6,636 research outputs found
Spectacular Role of Electron Correlation in the Hyperfine Interactions in States in Alkaline Earth Ions
The low-lying n(=3,4,5)d states alkaline earth ions are of vital
importance in a number of different physical applications. The hyperfine
structure constants of these states are characterized by unusually strong
electron correlation effects. Relativistic coupled-cluster theory has been
employed to carry out {\it ab initio} calculations of these constants. The role
of the all order core-polarization effects was found to be decisive in
obtaining good agreement of the results of our calculations with accurate
measurements. The present work is an apt demonstration of the power of the
coupled-cluster method to cope with strongly interacting configurations.Comment: Submitted to Physical Review Letters, 3 figures and 5 table
Gravitational Redshift in Einstein-Kalb-Ramond Spacetime and Randall-Sundrum Scenario
It is shown that the gravitational redshift as predicted by Einstein's
theory, is modified in presence of second rank antisymmetric tensor
(Kalb-Ramond) field in a string inspired background spacetime.In presence of
extra dimensions, the Randall-Sundrum brane world scenario is found to play a
crucial role in suppressing this additional shift. The bound on the value of
the warp factor is determined from the redshift data and is found to be in good
agreement with that determined from the requirements of Standard model.Comment: 4 Pages, Revtex, No figures, version thoroughly revise
The formation of supermassive black holes in the first galaxies
We discuss the formation of supermassive black holes in the early universe,
and how to probe their subsequent evolution with the upcoming mm/sub-mm
telescope ALMA. We first focus on the chemical and radiative conditions for
black hole formation, in particular considering radiation trapping and
molecular dissociation effects. We then turn our attention towards the magnetic
properties in the halos where the first black holes form, and show that the
presence of turbulence may lead to a magnetic dynamo, which could support the
black hole formation process by providing an efficient means of transporting
the angular momentum. We finally focus on observable properties of
high-redshift black holes with respect to ALMA, and discuss how to distinguish
between chemistry driven by the starburst and chemistry driven by X-rays from
the black hole.Comment: Contribution to AIP conference proceedings "First Stars and Galaxies:
Challenges in the Next Decade". 4 pages, 3 figure
Where are the degrees of freedom responsible for black hole entropy?
Considering the entanglement between quantum field degrees of freedom inside
and outside the horizon as a plausible source of black hole entropy, we address
the question: {\it where are the degrees of freedom that give rise to this
entropy located?} When the field is in ground state, the black hole area law is
obeyed and the degrees of freedom near the horizon contribute most to the
entropy. However, for excited state, or a superposition of ground state and
excited state, power-law corrections to the area law are obtained, and more
significant contributions from the degrees of freedom far from the horizon are
shown.Comment: 6 pages, 4 figures, Invited talk at Theory Canada III, Edmonton,
Alberta, Canada, June 16, 200
The universe dynamics in the tachyon cosmology with non-minimal coupling to matter
Recently, the tachyon cosmology has been represented as dark energy model to
support the current acceleration of the universe without phantom crossing. In
this paper, we study the dynamics of the tachyon cosmology in which the field
plays the role of tachyon field and also non--minimally coupled to the matter
lagrangian. The model shows current universe acceleration and also phantom
crossing in the future. Two cosmological tests are also performed to validate
the model; the difference in the distance modulus and the model independent
Cosmological Redshift Drift (CRD) test.Comment: 14 pages, 11 figure
A comprehensive set of UV and x-ray radiative transition rates for Fe XVI
Sodium-like Fe XVI is observed in collisionally ionized plasmas such as stellar coronae and coronal line regions of active galactic nuclei including black hole-accretion disc environments. Given its recombination edge from neon-like Fe XVII at ~25 Å, the Fe XVI bound-bound transitions lie in the soft x-ray and EUV (extreme ultraviolet) range. We present a comprehensive set of theoretical transition rates for radiative dipole allowed E1 transitions including fine structure for levels with nℓ(SLJ) ≤ 10, ℓ ≤ 9 using the relativistic Breit-Pauli R-matrix (BPRM) method. In addition, forbidden transitions of electric quadrupole (E2), electric octupole (E3), magnetic dipole (M1) and magnetic quadrupole (M2) type are presented for levels up to 5g(SLJ) from relativistic atomic structure calculations in the Breit-Pauli approximation using code SUPERSTRUCTURE. Some of the computed levels are autoionizing, and oscillator strengths among those are also provided. BPRM results have been benchmarked with the relativistic coupled cluster method and the atomic structure Dirac-Fock code GRASP. Levels computed with the electron collision BPRM codes in bound state mode were identified with a procedure based on the analysis of quantum defects and asymptotic wavefunctions. The total number of Fe XVI levels considered is 96, with 822 E1 transitions. Tabulated values are presented for the oscillator strengths f, line strengths S and Einstein radiative decay rates A. This extensive dataset should enable spectral modelings up to highly excited levels, including recombination-cascade matrices.This work was partially supported by the NASA Astronomy and Physics Research Program and the Astrophysical Theory Program. The computational work was carried out at the Ohio Supercomputer Center in Columbus, Ohio. CS acknowledges discussions with Professor B P Das, Professor D Mukherjee and Professor R K Chaudhuri
Functional imaging of visual cortical layers and subplate in awake mice with optimized three-photon microscopy
Two-photon microscopy is used to image neuronal activity, but has severe limitations for studying deeper cortical layers. Here, we developed a custom three-photon microscope optimized to image a vertical column of the cerebral cortex > 1 mm in depth in awake mice with low (<20 mW) average laser power. Our measurements of physiological responses and tissue-damage thresholds define pulse parameters and safety limits for damage-free three-photon imaging. We image functional visual responses of neurons expressing GCaMP6s across all layers of the primary visual cortex (V1) and in the subplate. These recordings reveal diverse visual selectivity in deep layers: layer 5 neurons are more broadly tuned to visual stimuli, whereas mean orientation selectivity of layer 6 neurons is slightly sharper, compared to neurons in other layers. Subplate neurons, located in the white matter below cortical layer 6 and characterized here for the first time, show low visual responsivity and broad orientation selectivity.National Institutes of Health (U.S.) (grant EY007023)National Institutes of Health (U.S.) (grant NS090473)National Institutes of Health (U.S.) (grant 4-P41-EB015871)National Science Foundation (U.S.) (grant EF1451125)Picower Institute for Learning and Memory (Engineering Collaboration Grant)Massachusetts Life Sciences Initiativ
Cutting the Cord in Virtual Reality
Today's virtual reality (VR) headsets require a cable connection to a PC or game console. This cable significantly limits the player’s mobility and hence her/his VR experience. The high data rate requirement of this link (multiple Gbps) precludes its replacement by WiFi. Thus, in this paper, we focus on using mmWave technology to deliver multi Gbps wireless communication between VR headsets and their game consoles. The challenge, however, is that mmWave signals can be easily blocked by the player's hand or head motion. We describe novel algorithms and system design that allow such mmWave links to sustain high data rates even in the presence of a blockage, enabling a high quality untethered VR experience.National Science Foundation (U.S.)Hong Kong University of Science and Technolog
Vector-Based Integration of Local and Long-Range Information in Visual Cortex
Integration of inputs by cortical neurons provides the basis for the complex information processing performed in the cerebral cortex. Here, we propose a new analytic framework for understanding integration within cortical neuronal receptive fields. Based on the synaptic organization of cortex, we argue that neuronal integration is a systems--level process better studied in terms of local cortical circuitry than at the level of single neurons, and we present a method for constructing self-contained modules which capture (nonlinear) local circuit interactions. In this framework, receptive field elements naturally have dual (rather than the traditional unitary influence since they drive both excitatory and inhibitory cortical neurons. This vector-based analysis, in contrast to scalarsapproaches, greatly simplifies integration by permitting linear summation of inputs from both "classical" and "extraclassical" receptive field regions. We illustrate this by explaining two complex visual cortical phenomena, which are incompatible with scalar notions of neuronal integration
Primary trisomic of rice: orgin, morphology, cytology and use in linkage mapping
Twelve primary trisomics of Oryza sativa L. were isolated from the progenies of spontaneous triploids and were transferred by backcrossing to the genetic background of IR36, a widely grown high yielding rice variety. Eleven trisomics can be identified morphologically from one another and from diploids. However, triplo 11 is difficult to distinguish from diploid sibs. -The extra chromosome of each trisomic was identified cytologically at pachytene stage of meiosis, and the chromosomes were numbered according to their length at this stage. The major distinguishing features of each pachytene chromosome were redescribed. -The female transmission rates varied from 15.5% for triplo 1, the longest chromosome, to 43.9% for triplo 12, the shortest chromosome. Seven of the 12 primary trisomics transmitted the extra chromosome through the male. The low level of chromosomal imbalance tolerated by rice and other evidence are interpreted to indicate that this species is a basic diploid. -Genetic segregation for 22 marker genes in the trisomic progenies was studied. Of a possible 264 combinations, involving 22 genes and 12 trisomics, 120 were examined. Marker genes for each of the 12 chromosomes were identified. The results helped establish associations between linkage groups and cytologically identifiable chromosomes of rice for the first time. Relationships between various systems of numbering chromosomes, trisomics, linkage groups and marker genes are described, and a revised linkage map of rice is presented
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