Phonon Anharmonicity Effects Research Articles

Quasi-harmonic theory for phonon thermal boundary conductance at high temperatures

We derive a theoretical model for phonon thermal boundary conductance across solid interfaces in the high temperature classical limit using quasi-harmonic thermodynamics, an approach that accounts for phonon anharmonicity effects on energy density changes via thermal expansion. Commonly used predictive models based on harmonic theory predict a thermal boundary conductance in the classical limit that is that constant and independent of temperature. Thus, these theories do not capture the increase in thermal boundary conductance with increasing temperature that has been reported in numerous molecular dynamics and anharmonic non-equilibrium Green’s function simulations. Our model accounts for anharmonic effects on the thermal boundary conductance via an increased internal energy of the material through an additional quasi-harmonic term that includes the material’s Grüneisen parameter. We show good agreement between our model calculations and the predicted thermal boundary conductance across a heavy argon/argon interface determined via molecular dynamics simulations. Further, our results also capture the contribution of inelastic scattering to thermal boundary conductance across a silicon/germanium interface predicted from anharmonic nonequilibrium Green’s functions simulations. Our quasi-harmonic thermodynamic-based theory suggests that an increase in thermal boundary conductance with an increase in temperature above the Debye temperature is due to anharmonicity in the materials adjacent to the interface, which is captured by the thermal expansion-driven phonon energy density changes in the materials. This theory is also consistent with prior molecular dynamics and anharmonic non-equilibrium Green’s function simulations that suggest that inelastic scattering effects on thermal boundary conductance are driven by phononic processes in materials near the interface and not at the interface. This model can help in screening materials for high interface density composites to increase thermal conductance and mitigate temperature in a range of applications.

Phase segregation in mixed-halide perovskites affects charge-carrier dynamics while preserving mobility

Mixed halide perovskites can provide optimal bandgaps for tandem solar cells which are key to improved cost-efficiencies, but can still suffer from detrimental illumination-induced phase segregation. Here we employ optical-pump terahertz-probe spectroscopy to investigate the impact of halide segregation on the charge-carrier dynamics and transport properties of mixed halide perovskite films. We reveal that, surprisingly, halide segregation results in negligible impact to the THz charge-carrier mobilities, and that charge carriers within the I-rich phase are not strongly localised. We further demonstrate enhanced lattice anharmonicity in the segregated I-rich domains, which is likely to support ionic migration. These phonon anharmonicity effects also serve as evidence of a remarkably fast, picosecond charge funnelling into the narrow-bandgap I-rich domains. Our analysis demonstrates how minimal structural transformations during phase segregation have a dramatic effect on the charge-carrier dynamics as a result of charge funnelling. We suggest that because such enhanced recombination is radiative, performance losses may be mitigated by deployment of careful light management strategies in solar cells.

Phonon anharmonicity: a pertinent review of recent progress and perspective

Anharmonic lattice vibrations govern the thermal dynamics in materials and present how the atoms interact and how they conduct heat. An indepth understanding of the microscopic mechanism of phonon anharmonicity in condensed systems is critical for developing better functional and energy materials. In recent years, a variety of novel behaviors in condense matters are driven by phonon anharmonic effects in some way or another, such as soft mode phase transition, negative thermal expansion, multiferroicity, ultralow thermal conductivity or high thermal resistance, and high-temperature superconductivity, etc. All these properties have endowed anharmonicity with many promising applications and provided remarkable opportunities for developing anharmonicity engineering, regulating heat transport towards excellent performance in materials. In this work, we review the recent development of the study on phonon anharmonic effect and summarize its origination, influence and mechanism, research methods, and applications. Besides, the remaining challenges, future trends, and prospects of phonon anharmonicity are also put forward.

Phonon anharmonicity of thermoelectric material HfTe5 studied by Raman spectroscopy

AbstractThe understanding of phonon–phonon anharmonic effect in HfTe5 is essential not only for fundamental scientific interest but also for its potential thermoelectric applications. Here, rectangular ribbon‐shaped HfTe5 single crystals have been grown. Raman spectroscopy is further utilized to investigate the phonon anharmonicity of HfTe5 by quantifying the temperature dependence of the phonon mode softening and broadening. We focus on the temperature range from 80 up to 400 K since obvious surface oxidation occurs at higher temperatures. It is found that four phonon anharmonic effects are non‐negligible. The phonon lifetime is greatly reduced by the enhancement of phonon scattering, resulting in the low thermal conductivity in HfTe5 single crystals.

Anharmonicity in Raman-active phonon modes in atomically thin MoS2

Phonon-phonon anharmonic effects have a strong influence on the phonon spectrum; most prominent manifestation of these effects are the softening (shift in frequency) and broadening (change in FWHM) of the phonon modes at finite temperature. Using Raman spectroscopy, we studied the temperature dependence of the FWHM and Raman shift of $\mathrm{E_{2g}^1}$ and $\mathrm{A_{1g}}$ modes for single-layer and natural bilayer MoS$_2$ over a broad range of temperatures ($8 < $T$ < 300$ K). Both the Raman shift and FWHM of these modes show linear temperature dependence for $T>100$ K, whereas they become independent of temperature for $T<100$ K. Using first-principles calculations, we show that three-phonon anharmonic effects intrinsic to the material can account for the observed temperature-dependence of the line-width of both the modes. It also plays an important role in determining the temperature-dependence of the frequency of the Raman modes. The observed evolution of the line-width of the A$_{1g}$ mode suggests that electron-phonon processes are additionally involved. From the analysis of the temperature-dependent Raman spectra of MoS$_2$ on two different substrates -- SiO$_2$ and hexagonal boron nitride, we disentangle the contributions of external stress and internal impurities to these phonon-related processes. We find that the renormalization of the phonon mode frequencies on different substrates is governed by strain and intrinsic doping. Our work establishes the role of intrinsic phonon anharmonic effects in deciding the Raman shift in MoS$_2$ irrespective of substrate and layer number.

The effect of phonon anharmonicity on the lattice thermal conductivity of rare-earth pyrochlores: A first-principles study

In thermal barrier coating materials, there are several conventional routes to suppress the lattice thermal conductivity in order to improve the thermal insulation efficiency, such as doping, increasing the mass of unit cells, or using complex lattice structures. However, in spite of the fact that the mass of unit cell of La2Sn2O7 is much larger than that of La2Zr2O7 and these two pyrochlores have very similar crystal structure, the thermal conductivity of the former is much higher. By comparing the phonon spectra, phonon density of states and Grüneisen parameter of each phonon mode calculated using first-principles methods, it is found that the La2Zr2O7 has stronger anharmonicity than La2Sn2O7 because of the anharmonic potential of La atoms in the (111) plane. The difference between La potentials of the two pyrochlores is attributed to the fact that the electron cloud around Zr atoms is more easily polarized than that around Sn by the displacement of La atoms, which induces stronger long-range interaction and decreases the potential energy of La. These results provide intuitive insights towards the interplay between the electronic structure, anharmonicity and lattice thermal conductivity of rare-earth pyrochlores.

Studies on structural, optical, thermal and vibrational properties of thienyl chalcone derivative: 1-(4-Nitrophenyl)-3-(2-thienyl)prop-2-en-1-one

The structural, optical, thermal and vibrational properties of thienyl chalcone derivative 1-(4-Nitrophenyl)-3-(2-thienyl)prop-2-en-1-one, C13H9NO3S were investigated combining nuclear magnetic resonance (1H and 13C NMR), X-ray diffraction (XRD), Fourier transform infrared (FTIR), UV–vis spectroscopy at room temperature assisted by density functional theory (DFT) calculations and Raman scattering at the temperature range 303–463 K. The electronic properties, including excitation energies, oscillator strengths, HOMO and LUMO energies were calculated by time-dependent DFT (TD-DFT) to complement the experimental findings. The B3LYP/6-311G (d,p) (B3LYP/cc-pVTZ) calculations led to the identification of ‘two minima on the molecules’ potential energy surfaces. From these calculations, it was predicted that the most stable conformer for C13H9NO3S in the gas phase is founded at 0 K relationship to dihedral angle C8C9C10S1, in agreement with XRD results. The molecular plot showed that the electrical charge mobility in the molecule occurs from thiophene to benzene ring. The optical band gap energy calculated from the difference between HOMO and LUMO orbitals was founded to be ∼3.87 (3.82) eV, in close agreement with the experimental value of 2.94 eV. The comparison between experimental and theoretical vibrational spectra gives a precise knowledge of the fundamental vibrational modes and leads to a better interpretation of the experimental Raman and infrared spectra. As temperature increases from room temperature to 443 K, it was observed the current phonon anharmonicity effects associated to changes in the Raman line intensities, line-widths and red-shift, in special in the external modes region, whereas the internal modes region remains almost unchanged due its strong chemical bonds. Furthermore, C13H9NO3S goes to phase transition in the temperature range 453–463 K. This thermal phenomenon was attributed to the disappearance of the lattice (∼10-200 cm−1) and molecular (∼300-4000 cm−1) modes in the Raman spectra. Finally, the vibrational mode assignment given in terms of potential energy distribution (PED) analysis leads to a more comprehensive interpretation of the vibrational spectra and origin of instability the investigated material.

Thermodynamic simulation of thermophysical and elastic properties of plutonium

A self-consistent thermodynamic model taking into account phonon anharmonicity effects is used for calculating the temperature dependences of the lattice components of thermophysical and elastic properties of the alpha-phase of plutonium. The theoretical curves are in good agreement with experimental data and make it possible to compare the mechanisms of formation of thermophysical and elastic properties of the alpha- and delta-phases of plutonium. The entropy of both phases is estimated, the role of the lattice magnetic anharmonicity is analyzed, and the key question concerning the reasons for stabilization of the delta-phase of plutonium relative to its α-phase is considered.

Effect of phonon anharmonicity on the thermophysical and elastic properties of platinum

A self-consistent thermodynamic model is constructed, which describes the effect of anharmonicity of crystal lattice on heat capacity, coefficient of thermal expansion, density, and bulk modulus of palladium. Fairly good agreement with experimental data is obtained within the model being developed, and the contribution to heat capacity associated with the effect of anharmonicity is identified. Significant increase is demonstrated of the effect made by lattice anharmonicity on the thermo-physical properties in the region of temperatures significantly exceeding the Debye temperature.

Effect of the phonon and magnetic anharmonicity on the thermal and elastic properties of nearly magnetic δ-plutonium

The temperature dependences of the total heat capacity and the lattice components of the bulk modulus, the volume thermal expansion coefficient, and the mean-square deviation of atoms from the equilibrium positions of nearly magnetic δ-plutonium (using the Pu0.96Ga0.04 alloy as an example) have been calculated within the framework of the self-consistent thermodynamic model. The electronic heat capacity has been calculated using the results obtained in terms of the self-consistent spin-fluctuation theory based on the inclusion of the strong magnetic anharmonicity, which leads to a splitting of the electronic spectra by fluctuating exchange fields. On this basis, the effect of phonon anharmonicity not only on the lattice heat capacity but also on other thermal and elastic properties has been considered.

The effect of phonon anharmonicity on the thermophysical and elastic properties of palladium

The effect of phonon anharmonicity on the thermophysical and elastic properties of palladium

Effect of phonon anharmonicity on the thermal and elastic properties of stabilized δ-plutonium

The Debye temperature, bulk modulus, density, and molar heat capacity of a Pu0.95Al0.05 alloy have been calculated as functions of the temperature using a self-consistent thermodynamic model developed according to a quasi-classical approach that takes into account the effect of phonon anharmonicity on the acoustic and thermodynamic properties of metals. The results of self-consistent numerical simulation of the thermal and elastic properties are compared to the available experimental data and theoretical results obtained using alternative approaches. It is established that the phonon anharmonicity plays a significant role in the formation of elastic and thermal properties of the system under consideration.

The Effect of Phonon Anharmonicity on the Thermodynamic Properties of Nonmetallic Solid

Germanium is used as an example for developing a self-consistent thermodynamic approach to the construction of a model of crystal lattice of a nonmetallic nonmagnetic solid. This approach takes into account the effect of anharmonicity of thermally excited phonons. The calculations of temperature dependences of basic thermodynamic functions reveal, for a model substance, very adequate agreement with the available experimental data in a wide temperature range of hundreds of kelvins.

Exciton-LO phonon dynamics inInAs∕GaAsquantum dots: Effects of zone-edge phonon damping

The dynamics of an exciton-LO phonon system after an ultrafast optical excitation in an $\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ quantum dot is studied theoretically. Influence of anharmonic phonon damping and its interplay with the phonon dispersion is analyzed. The signatures of the zone-edge decay process in the absorption spectrum and time evolution are highlighted, providing a possible way of experimental investigation on phonon anharmonicity effects.

Melting of the cooperative Jahn-Teller distortion in LaMnO 3 single crystal studied by Raman spectroscopy

We have studied the behavior of the Raman phonons of a stoichiometric LaMnO3 single crystal as a function of temperature in the range between 77 K and 900 K. We focus on the three main phonon peaks of the Pbnm structure, related to the tilt, antisymmetric stretching (Jahn-Teller mode) and stretching modes of Mn-O octahedra. The phonon frequencies show a strong softening that can be fit taking into account their renormalization because of three phonon anharmonic effects in the pseudoharmonic approximation. Thermal expansion effects, in particular the variation of Mn-O bond lengths with temperature, are not relevant above 300 K. On the contrary, phonon width behavior deviates from the three phonon scattering processes well bellow T c . The correlation between the magnitude of the cooperative Jahn-Teller distortion, that disappears at 800 K, and the amplitude of the Raman phonons in the orthorhombic phase is shown. Nevertheless, Pbnm phonons are still observable above this temperature. Phonon width and intensity behavior around T c can be explained by local melting of the orbital order that begins quite below T c and by fluctuations of the regular Mn-O octahedra that correspond to dynamic Jahn-Teller distortions.

Experimental Raman scattering investigation of phonon anharmonicity effects in

Experiments in which the Raman linewidth was measured as a function of temperature (7-1183 K) and pressure (0-400 bar) were performed on the (111) and (100) planes of single crystals of the cubic anti-fluorite . The temperature dependence of the lattice constant was determined by x-ray diffraction (11-295 K). From these results and published Brillouin scattering data for this host, the volume thermal expansion coefficient as a function of temperature was obtained as well as the isothermal compressibility and the isothermal Raman mode Grüneisen parameter. Using the thermodynamic approach within the quasi-harmonic approximation, we show that below 400 K the volume effects describe well the temperature dependence of the Raman linewidth whereas above this temperature there are direct anharmonic effects appearing. Above approximately 850 K new Raman lines appear that are and E polarized.

A tight-binding molecular dynamics study of phonon anharmonic effects in diamond and graphite

We study the temperature dependence of phonon frequency shifts and phonon linewidths in diamond and graphite using tight-binding molecular dynamics simulations. The calculation of one-phonon spectral intensities of several modes through velocity - velocity correlation functions allows a quantitative and nonperturbative study of these anharmonic effects. Our results for the zone-centre optical mode of diamond, for which experimental data are available, agree very well with first-order Raman-scattering measurements.

Tight-binding molecular-dynamics study of phonon anharmonic effects in silicon and diamond.

The anharmonic effects on phonons in silicon and diamond have been studied by molecular-dynamics simulations using an empirical tight-binding Hamiltonian. One-phonon spectral intensities of the zone-center and zone-boundary ({ital X}) modes have been calculated through the Fourier transform of the velocity-velocity correlation functions. This scheme allows a quantitative and nonperturbative study of phonon frequency shift and phonon linewidth as a function of temperature. The results obtained are in good agreement with experimental data.