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Search Results: 1 - 10 of 5939 matches for " Douglas Vanderbilt "
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“Your Own Sensitivity, At Least”: Remembering the Postwar Poet Ibaragi Noriko, an Appreciation and Four Translations
Greg Vanderbilt
Asia-Pacific Journal : Japan Focus , 2011,
Abstract: This month marks the fifth anniversary of the death of the postwar poet Ibaragi Noriko (1926-2006). She was prepared: three months earlier, at age 79, she had written out a farewell letter and had it printed, ready to send to some two hundred of her friends and correspondents. Leaving blanks for the date (February 17, 2006, though it was two days later when she was found in her bed) and cause of her death (a brain hemorrhage), she expressed her wishes that there be no funeral or memorial and that no flowers be sent to her now vacant suburban home. Instead she made one request: “If you will pause for a moment, just a moment, and say to yourself ‘So now she is gone…,’ that will be enough.”Among the first poets to emerge in a new generation (and often considered the first and best-known woman among them) after the 1945 defeat, Ibaragi was sui generis in a time when poets were part of rebuilding the imagination of a citizenry, seeking to “cultivate” (tagayasu, her favorite verb, she said) in the language, place, and time where they happened to make their homes. With her beret and dark-rimmed glasses, her ever-present slim cigarettes and mellow voice, and her keen, youth-filled observations, she cut an unforgettable figure to the end of her life. A comment she made in her last months may well be a fitting summation: “I never thought I would have any affiliation, but in the end I can say I was affiliated with the Japanese language.”
Nonlocality of Kohn-Sham exchange-correlation fields in dielectrics
David Vanderbilt
Physics , 1997, DOI: 10.1103/PhysRevLett.79.3966
Abstract: The theory of the macroscopic field appearing in the Kohn-Sham exchange-correlation potential for dielectric materials, as introduced by Gonze, Ghosez and Godby, is reexamined. It is shown that this Kohn-Sham field cannot be determined from a knowledge of the local state of the material (local crystal potential, electric field, and polarization) alone. Instead, it has an intrinsically nonlocal dependence on the global electrostatic configuration. For example, it vanishes in simple transverse configurations of a polarized dielectric, but not in longitudinal ones.
Berry-phase theory of proper piezoelectric response
David Vanderbilt
Physics , 1999,
Abstract: Recent theoretical advances have established that the electric polarization in an insulating crystal can be viewed as a multivalued quantity that is determined by certain Berry phases associated with the occupied Bloch bands. The application of this approach to the computation of piezoelectric coefficients is not entirely straightforward, since a naive determination of the (``improper'') piezoelectric coefficients from finite differences of the polarization at nearby strain states leads to a dependence upon the choice of ``branch'' of the polarization. The purpose of the present paper is to clarify that if one calculates instead the ``proper'' piezoelectric response, the branch dependence is eliminated. From this analysis, a simplified recipe for the direct finite-difference computation of the proper piezoelectric coefficients emerges naturally.
A Multivariate Surface-Based Analysis of the Putamen in Premature Newborns: Regional Differences within the Ventral Striatum
Jie Shi, Yalin Wang, Rafael Ceschin, Xing An, Yi Lao, Douglas Vanderbilt, Marvin D. Nelson, Paul M. Thompson, Ashok Panigrahy, Natasha Leporé
PLOS ONE , 2013, DOI: 10.1371/journal.pone.0066736
Abstract: Many children born preterm exhibit frontal executive dysfunction, behavioral problems including attentional deficit/hyperactivity disorder and attention related learning disabilities. Anomalies in regional specificity of cortico-striato-thalamo-cortical circuits may underlie deficits in these disorders. Nonspecific volumetric deficits of striatal structures have been documented in these subjects, but little is known about surface deformation in these structures. For the first time, here we found regional surface morphological differences in the preterm neonatal ventral striatum. We performed regional group comparisons of the surface anatomy of the striatum (putamen and globus pallidus) between 17 preterm and 19 term-born neonates at term-equivalent age. We reconstructed striatal surfaces from manually segmented brain magnetic resonance images and analyzed them using our in-house conformal mapping program. All surfaces were registered to a template with a new surface fluid registration method. Vertex-based statistical comparisons between the two groups were performed via four methods: univariate and multivariate tensor-based morphometry, the commonly used medial axis distance, and a combination of the last two statistics. We found statistically significant differences in regional morphology between the two groups that are consistent across statistics, but more extensive for multivariate measures. Differences were localized to the ventral aspect of the striatum. In particular, we found abnormalities in the preterm anterior/inferior putamen, which is interconnected with the medial orbital/prefrontal cortex and the midline thalamic nuclei including the medial dorsal nucleus and pulvinar. These findings support the hypothesis that the ventral striatum is vulnerable, within the cortico-stiato-thalamo-cortical neural circuitry, which may underlie the risk for long-term development of frontal executive dysfunction, attention deficit hyperactivity disorder and attention-related learning disabilities in preterm neonates.
Dependence of electronic polarization on octahedral rotations in TbMnO3 from first principles
Andrei Malashevich,David Vanderbilt
Physics , 2009, DOI: 10.1103/PhysRevB.80.224407
Abstract: The electronic contribution to the magnetically induced polarization in orthorhombic TbMnO3 is studied from first principles. We compare the cases in which the spin cycloid, which induces the electric polarization via the spin-orbit interaction, is in either the b-c or a-b plane. We find that the electronic contribution is negligible in the first case, but much larger, and comparable to the lattice-mediated contribution, in the second case. However, we how that this behavior is an artifact of the particular pattern of octahedral rotations characterizing the structurally relaxed Pbnm crystal structure. To do so, we explore how the electronic contribution varies for a structural model of rigidly rotated MnO6 octahedra, and demonstrate that it can vary over a wide range, comparable with the lattice-mediated contribution, for both b-c and a-b spirals. We introduce a phenomenological model that is capable of describing this behavior in terms of sums of symmetry-constrained contributions arising from the displacements of oxygen atoms from the centers of the Mn-Mn bonds.
First-principles theory of magnetically induced ferroelectricity in TbMnO3
Andrei Malashevich,David Vanderbilt
Physics , 2009, DOI: 10.1140/epjb/e2009-00208-2
Abstract: We study the polarization induced via spin-orbit interaction by a magnetic cycloidal order in orthorhombic TbMnO3 using first-principle methods. The case of magnetic spiral lying in the b-c plane is analyzed, in which the pure electronic contribution to the polarization is shown to be small. We focus our attention on the lattice-mediated contribution, and study it's dependence on the Coulomb interaction parameter U in the LDA+U method and on the wave-vector of the spin spiral. The role of the spin-orbit interaction on different sites is also analyzed.
Effect of quantum fluctuations on structural phase transitions in SrTiO_3 and BaTiO_3
W. Zhong,David Vanderbilt
Physics , 1995, DOI: 10.1103/PhysRevB.53.5047
Abstract: Using path-integral Monte Carol simulations and an ab initio effective Hamiltonian, we study the effects of quantum fluctuations on structural phase transitions in the cubic perovskite compounds SrTiO3 and BaTiO3. We find quantum fluctuations affect ferroelectric (FE) transitions more strongly than antiferrodistortive (AFD) ones, even though the effective mass of a single FE local mode is larger. For SrTiO3 we find that the quantum fluctuations suppress the FE transition completely, and reduce the AFD transition temperature from 130K to 110K. For BaTiO3, quantum fluctuations do not affect the order of the transition, but do reduce the transition temperature by 35-50 K. The implications of the calculations are discussed.
First-principles study of stability and vibrational properties of tetragonal PbTiO_3
Alberto Garcia,David Vanderbilt
Physics , 1996, DOI: 10.1103/PhysRevB.54.3817
Abstract: A first-principles study of the vibrational modes of PbTiO_3 in the ferroelectric tetragonal phase has been performed at all the main symmetry points of the Brillouin zone (BZ). The calculations use the local-density approximation and ultrasoft pseudopotentials with a plane-wave basis, and reproduce well the available experimental information on the modes at the Gamma point, including the LO-TO splittings. The work was motivated in part by a previously reported transition to an orthorhombic phase at low temperatures [(J. Kobayashi, Y. Uesu, and Y. Sakemi, Phys. Rev. B {\bf 28}, 3866 (1983)]. We show that a linear coupling of orthorhombic strain to one of the modes at Gamma plays a role in the discussion of the possibility of this phase transition. However, no mechanical instabilities (soft modes) are found, either at Gamma or at any of the other high-symmetry points of the BZ.
Semiconductor effective charges from tight-binding theory
J. Bennetto,David Vanderbilt
Physics , 1996, DOI: 10.1103/PhysRevB.53.15417
Abstract: We calculate the transverse effective charges of zincblende compound semiconductors using Harrison's tight-binding model to describe the electronic structure. Our results, which are essentially exact within the model, are found to be in much better agreement with experiment than previous perturbation-theory estimates. Efforts to improve the results by using more sophisticated variants of the tight-binding model were actually less successful. The results underline the importance of including quantities that are sensitive to the electronic wavefunctions, such as the effective charges, in the fitting of tight-binding models.
Theory of hypothetical ferroelectric superlattices incorporating head-to-head and tail-to-tail 180$^\circ$ domain walls
Xifan Wu,David Vanderbilt
Physics , 2005, DOI: 10.1103/PhysRevB.73.020103
Abstract: While electrical compatibility constraints normally prevent head-to-head (HH) and tail-to-tail (TT) domain walls from forming in ferroelectric materials, we propose that such domain walls could be stabilized by intentional growth of atomic layers in which the cations are substituted from a neighboring column of the periodic table. In particular, we carry out predictive first-principles calculations of superlattices in which Sc, Nb, or other substitutional layers are inserted periodically into PbTiO$_3$. We confirm that this gives rise to a domain structure with the longitudinal component of the polarization alternating from domain to domain, and with the substitutional layers serving as HH and TT domain walls. We also find that a substantial transverse component of the polarization can also be present.
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