@article{Li2020,
title = {Giant low-temperature anharmonicity in silicon nanocrystals},
author = {Shuonan Chen and Devin Coleman and Douglas L. Abernathy and Arnab Banerjee and Luke L. Daemen and Lorenzo Mangolini and Chen W. Li},
url = {https://journals.aps.org/prmaterials/pdf/10.1103/PhysRevMaterials.4.056001},
doi = {10.1103/PhysRevMaterials.4.056001},
year = {2020},
date = {2020-05-04},
journal = {Phys. Rev. Materials},
volume = {4},
pages = {056001},
abstract = {The phonon density of states of silicon nanocrystals with size between 4 and 7.5 nm was measured by inelastic neutron scattering in the 5-600 K temperature range. The narrow particle size distributions enable the study of size effects on phonon dynamics. Giant softening of phonon features below 30 meV, universal broadening of phonon features, and the disappearance of intermediate-energy phonons were observed with decreasing nanocrystals size. Such size effects are mostly attributed to the structure variations within the nanocrystals. The phonons below 30 meV in silicon nanocrystals show temperature dependence opposite to the bulk silicon, explained by the large anharmonicity of the under-constrained near-surface phonons. This is supported by the abnormal atomic mean-square-displacement, and low energy phonon population in small silicon nanocrystals. This work provides crucial information on the phonon dynamics in spatially confined materials.},
keywords = {anharmonicity, high temperature, lifetime, neutron},
pubstate = {published},
tppubtype = {article}
}

@article{Smith2018,
title = {Temperature dependence of phonons in FeGe2},
author = {Hillary L. Smith and Yang Shen and Dennis S. Kim and Fred C. Yang and C.P. Adams and Chen W. Li and D.L. Abernathy and M.B. Stone and B. Fultz},
url = {https://journals.aps.org/prmaterials/abstract/10.1103/PhysRevMaterials.2.103602},
doi = {10.1103/PhysRevMaterials.2.103602},
year = {2018},
date = {2018-10-09},
journal = {Physical Review Materials},
volume = {2},
pages = {103602},
abstract = {Inelastic neutron scattering was used to measure phonon dispersions in a single crystal of FeGe2 with the C16 structure at 300, 500, and 635 K. Phonon densities of states (DOS) were also measured on polycrystalline FeGe2 from 325 to 1050 K, and the Fe partial DOS was obtained from polycrystalline
57 FeGe2 at 300 K using nuclear resonant inelastic x-ray scattering. The dominant feature in the temperature dependence of the phonon spectrum is thermal broadening of high-energy modes. The energy shifts of the low- and high-energy parts of the spectrum were almost the same. DFT calculations performed with the quasiharmonic approximation gave results in moderate agreement with the experimental thermal energy shifts, although the isobaric Grüneisen parameter calculated from the quasiharmonic model was smaller than that from measurements. The thermal broadening of the phonon spectrum and dispersions, especially at high energies, indicates a cubic anharmonicity to second order that should also induce phonon shifts. We show that different anharmonic contributions cancel out, giving average phonon shifts in moderate agreement to calculations with the quasiharmonic approximation. The different parts of the large phonon contribution to the entropy are separated for FeGe2, showing modest but interpretable anharmonic contributions.},
keywords = {anharmonicity, high temperature, lattice expansion, magnetism, phonon, vibrational entropy},
pubstate = {published},
tppubtype = {article}
}

@article{Kim2018,
title = {Nuclear quantum effect with pure anharmonicity and the anomalous thermal expansion of silicon},
author = {D. S. Kim and O. Hellman and J. Herriman and H. L. Smith and J. Y. Y. Lin and N. Shulumba and J. L. Niedziela and C. W. Li and D. L. Abernathy and B. Fultz},
url = {https://www.pnas.org/content/115/9/1992},
doi = {10.1073/pnas.1707745115},
year = {2018},
date = {2018-02-27},
journal = {Proceedings of the National Academy of Sciences},
volume = {115},
number = {9},
pages = {1992},
abstract = {Despite the widespread use of silicon in modern technology, its peculiar thermal expansion is not well understood. Adapt- ing harmonic phonons to the specific volume at temperature, the quasiharmonic approximation, has become accepted for sim- ulating the thermal expansion, but has given ambiguous inter- pretations for microscopic mechanisms. To test atomistic mech- anisms, we performed inelastic neutron scattering experiments from 100 K to 1,500 K on a single crystal of silicon to mea- sure the changes in phonon frequencies. Our state-of-the-art ab initio calculations, which fully account for phonon anharmonic- ity and nuclear quantum effects, reproduced the measured shifts of individual phonons with temperature, whereas quasiharmonic shifts were mostly of the wrong sign. Surprisingly, the accepted quasiharmonic model was found to predict the thermal expan- sion owing to a large cancellation of contributions from individual phonons.},
keywords = {anharmonicity, first-principles, high temperature, lattice expansion},
pubstate = {published},
tppubtype = {article}
}

@article{markovskiy_nonharmonic_2011,
title = {Nonharmonic phonons in MgB 2at elevated temperatures},
author = {N D Markovskiy and J A Munoz and M S Lucas and Chen W Li and O Delaire and M B Stone and D L Abernathy and B Fultz},
url = {https://link.aps.org/doi/10.1103/PhysRevB.83.174301},
doi = {10.1103/PhysRevB.83.174301},
year = {2011},
date = {2011-01-01},
journal = {Physical Review B},
volume = {83},
number = {17},
pages = {174301},
abstract = {Inelastic neutron scattering was used to measure phonon spectra in MgB$_2$ and Mg$_0.75$Al$_0.25$B$_2$ from 7 to 750 K to investigate anharmonicity and adiabatic electron-phonon coupling. First-principles calculations of phonons with a linear response method were performed at multiple unit cell volumes, and the Helmholtz free energy was minimized to obtain the lattice parameters and phonon dynamics at elevated temperature in the quasiharmonic approximation. Most of the temperature dependence of the phonon density of states could be understood with the quasiharmonic approximation, although there was also significant thermal broadening of the phonon spectra. In comparison to Mg$_0.75$Al$_0.25$B$_2$, in the energy range of 60 to 80 meV the experimental phonon spectra from MgB$_2$ showed a nonmonotonic change with temperature around 500 K. This may originate from a change with temperature of the adiabatic electron-phonon coupling.},
keywords = {anharmonicity, high temperature, MgB2, phonon},
pubstate = {published},
tppubtype = {article}
}

@article{tang_anharmonicity-induced_2010,
title = {Anharmonicity-induced phonon broadening in aluminum at high temperatures},
author = {X Tang and Chen W Li and B Fultz},
url = {https://link.aps.org/doi/10.1103/PhysRevB.82.184301},
doi = {10.1103/PhysRevB.82.184301},
year = {2010},
date = {2010-11-01},
journal = {Physical Review B},
volume = {82},
number = {18},
pages = {184301},
abstract = {Thermal phonon broadening in aluminum was studied by theoretical and experimental methods. Using second-order perturbation theory, phonon linewidths from the third-order anharmonicity were calculated from first-principles density-functional theory (DFT) with the supercell finite-displacement method. The importance of all three-phonon processes were assessed and individual phonon broadenings are presented. The good agreement between calculations and prior measurements of phonon linewidths at 300 K and new measurements of the phonon density of states to 750 K indicates that the third-order phonon-phonon interactions calculated from DFT can account for the lifetime broadenings of phonons in aluminum to at least 80% of its melting temperature.},
keywords = {Al, anharmonicity, high temperature, lifetime, phonon},
pubstate = {published},
tppubtype = {article}
}

@article{li_raman_2010,
title = {A Raman Spectrometry Study of Phonon Anharmonicity of Zirconia at Elevated Temperatures},
author = {Chen W Li and M M McKerns and B Fultz},
url = {http://doi.wiley.com/10.1111/j.1551-2916.2010.04057.x},
doi = {10.1111/j.1551-2916.2010.04057.x},
year = {2010},
date = {2010-01-01},
journal = {Journal of the American Ceramic Society},
volume = {94},
number = {1},
pages = {224-229},
abstract = {Raman spectra of monoclinic zirconia (ZrO2) were measured at temperatures of up to 950 K. Temperature‐dependent Raman peak shifts and broadenings were reported and compared with prior results on hafnia...},
keywords = {anharmonicity, high temperature, oxide, phonon, Raman},
pubstate = {published},
tppubtype = {article}
}

@article{li_raman_2009,
title = {Raman spectrometry study of phonon anharmonicity of hafnia at elevated temperatures},
author = {Chen W Li and M M McKerns and B Fultz},
url = {https://link.aps.org/doi/10.1103/PhysRevB.80.054304},
doi = {10.1103/PhysRevB.80.054304},
year = {2009},
date = {2009-08-01},
journal = {Physical Review B},
volume = {80},
number = {5},
pages = {1745},
abstract = {Raman spectra of monoclinic hafnium oxide $(textbackslashtextHfO_2)$ were measured at temperatures up to 1100 K. Raman peak shifts and broadenings are reported. Phonon dynamics calculations were performed with the shell model to obtain the total and partial phonon density of states, and to identify the individual motions of Hf and O atoms in the Raman modes. Correlating these motions to the thermal peak shifts and broadenings, it was found that modes involving changes in oxygen-oxygen bond length were the most anharmonic. The hafnium-dominated modes were more quasiharmonic and showed less broadening with temperature. Comparatively, the oxygen-dominated modes were more influenced by the cubic term in the interatomic potential than the hafnium-dominated modes. An approximately quadratic correlation was found between phonon-line broadening and softening.},
keywords = {anharmonicity, high temperature, oxide, phonon, Raman},
pubstate = {published},
tppubtype = {article}
}