pure YNbO4 as well as other mixed matrices. The QC efficiency of NIR emission for YVO4:5 Yb has been calculated and found to 129%. Therefore, the prepared phosphor material is highly applicable to increase the efficiency of Si- solar cell by QC NIR emission.

Thermal Transport Study in a Strained Carbon Nanotube and Graphene Junction Using Phonon Wavepacket Analysis

This study investigates single-mode phonon scattering from a junction structure consisting of a (6,6) single-walled carbon nanotube (SWCNT) and graphene, subject to mechanical deformation, using phonon wavepacket analysis. Results show that longitudinal acoustic (LA) and transverse acoustic (TA) phonons at low frequencies are transmitted more effectively through the SWCNT–graphene junction when the junction is deformed. As low-frequency phonons in LA and TA modes are major energy carriers, it is expected that thermal transport across the SWCNT–graphene junction will be more efficient when the junction is deformed. Interfacial thermal resistance across the SWCNT-graphene junction was calculated using reverse nonequilibrium molecular dynamics (RNEMD). The RNEMD results show that the interfacial thermal resistance decreases when the structure is elongated, deforming the junction between the SWCNT and graphene. However, there was no notable difference in the transmission of twisting (TW) and flexural (FO) phonons when the junction was deformed. The study also showed that the transmission of phonon energy through the SWCNT–graphene junction has a slight dependence on the group velocity of phonons, with phonons having higher group velocities transmitting the junction more effectively. The findings of this research will play a significant role in advancing the development of futuristic electronics by providing a tool for developing 3D carbon nanostructures with high thermal performance under mechanical deformation.

Phonon hardening and the effect of phonon transport in cubic antiperovskites [formula omitted] (A = Li, Na; B = Se, Te) induced by quartic anharmonicity

In this article, we aim at the structural stability, lattice thermal conductivity, transport properties of four cubic antiperovskites A3FB using first-principle calculations combined with self-consistent phonon theory(SCP), compressive sensing techniques (CS), and Boltzmann transport equation (BTE), with a particular focus on the 4 phonon scattering rates and phonon frequency shifts caused by quartic anharmonicity. The results indicate that the strong quartic anharmonicity of alkali metal atoms Na atom harden low-frequency phonon branch to ensure dynamic stability of the material. We also find that the quartic anharmonicity play a crucial role in obtaining the reasonable temperature dependence of κL. In addition, we analyze the impact of phonon group velocities, the scattering rate and scattering phase space of 3ph and 4ph on phonon transport, revealing that the low κL of Na3FSe is due to the strong 4ph SRs which is dominated by redistribution and Umklapp processes. Our studies provide the microscopic understanding of the quartic anharmonicity together with their impact on lattice dynamics and phonon transport is not only of indispensable interest but also essential to design and explore materials with ultralow thermal conductivity.

Delocalized Bi-tetrahedral cluster induced ultralow lattice thermal conductivity in Bi3Ir3O11

Low-frequency cluster vibration plays an important role in reducing the lattice thermal conductivity of a system. Here, we show a new type of cluster vibration called “delocalized cluster vibration” that involves a highly symmetric local structure with lone pair electrons and marked impacts on lattice thermal transport. We take a transition-metal oxide Bi3Ir3O11 (two specific Wyckoff positions for Bi: corner-Bi1 and inner-Bi2) as an example, which exhibits counterintuitive low lattice thermal conductivity in metallic materials. Based on first-principles simulations of phonon transport in the Boltzmann transport formulation, this study shows that the four fourth neighbor Bi2 atoms form a regular Bi-tetrahedral cluster (Bi24), which collectively vibrates in certain intercorrelated patterns strongly and induces low-frequency optical phonon modes (∼0.77 and ∼1.06 THz) along with low group velocities. The Bi24-related optical modes interrupt the acoustic vibrations and thus suppress the lattice thermal conductivity to an ultralow value (calculated value is 1.0 W/m·K at 300 K). Surprisingly, these Bi2–Bi2 have relatively large distances but significant interatomic force constants, indicating delocalized interactions among Bi24. This work demonstrates that the delocalized cluster vibration plays a significant role in lowering lattice thermal conductivity.

High-frequency phonons drive large phonon-drag thermopower in semiconductors at high carrier density

It has been well established that (i) the thermopower of semiconductors can be enhanced through a phenomenon known as the drag effect, and (ii) the drag enhancement involves only low-frequency acoustic phonons and benefits from low electron densities and low temperatures. Using first-principles calculations we show that large drag enhancements to the thermopower are possible at high carrier density even at room temperature and arise from high-frequency acoustic phonons. A fascinating example is cubic boron arsenide (BAs) for which the calculated room temperature drag enhancement of the thermopower exceeds an order of magnitude at a high hole density of ${10}^{21}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$. This remarkable behavior stems from the simultaneously weak phonon-phonon and phonon-hole scattering of the high-frequency phonons in BAs that become drag active at high carrier densities through electron-phonon interactions. This work advances our understanding of coupled electron-phonon nanoscale transport and introduces an unexpected paradigm for achieving large thermopowers.

Electro-optic properties of ZrO2 , HfO2 , and LiNbO3 ferroelectric phases: A comparative density functional study

We report the Pockels electro-optic properties of ${\mathrm{ZrO}}_{2}$ and ${\mathrm{HfO}}_{2}$ orthorhombic $Pbc{2}_{1}$ and rhombohedral $R3m$ ferroelectric phases, and we compare them to the well-known rhombohedral $R3c$ ${\mathrm{LiNbO}}_{3}$ Pockels material from density functional theory calculations using the crystal suite of programs. Specifically, three essential processes are explicitly investigated: The electronic, the ionic (or vibrational), and the piezoelectric contributions. Our calculations reveal that the ionic part coming from the low-frequency phonon modes contributes the most to the electro-optic coefficients of rhombohedral ${\mathrm{LiNbO}}_{3}$ and of orthorhombic ${\mathrm{ZrO}}_{2}$ and ${\mathrm{HfO}}_{2}$. Additionally, the low-frequency phonon modes exhibit zero contribution to the Pockels coefficients of ${\mathrm{ZrO}}_{2}$ and ${\mathrm{HfO}}_{2}$ rhombohedral $R3m$ phase.