CaltechAUTHORS: Combined
https://feeds.library.caltech.edu/people/Li-Chen-W/combined.rss
A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenWed, 18 Sep 2024 19:10:05 -0700Low Excited States of F19. III. Coulomb Excitation by α Particles
https://resolver.caltech.edu/CaltechAUTHORS:SHEpr54a
Year: 1954
DOI: 10.1103/PhysRev.94.1076.2
We have investigated the yield of gamma rays resulting from the bombardment of F19 by α particles [1]. Up to an α-particle energy 2.8 Mev we observed only the 1.28 Mev γ ray of the reaction F19(α,p)Ne22*, and the 114-kev and 200-kev radiations from the first and second excited states of F19 produced by inelastic scattering of the α particles. The γ rays were detected with a 1 1/2 in. X 1 1/2 in. sodium iodide scintillation spectrometer. The pulse spectrum was recorded with a 10-channel analyzer.https://resolver.caltech.edu/CaltechAUTHORS:SHEpr54aCoulomb Excitation of F19 by Alpha Particles
https://resolver.caltech.edu/CaltechAUTHORS:SHEpr54b
Year: 1954
DOI: 10.1103/PhysRev.96.1258
Gamma rays emitted in the excitation of F19 by α particles of 0.6 to 2.8 Mev have been studied. Resonances are found in the reaction F19(α, p)Ne22* at α particle energies greater than 1.3 Mev and in the inelastic excitation of 109-kev and 196-kev levels in F19 at energies greater than 2.2 Mev. At bombarding energies below 2 Mev, the cross sections for inelastic excitation of F19 decrease much too slowly for compound nucleus formation and are identified as being due to Coulomb excitation. The observed cross sections in the region 0.6 Mev to 2 Mev agree well with the theory for Coulomb excitation. The electromagnetic transition probabilities for decay of these states deduced from the excitation cross sections are in good agreement with those found from direct measurement of the lifetimes by Thirion, Barnes, and Lauritsen. Together with the results of Peterson, Barnes, Fowler, and Lauritsen on the inelastic excitation of fluorine by protons, these experiments lead to spin and parity assignment of ½- for the 109 kev state and 5/2+ for the 196-kev state of F19. The observed angular distributions of the γ rays from Coulomb excitation by α particles are also in accord with theory.https://resolver.caltech.edu/CaltechAUTHORS:SHEpr54bRaman spectrometry study of phonon anharmonicity of hafnia at elevated temperatures
https://resolver.caltech.edu/CaltechAUTHORS:20090923-143134720
Year: 2009
DOI: 10.1103/PhysRevB.80.054304
Raman spectra of monoclinic hafnium oxide (HfO_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.https://resolver.caltech.edu/CaltechAUTHORS:20090923-143134720Effects of chemical composition and B2 order on phonons in bcc Fe–Co alloys
https://resolver.caltech.edu/CaltechAUTHORS:20100831-094242871
Year: 2010
DOI: 10.1063/1.3456500
The phonon density of states (DOS) gives insight into interatomic forces and provides the vibrational entropy, making it a key thermodynamic function for understanding alloy phase transformations. Nuclear resonant inelastic x-ray scattering and inelastic neutron scattering were used to measure the chemical dependence of the DOS of bcc Fe–Co alloys. For the equiatomic alloy, the A2→B2 (chemically disordered→chemically ordered) phase transformation caused measurable changes in the phonon spectrum. The measured change in vibrational entropy upon ordering was −0.02±0.02 k_B/atom, suggesting that vibrational entropy results in a reduction in the order–disorder transition temperature by 60±60 K. The Connolly–Williams cluster inversion method was used to obtain interaction DOS (IDOS) curves that show how point and pair variables altered the phonon DOS of disordered bcc Fe–Co alloys. These IDOS curves accurately captured the change in the phonon DOS and vibrational entropy of the B2 ordering transition.https://resolver.caltech.edu/CaltechAUTHORS:20100831-094242871Anharmonicity-induced phonon broadening in aluminum at high temperatures
https://resolver.caltech.edu/CaltechAUTHORS:20101206-143055391
Year: 2010
DOI: 10.1103/PhysRevB.82.184301
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.https://resolver.caltech.edu/CaltechAUTHORS:20101206-143055391AtomSim: web-deployed atomistic dynamics simulator
https://resolver.caltech.edu/CaltechAUTHORS:20110104-084939003
Year: 2010
DOI: 10.1107/S0021889810037209
AtomSim, a collection of interfaces for computational crystallography simulations, has been developed. It uses forcefield-based dynamics through physics engines such as the General Utility Lattice Program, and can be integrated into larger computational frameworks such as the Virtual Neutron Facility for processing its dynamics into scattering functions, dynamical functions etc. It is also available as a Google App Engine-hosted web-deployed interface. Examples of a quartz molecular dynamics run and a hafnium dioxide phonon calculation are presented.https://resolver.caltech.edu/CaltechAUTHORS:20110104-084939003A Raman Spectrometry Study of Phonon Anharmonicity of Zirconia at Elevated Temperatures
https://resolver.caltech.edu/CaltechAUTHORS:20110317-102805572
Year: 2011
DOI: 10.1111/j.1551-2916.2010.04057.x
Raman spectra of monoclinic zirconia (ZrO_2) 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
(HfO_2). Lattice dynamics calculations were performed with both shell
model and density functional theory to obtain Raman frequencies, and
the total and partial phonon density of states. These calculations were
also used to identify the individual motions of metal and oxygen atoms
in the different Raman modes. By correlating these motions to the
thermal peak shifts and broadenings, it was confirmed that modes
involving changes in oxygen-oxygen bond length were the most
anharmonic. The metal-dominated modes were found to be more
quasiharmonic, and thus showed less broadening with temperature. Mass
effects were evident by comparing the mode softening and shifting
between zirconia and hafnia.https://resolver.caltech.edu/CaltechAUTHORS:20110317-102805572Nonharmonic phonons in MgB_2 at elevated temperatures
https://resolver.caltech.edu/CaltechAUTHORS:20120530-104815505
Year: 2011
DOI: 10.1103/PhysRevB.83.174301
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.https://resolver.caltech.edu/CaltechAUTHORS:20120530-104815505Positive Vibrational Entropy of Chemical Ordering in FeV
https://resolver.caltech.edu/CaltechAUTHORS:20110928-101310590
Year: 2011
DOI: 10.1103/PhysRevLett.107.115501
Inelastic neutron scattering and nuclear resonant inelastic x-ray scattering were used to measure phonon spectra of FeV as a B2 ordered compound and as a bcc solid solution. The two data sets were combined to give an accurate phonon density of states, and the phonon partial densities of states for V and Fe atoms. Contrary to the behavior of ordering alloys studied to date, the phonons in the B2 ordered phase are softer than in the solid solution. Ordering increases the vibrational entropy by +0.22±0.03k_B/atom, which stabilizes the ordered phase to higher temperatures. First-principles calculations show that the number of electronic states at the Fermi level increases upon ordering, enhancing the screening between ions, and reducing the interatomic force constants. The effect of screening is larger at the V atomic sites than at the Fe atomic sites.https://resolver.caltech.edu/CaltechAUTHORS:20110928-101310590Structural Relationship between Negative Thermal Expansion and Quartic Anharmonicity of Cubic ScF_3
https://resolver.caltech.edu/CaltechAUTHORS:20111212-121400317
Year: 2011
DOI: 10.1103/PhysRevLett.107.195504
Cubic scandium trifluoride (ScF_3) has a large negative thermal expansion over a wide range of temperatures. Inelastic neutron scattering experiments were performed to study the temperature dependence of the lattice dynamics of ScF3 from 7 to 750 K. The measured phonon densities of states show a large anharmonic contribution with a thermal stiffening of modes around 25 meV. Phonon calculations with first-principles methods identified the individual modes in the densities of states, and frozen phonon calculations showed that some of the modes with motions of F atoms transverse to their bond direction behave as quantum quartic oscillators. The quartic potential originates from harmonic interatomic forces in the DO_9 structure of ScF_3, and accounts for phonon stiffening with the temperature and a significant part of the negative thermal expansion.https://resolver.caltech.edu/CaltechAUTHORS:20111212-121400317Phonon anharmonicity of rutile SnO_2 studied by Raman spectrometry and first principles calculations of the kinematics of phonon-phonon interactions
https://resolver.caltech.edu/CaltechAUTHORS:20121115-091306052
Year: 2012
DOI: 10.1103/PhysRevB.86.134302
Raman spectra of rutile tin dioxide (SnO_2) were measured at temperatures from 83 to 873 K. The pure anharmonicity from phonon-phonon interactions was found to be large and comparable to the quasiharmonicity. First-principles calculations of phonon dispersions were used to assess the kinematics of three-phonon and four-phonon processes. These kinematics were used to generate Raman peak widths and shifts, which were fit to measured data to obtain the cubic and quartic components of the anharmonicity for each Raman mode. The B_(2g) mode had a large quartic component, consistent with the symmetry of its atom displacements. The broadening of the B_(2g) mode with temperature showed an unusual concave-downwards curvature. This curvature is caused by a change with temperature in the number of down-conversion decay channels, originating with the wide band gap in the phonon dispersions.https://resolver.caltech.edu/CaltechAUTHORS:20121115-091306052Anharmonic lattice dynamics of Ag_2O studied by inelastic neutron scattering and first principles molecular dynamics simulations
https://resolver.caltech.edu/CaltechAUTHORS:20140224-090219387
Year: 2014
DOI: 10.1103/PhysRevB.89.054306
Inelastic neutron scattering measurements on silver oxide (Ag_2O) with the cuprite structure were
Performed at temperatures from40 to 400 K, and Fourier transform far-infrared spectra were measured
From 100 to 300K. The measured phonon densities of states and the infrared spectra showed unusually
large energy shifts with temperature, and large linewidth broadenings. First principles molecular
dynamics (MD) calculations were performed at various temperatures, successfully accounting for
the negative thermal expansion (NTE) and local dynamics. Using the Fourier-transformed velocity
autocorrelation method, the MD calculations reproduced the large anharmonic effects of Ag_2O, and
were in excellent agreement with the neutron scattering data. The quasiharmonic approximation
(QHA) was less successful in accounting for much of the phonon behavior. The QHA could account
for some of the NTE below 250 K, although not at higher temperatures. Strong anharmonic effects
were found for both phonons and for the NTE. The lifetime broadenings of Ag_2O were explained by
anharmonic perturbation theory, which showed rich interactions between the Ag-dominated modes
and the O-dominated modes in both up- and down-conversion processes.https://resolver.caltech.edu/CaltechAUTHORS:20140224-090219387Phonon anharmonicity in silicon from 100 to 1500 K
https://resolver.caltech.edu/CaltechAUTHORS:20150303-083220958
Year: 2015
DOI: 10.1103/PhysRevB.91.014307
Inelastic neutron scattering was performed on silicon powder to measure the phonon density of states (DOS) from 100 to 1500 K. The mean fractional energy shifts with temperature of the modes were ⟨Δɛ_i/ɛ_iΔT⟩=−0.07, giving a mean isobaric Grüneisen parameter of +6.95±0.67, which is significantly different from the isothermal parameter of +0.98. These large effects are beyond the predictions from quasiharmonic models using density functional theory or experimental data, demonstrating large effects from phonon anharmonicity. At 1500 K the anharmonicity contributes 0.15k_B/atom to the vibrational entropy, compared to 0.03k_B/atom from quasiharmonicity. Excellent agreement was found between the entropy from phonon DOS measurements and the reference NIST-JANAF thermodynamic entropy from calorimetric measurements.https://resolver.caltech.edu/CaltechAUTHORS:20150303-083220958Phonon anharmonicity of monoclinic zirconia and yttrium-stabilized zirconia
https://resolver.caltech.edu/CaltechAUTHORS:20150430-134245369
Year: 2015
DOI: 10.1103/PhysRevB.91.144302
Inelastic neutron scattering measurements on monoclinic zirconia (ZrO_2) and 8 mol% yttrium-stabilized zirconia were performed at temperatures from 300 to 1373wK. Temperature-dependent phonon densities of states (DOS) are reported, as are Raman spectra obtained at elevated temperatures. First-principles lattice dynamics calculations with density functional theory gave total and partial phonon DOS curves and mode Grüneisen parameters. These mode Grüneisen parameters were used to predict the experimental temperature dependence of the phonon DOS with partial success. However, substantial anharmonicity was found at elevated temperatures, especially for phonon modes dominated by the motions of oxygen atoms. Yttrium-stabilized zirconia (YSZ) was somewhat more anharmonic and had a broader phonon spectrum at low temperatures, owing in part to defects in its structure. YSZ also has a larger vibrational entropy than monoclinic zirconia.https://resolver.caltech.edu/CaltechAUTHORS:20150430-134245369Phonon quarticity induced by changes in phonon-tracked hybridization during lattice expansion, and its stabilization of rutile TiO_2
https://resolver.caltech.edu/CaltechAUTHORS:20150511-083811430
Year: 2015
DOI: 10.1103/PhysRevB.92.054304
Although the rutile structure of TiO_2 is stable at high temperatures, the conventional quasiharmonic approximation predicts that several acoustic phonons decrease anomalously to zero frequency with thermal expansion, incorrectly predicting a structural collapse at temperatures well below 1000 K. Inelastic neutron scattering was used to measure the temperature dependence of the phonon density of states (DOS) of rutile TiO_2 from 300 to 1373 K. Surprisingly, these anomalous acoustic phonons were found to increase in frequency with temperature. First-principles calculations showed that with lattice expansion, the potentials for the anomalous acoustic phonons transform from quadratic to quartic, stabilizing the rutile phase at high temperatures. In these modes, the vibrational displacements of adjacent Ti and O atoms cause variations in hybridization of 3d electrons of Ti and 2p electrons of O atoms. With thermal expansion, the energy variation in this "phonon-tracked hybridization" flattens the bottom of the interatomic potential well between Ti and O atoms, and induces a quarticity in the phonon potential.https://resolver.caltech.edu/CaltechAUTHORS:20150511-083811430Phonon anharmonicity and components of the entropy in palladium and platinum
https://resolver.caltech.edu/CaltechAUTHORS:20160606-100905733
Year: 2016
DOI: 10.1103/PhysRevB.93.214303
Inelastic neutron scattering was used to measure the phonon density of states in fcc palladium and platinum metal at temperatures from 7 K to 1576 K. Both phonon-phonon interactions and electron-phonon interactions were calculated by methods based on density functional theory (DFT) and were consistent with the measured shifts and broadenings of phonons with temperature. Unlike the longitudinal modes, the characteristic transverse modes had a nonlinear dependence on temperature owing to the requirement for a population of thermal phonons for upscattering. Kohn anomalies were observed in the measurements at low temperature and were reproduced by calculations based on DFT. Contributions to the entropy from phonons and electrons were assessed and summed to obtain excellent agreement with prior calorimetric data. The entropy from thermal expansion is positive for both phonons and electrons but larger for phonons. The anharmonic phonon entropy is negative in Pt, but in Pd it changes from positive to negative with increasing temperature. Owing to the position of the Fermi level on the electronic DOS, the electronic entropy was sensitive to the adiabatic electron-phonon interaction in both Pd and Pt. The adiabatic EPI depended strongly on thermal atom displacements.https://resolver.caltech.edu/CaltechAUTHORS:20160606-100905733Pure phonon anharmonicity and the anomalous thermal expansion of silicon
https://resolver.caltech.edu/CaltechAUTHORS:20161212-143546481
Year: 2016
DOI: 10.48550/arXiv.1610.08737
Despite the widespread use of silicon in modern technology, its peculiar thermal expansion is not well-understood. Harmonic phonons adapted to the specific volume at temperature, quasiharmonic approximation, has become accepted for simulating the thermal expansion, but has given ambiguous interpretations for microscopic mechanisms. To test the atomistic mechanisms, we performed inelastic
neutron scattering experiments on a single crystal of silicon to measure the changes in lattice dynamics
from 100 to 1500 K. Our state-of-the-art ab initio calculations, which fully account for phonon anharmonicity, reproduced the measured shifts of individual phonons with temperature, whereas the quasiharmonic approximation typically gave results of the wrong sign. Surprisingly, the accepted quasiharmonic model was found to predict the thermal expansion owing to a fortuitous cancellation of
contributions from individual phonons.https://resolver.caltech.edu/CaltechAUTHORS:20161212-143546481Separating the configurational and vibrational entropy contributions in metallic glasses
https://resolver.caltech.edu/CaltechAUTHORS:20170411-135835964
Year: 2017
DOI: 10.1038/nphys4142
Glassy materials exist in nature and play a critical role in technology, but key differences between the glass, liquid and crystalline phases are not well understood. Over several decades there has been controversy about the specific heat absorbed as a glass transforms to a liquid—does this originate from vibrational entropy or configurational entropy? Here we report direct in situ measurements of the vibrational spectra of strong and fragile metallic glasses in the glass, liquid and crystalline phases. For both types of material, the measured vibrational entropies of the glass and liquid show a tiny excess over the crystal, representing less than 5% of the total excess entropy measured with step calorimetry. These results reveal that the excess entropy of metallic glasses is almost entirely configurational in origin, consistent with the early theories of Gibbs and co-workers describing the glass transition as a purely configurational transition.https://resolver.caltech.edu/CaltechAUTHORS:20170411-135835964Nuclear quantum effect with pure anharmonicity and the anomalous thermal expansion of silicon
https://resolver.caltech.edu/CaltechAUTHORS:20180214-150839490
Year: 2018
DOI: 10.1073/pnas.1707745115
PMCID: PMC5834665
Despite the widespread use of silicon in modern technology, its peculiar thermal expansion is not well understood. Adapting harmonic phonons to the specific volume at temperature, the quasiharmonic approximation, has become accepted for simulating the thermal expansion, but has given ambiguous interpretations for microscopic mechanisms. To test atomistic mechanisms, we performed inelastic neutron scattering experiments from 100 K to 1,500 K on a single crystal of silicon to measure the changes in phonon frequencies. Our state-of-the-art ab initio calculations, which fully account for phonon anharmonicity 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 expansion owing to a large cancellation of contributions from individual phonons.https://resolver.caltech.edu/CaltechAUTHORS:20180214-150839490Temperature dependence of phonons in FeGe_2
https://resolver.caltech.edu/CaltechAUTHORS:20181009-094209133
Year: 2018
DOI: 10.1103/PhysRevMaterials.2.103602
Inelastic neutron scattering was used to measure phonon dispersions in a single crystal of FeGe_2 with the C16 structure at 300, 500, and 635 K. Phonon densities of states (DOS) were also measured on polycrystalline FeGe_2 from 325 to 1050 K, and the Fe partial DOS was obtained from polycrystalline ^(57)FeGe_2 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 FeGe_2, showing modest but interpretable anharmonic contributions.https://resolver.caltech.edu/CaltechAUTHORS:20181009-094209133Temperature-dependent phonon lifetimes and thermal conductivity of silicon by inelastic neutron scattering and ab initio calculations
https://resolver.caltech.edu/CaltechAUTHORS:20201117-105350111
Year: 2020
DOI: 10.1103/physrevb.102.174311
Inelastic neutron scattering on a single crystal of silicon was performed at temperatures from 100 to 1500 K. These experimental data were reduced to obtain phonon spectral intensity at all wave vectors →Q and frequencies ω in the first Brillouin zone. Thermal broadenings of the phonon peaks were obtained by fitting and by calculating with an iterative ab initio method that uses thermal atom displacements on an ensemble of superlattices. Agreement between the calculated and experimental broadenings was good, with possible discrepancies at the highest temperatures. Distributions of phonon widths versus phonon energy had similar shapes for computation and experiment. These distributions grew with temperature but maintained similar shapes. Parameters from the ab initio calculations were used to obtain the thermal conductivity from the Boltzmann transport equation, which was in good agreement with experimental data. Despite the high group velocities of longitudinal acoustic phonons, their shorter lifetimes reduced their contribution to the thermal conductivity, which was dominated by transverse acoustic modes.https://resolver.caltech.edu/CaltechAUTHORS:20201117-105350111