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A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenSat, 13 Apr 2024 01:04:32 +0000Tight-binding based non-collinear spin model and magnetic correlations in iron
https://resolver.caltech.edu/CaltechAUTHORS:20190923-104220495
Authors: {'items': [{'id': 'Mukherjee-Sonali', 'name': {'family': 'Mukherjee', 'given': 'Sonali'}}, {'id': 'Cohen-R-E', 'name': {'family': 'Cohen', 'given': 'R. E.'}, 'orcid': '0000-0001-5871-2359'}]}
Year: 2001
DOI: 10.1023/a:1020070028021
We have extended a first principles tight-binding total energy model to include magnetic correlations. We have tested the validity of the model against ab-initio calculations for bcc and fcc Fe. We find that our model can quantitatively describe the pressure dependence of magnetic moments and magnetization energy for bcc and fcc Fe. Moreover, in fcc Fe with increasing pressures it is able to capture the transitions from ferromagnetic to non-collinear spin to antiferromagnetic and finally to the non-magnetic state.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/3nxqb-3ps67First-principles computation of elasticity of
Pb(Zr,Ti)O_3: The importance of elasticity in
piezoelectrics
https://resolver.caltech.edu/CaltechAUTHORS:20111117-075112891
Authors: {'items': [{'id': 'Cohen-R-E', 'name': {'family': 'Cohen', 'given': 'R. E.'}, 'orcid': '0000-0001-5871-2359'}, {'id': 'Heifets-E', 'name': {'family': 'Heifets', 'given': 'E.'}}, {'id': 'Fu-H', 'name': {'family': 'Fu', 'given': 'H.'}}]}
Year: 2001
DOI: 10.1063/1.1399685
We have computed the elastic constant tensor for PbZrTiO_6, PZT 50/50, in the rhombohedral structure. We find it to be very soft elastically, and the Young's modulus to be very anisotropic. Our results for PZT are compared with literature values on other oxide perovskites, and the new experimental results on PMN-PT and PZN-PT. The elastic response is an important part of the electromechanical response, and should be studied experimentally and theoretically to help build up a systemic understanding of electromechanical materials. We find that anisotropy in Young's modulus and elastic softness may be key discriminators in evaluating or improving electromechanical properties.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/3q4be-gaw07Elasticity of iron at the temperature of the Earth's inner core
https://resolver.caltech.edu/CaltechAUTHORS:20150331-084539570
Authors: {'items': [{'id': 'Steinle-Neumann-G', 'name': {'family': 'Steinle-Neumann', 'given': 'Gerd'}}, {'id': 'Stixrude-L', 'name': {'family': 'Stixrude', 'given': 'Lars'}}, {'id': 'Cohen-R-E', 'name': {'family': 'Cohen', 'given': 'R. E.'}, 'orcid': '0000-0001-5871-2359'}, {'id': 'Gülseren-O', 'name': {'family': 'Gülseren', 'given': 'Oguz'}}]}
Year: 2001
DOI: 10.1038/35092536
Seismological body-wave and free-oscillation studies of the Earth's solid inner core have revealed that compressional waves traverse the inner core faster along near-polar paths than in the equatorial plane. Studies have also documented local deviations from this first-order pattern of anisotropy on length scales ranging from 1 to 1,000 km. These observations, together with reports of the differential rotation of the inner core, have generated considerable interest in the physical state and dynamics of the inner core, and in the structure and elasticity of its main constituent, iron, at appropriate conditions of pressure and temperature. Here we report first-principles calculations of the structure and elasticity of dense hexagonal close-packed (h.c.p.) iron at high temperatures. We find that the axial ratio c/a of h.c.p. iron increases substantially with increasing temperature, reaching a value of nearly 1.7 at a temperature of 5,700 K, where aggregate bulk and shear moduli match those of the inner core. As a consequence of the increasing c/a ratio, we have found that the single-crystal longitudinal anisotropy of h.c.p. iron at high temperature has the opposite sense from that at low temperature. By combining our results with a simple model of polycrystalline texture in the inner core, in which basal planes are partially aligned with the rotation axis, we can account for seismological observations of inner-core anisotropy.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/dj512-xhp35Hard superconducting nitrides
https://resolver.caltech.edu/CaltechAUTHORS:CHEpnas05
Authors: {'items': [{'id': 'Chen-Xiao-Jia', 'name': {'family': 'Chen', 'given': 'Xiao-Jia'}}, {'id': 'Struzhkin-V-V', 'name': {'family': 'Struzhkin', 'given': 'Viktor V.'}, 'orcid': '0000-0002-3468-0548'}, {'id': 'Wu-Zhigang', 'name': {'family': 'Wu', 'given': 'Zhigang'}, 'orcid': '0000-0001-8959-2345'}, {'id': 'Somayazulu-M-S', 'name': {'family': 'Somayazulu', 'given': 'Maddury'}}, {'id': 'Qian-Jiangu', 'name': {'family': 'Qian', 'given': 'Jiangu'}}, {'id': 'Kung-Simon', 'name': {'family': 'Kung', 'given': 'Simon'}}, {'id': 'Christensen-A-N', 'name': {'family': 'Christensen', 'given': 'Axel Nørlund'}}, {'id': 'Zhao-Yusheng', 'name': {'family': 'Zhao', 'given': 'Yusheng'}, 'orcid': '0000-0003-1640-5216'}, {'id': 'Cohen-R-E', 'name': {'family': 'Cohen', 'given': 'Ronald E.'}, 'orcid': '0000-0001-5871-2359'}, {'id': 'Mao-Ho-kwang', 'name': {'family': 'Mao', 'given': 'Ho-kwang'}}, {'id': 'Hemley-R-J', 'name': {'family': 'Hemley', 'given': 'Russell J.'}}]}
Year: 2005
DOI: 10.1073/pnas.0500174102
PMCID: PMC552926
Detailed study of the equation of state, elasticity, and hardness of selected superconducting transition-metal nitrides reveals interesting correlations among their physical properties. Both the bulk modulus and Vickers hardness are found to decrease with increasing zero-pressure volume in NbN, HfN, and ZrN. The computed elastic constants from first principles satisfy c(11) > c(12) > c(44) for NbN, but c(11) > c(44) > c(12) for HfN and ZrN, which are in good agreement with the neutron scattering data. The cubic delta-NbN superconducting phase possesses a bulk modulus of 348 GPa, comparable to that of cubic boron nitride, and a Vickers hardness of 20 GPa, which is close to sapphire. Theoretical calculations for NbN show that all elastic moduli increase monotonically with increasing pressure. These results suggest technological applications of such materials in extreme environments.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/4xgd0-tky19Absence of Magnetism in Hcp Iron-Nickel at 11 K
https://resolver.caltech.edu/CaltechAUTHORS:PAPprl06
Authors: {'items': [{'id': 'Papandrew-A-B', 'name': {'family': 'Papandrew', 'given': 'A. B.'}}, {'id': 'Lucas-M-S', 'name': {'family': 'Lucas', 'given': 'M. S.'}}, {'id': 'Stevens-R', 'name': {'family': 'Stevens', 'given': 'R.'}}, {'id': 'Halevy-I', 'name': {'family': 'Halevy', 'given': 'I.'}}, {'id': 'Fultz-B', 'name': {'family': 'Fultz', 'given': 'B.'}, 'orcid': '0000-0002-6364-8782'}, {'id': 'Hu-Michael-Y', 'name': {'family': 'Hu', 'given': 'M. Y.'}, 'orcid': '0000-0002-3718-7169'}, {'id': 'Chow-Paul', 'name': {'family': 'Chow', 'given': 'P.'}}, {'id': 'Cohen-R-E', 'name': {'family': 'Cohen', 'given': 'R. E.'}, 'orcid': '0000-0001-5871-2359'}, {'id': 'Somayazulu-M-S', 'name': {'family': 'Somayazulu', 'given': 'M.'}}]}
Year: 2006
DOI: 10.1103/PhysRevLett.97.087202
Synchrotron Mössbauer spectroscopy (SMS) was performed on an hcp-phase alloy of composition Fe92Ni8 at a pressure of 21 GPa and a temperature of 11 K. Density functional theoretical calculations predict antiferromagnetism in both hcp Fe and hcp Fe-Ni. For hcp Fe, these calculations predict no hyperfine magnetic field, consistent with previous experiments. For hcp Fe-Ni, however, substantial hyperfine magnetic fields are predicted, but these were not observed in the SMS spectra. Two possible explanations are suggested. First, small but significant errors in the generalized gradient approximation density functional may lead to an erroneous prediction of magnetic order or of erroneous hyperfine magnetic fields in antiferromagnetic hcp Fe-Ni. Alternately, quantum fluctuations with periods much shorter than the lifetime of the nuclear excited state would prohibit the detection of moments by SMS.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/4y5a0-j7021First Principles Force Field for Metallic Tantalum
https://resolver.caltech.edu/CaltechAUTHORS:20190702-100852563
Authors: {'items': [{'id': 'Strachan-Alejandro', 'name': {'family': 'Strachan', 'given': 'Alejandro'}, 'orcid': '0000-0002-4174-9750'}, {'id': 'Çağin-T', 'name': {'family': 'Çağin', 'given': 'Tahir'}, 'orcid': '0000-0002-3665-0932'}, {'id': 'Gülseren-Oğuz', 'name': {'family': 'Gülseren', 'given': 'Oğuz'}}, {'id': 'Mukherjee-Sonali', 'name': {'family': 'Mukherjee', 'given': 'Sonali'}}, {'id': 'Cohen-R-E', 'name': {'family': 'Cohen', 'given': 'Ronald E.'}, 'orcid': '0000-0001-5871-2359'}, {'id': 'Goddard-W-A-III', 'name': {'family': 'Goddard', 'given': 'William A., III'}, 'orcid': '0000-0003-0097-5716'}]}
Year: 2019
DOI: 10.48550/arXiv.0208027
We propose a general strategy to develop accurate Force Fields (FF) for metallic systems derived from ab initio quantum mechanical (QM) calculations; we illustrate this approach for tantalum. As input data to the FF we use the linearized augmented plane wave method (LAPW) with the generalized gradient approximation (GGA) to calculate: (i) the zero temperature equation of state (EOS) of Ta for bcc, fcc, and hcp crystal structures for pressures up to ~500 GPa. (ii) Elastic constants. (iii) We use a mixed-basis pseudopotential code to calculate volume relaxed vacancy formation energy also as a function of pressure. In developing the Ta FF we also use previous QM calculations of: (iv) the equation of state for the A15 structure. (v) the surface energy bcc (100). (vi) energetics for shear twinning of the bcc crystal. We find that withappropriate parameters an embedded atom model force field (denoted as qEAM FF) is able to reproduce all this QM data. Thus, the same FF describes with good accuracy the bcc, fcc, hcp and A15 phases of Ta for pressures from ~ -10 GPa to ~ 500 GPa, while also describing the vacancy, surface energy, and shear transformations. The ability of this single FF to describe such a range of systems with a variety of coordinations suggests that it would be accurate for describing defects such as dislocations, grain boundaries, etc.
We illustrate the use of the qEAM FF with molecular dynamics to calculate such finite temperature properties as the melting curve up to 300 GPa; we obtain a zero pressure melting temperature of T_(melt)=3150 +/- 50 K in good agreement with experiment (3213-3287 K). We also report on the thermal expansion of Ta in a wide temperature range; our calculated thermal expansivity agrees well with experimental data.https://authors.library.caltech.eduhttps://authors.library.caltech.edu/records/zechb-q7g14