Under the framework of Taylor series expansion for potential energy, we propose a simple and robust metric, dubbed ``regular residual analysis,'' to measure the fourth-order phonon anharmonicity in crystals. The method is verified by studying the intrinsic strong higher-order anharmonic effects in ${\mathrm{UO}}_{2}$ and ${\mathrm{CeO}}_{2}$. Comparison of the thermal conductivity results, which calculated by the anharmonic lattice dynamics method coupled with the Boltzmann transport equation and the spectral energy density method coupled with ab initio molecular dynamics simulation further validates our analysis. Analysis of the bulk Si and Ge systems confirms that the fourth-order phonon anharmonicity is enhanced and cannot be neglected at high enough temperatures, which agrees with a previous study where the four-phonon scattering was explicitly determined. This metric will facilitate evaluating and interpreting the lattice thermal conductivity of crystals with strong fourth-order phonon anharmonicity.
Coastal seagrass beds are pivotal but threatened marine ecosystems throughout the world. The seagrass Zostera japonica Asch. & Graebn. is an endangered species in its native range along the northwestern Pacific coast. In this study, we used ecological survey methods and microsatellite analysis to evaluate sexual reproduction and its role in recruitment of Z. japonica populations at Swan Lake lagoon (SLL) and Huiquan Bay (HQB) in northern China. Mixed annual and continuous meadows of Z. japonica at SLL produced a high number of seeds (mean ± SD: 40244 ± 18666 seeds m -2 ) and formed a relatively stable seed bank (1460 ± 417 seeds m -2 ) in the sediment. About 41% of the seed bank and 6% of shoots survived over winter, and recruitment from seeds accounted for 41 ± 24%. In contrast, perennial and fragmented Z. japonica at HQB had lower seed production (12501 ± 5748 seeds m -2 ) and a much smaller seed bank (10 ± 6 seeds m -2 ). About 66% of shoots survived over winter, but seedling recruitment was rare at HQB. Thus, relatively large differences in genetic and clonal diversity were predicted between SLL and HQB. Results of the microsatellite analysis of samples collected in 2012 and 2015 showed higher clonal ( R ) and genetic diversity ( H o ) at SLL (2015: R = 1; H o = 0.55) than at HQB (2015: R = 0.40; H o = 0.42). These results highlight the role of sexual and asexual reproduction in maintenance and evolutionary connectivity of seagrass populations and emphasize the need to understand local recruitment strategies before starting restoration and management projects.
Anharmonic phonon-phonon scattering serves a critical role in heat conduction in solids. Previous studies have identified many selection rules for possible phonon-phonon scattering channels imposed by phonon energy and momentum conservation conditions and crystal symmetry. However, the crystal-symmetry-based selection rules have mostly been ad hoc so far in selected materials, and a general formalism that can summarize known selection rules and lead to new ones in any given crystal is still lacking. In this work, we apply a general formalism for symmetry-based scattering selection rules based on the group theory to anharmonic phonon-phonon scatterings, which can reproduce known selection rules and guide the discovery of new selection rules between phonon branches imposed by the crystal symmetry. We apply this formalism to analyze the phonon-phonon scattering selection rules imposed by the in-plane symmetry of graphene, and demonstrate the significant impact of symmetry-breaking strain on the lattice thermal conductivity. Our work quantifies the critical influence of the crystal symmetry on the lattice thermal conductivity in solids and suggests routes to engineer heat conduction by tuning the crystal symmetry.
Considerable discussions have occurred about the critical role played by free electrons in the transport of heat in pure metals. In principle, any environment that can influence the dynamical behaviors of electrons would have impact on electronic thermal conductivity $({\ensuremath{\kappa}}_{\mathrm{el}})$ of metals. Over the past decades, significant progress and comprehensive understanding have been gained from theoretical, as well as experimental, investigations by taking into account the effects of various conditions, typically temperature, impurities, strain, dimensionality, interface, etc. However, the effect of external magnetic field has received less attention. In this paper, the magnetic-field dependence of electron-phonon scattering, the electron's lifetime, and ${\ensuremath{\kappa}}_{\mathrm{el}}$ of representative metals (Al, Ni, and Nb) are investigated within the framework of all-electron spin-density functional theory. For Al and Ni, the induced magnetization vector field and difference in electron density under external magnetic-field aggregate toward the center of unit cell, leading to the enhanced electron-phonon scattering, the damped electron's lifetime, and thus the reduced ${\ensuremath{\kappa}}_{\mathrm{el}}$. On the contrary, for Nb with strong intrinsic electron-phonon interaction, the electron's lifetime and ${\ensuremath{\kappa}}_{\mathrm{el}}$ slightly increase as external magnetic field is enhanced. This is mainly attributed to the separately distributed magnetization vector field and difference in electron density along the corner of unit cell. This paper sheds light on the origin of influence of external magnetic field on ${\ensuremath{\kappa}}_{\mathrm{el}}$ for pure metals and offers a new route for robust manipulation of electronic thermal transport via applying external magnetic field.
New classes of two-dimensional (2D) materials beyond graphene, including layered and non-layered, and their heterostructures, are currently attracting increasing interest due to their promising applications in nanoelectronics, optoelectronics and clean energy, where thermal transport is a fundamental physical parameter. In this paper, we systematically investigated the phonon transport properties of the 2D orthorhombic group IV–VI compounds of GeS, GeSe, SnS and SnSe by solving the Boltzmann transport equation (BTE) based on first-principles calculations. Despite their similar puckered (hinge-like) structure along the armchair direction as phosphorene, the four monolayer compounds possess diverse anisotropic properties in many aspects, such as phonon group velocity, Young's modulus and lattice thermal conductivity (κ), etc. Especially, the κ along the zigzag and armchair directions of monolayer GeS shows the strongest anisotropy while monolayer SnS and SnSe show almost isotropy in phonon transport. The origin of the diverse anisotropy is fully studied and the underlying mechanism is discussed in details. With limited size, the κ could be effectively lowered, and the anisotropy could be effectively modulated by nanostructuring, which would extend the applications to nanoscale thermoelectrics and thermal management. Our study offers fundamental understanding of the anisotropic phonon transport properties of 2D materials, and would be of significance for further study, modulation and applications in emerging technologies.
Topological phase transitions occur when the electronic bands change their topological properties, typically featuring the closing of the bandgap. While the influence of topological phase transitions on electronic and optical properties has been extensively studied, its implication on phononic properties and thermal transport remains unexplored. In this work, we use first-principles simulations to show that certain phonon modes are significantly softened near topological phase transitions, leading to increased phonon-phonon scattering and reduced lattice thermal conductivity. We demonstrate this effect using two model systems: pressure-induced topological phase transition in $\rm ZrTe_5$ and chemical composition induced topological phase transition in $\rm{Hg_{1-x}Cd_{x}Te}$. We attribute the phonon softening to emergent Kohn anomalies associated with the closing of the bandgap. Our study reveals the strong connection between electronic band structures and lattice instabilities and opens up a potential direction towards controlling heat conduction in solids.
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