Article records
https://feeds.library.caltech.edu/people/Cohen-R-E/article.rss
A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenTue, 16 Apr 2024 13:29:16 +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.edu/records/3nxqb-3ps67Elasticity 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.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.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.edu/records/4y5a0-j7021