8,237 research outputs found
Three-Dimensional Topological Insulator in a Magnetic Field: Chiral Side Surface States and Quantized Hall Conductance
Low energy excitation of surface states of a three-dimensional topological
insulator (3DTI) can be described by Dirac fermions. By using a tight-binding
model, the transport properties of the surface states in a uniform magnetic
field is investigated. It is found that chiral surface states parallel to the
magnetic field are responsible to the quantized Hall (QH) conductance
multiplied by the number of Dirac cones. Due to the
two-dimension (2D) nature of the surface states, the robustness of the QH
conductance against impurity scattering is determined by the oddness and
evenness of the Dirac cone number. An experimental setup for transport
measurement is proposed
Turbo-FLASH based arterial spin labeled perfusion MRI at 7 T.
Motivations of arterial spin labeling (ASL) at ultrahigh magnetic fields include prolonged blood T1 and greater signal-to-noise ratio (SNR). However, increased B0 and B1 inhomogeneities and increased specific absorption ratio (SAR) challenge practical ASL implementations. In this study, Turbo-FLASH (Fast Low Angle Shot) based pulsed and pseudo-continuous ASL sequences were performed at 7T, by taking advantage of the relatively low SAR and short TE of Turbo-FLASH that minimizes susceptibility artifacts. Consistent with theoretical predictions, the experimental data showed that Turbo-FLASH based ASL yielded approximately 4 times SNR gain at 7T compared to 3T. High quality perfusion images were obtained with an in-plane spatial resolution of 0.85×1.7 mm(2). A further functional MRI study of motor cortex activation precisely located the primary motor cortex to the precentral gyrus, with the same high spatial resolution. Finally, functional connectivity between left and right motor cortices as well as supplemental motor area were demonstrated using resting state perfusion images. Turbo-FLASH based ASL is a promising approach for perfusion imaging at 7T, which could provide novel approaches to high spatiotemporal resolution fMRI and to investigate the functional connectivity of brain networks at ultrahigh field
Spectroastrometric Reverberation Mapping of Broad-line Regions
Spectroastrometry measures source astrometry as a function of
wavelength/velocity. Reverberations of spectroastrometric signals naturally
arise in broad-line regions (BLRs) of active galactic nuclei as a result of the
continuum variations that drive responses of the broad emission lines with time
delays. Such signals provide a new diagnostic for mapping BLR kinematics and
geometry, complementary to the traditional intensity reverberation mapping (RM)
technique. We present the generic mathematical formulism for spectroastrometric
RM and show that under realistic parameters of a phenomenological BLR model,
the spectroastrometric reverberation signals vary on a level of several to tens
of microarcseconds, depending on the BLR size, continuum variability, and
angular-size distance. We also derive the analytical expressions of
spectroastrometric RM for an inclined ring-like BLR. We develop a Bayesian
framework with a sophisticated Monte-Carlo sampling technique to analyze
spectroastrometic data and infer the BLR properties, including the central
black hole mass and angular-size distance. We demonstrate the potential of
spectroastrometric RM in spatially resolving BLR kinematics and geometry
through a suite of simulation tests. An application to realistic observation
data of 3C~273 obtains tentative, but enlightening results, reinforcing the
practical feasibility of conducting spectroastrometric RM experiments on bright
AGNs with the operating Very Large Telescope Interferometer as well as possibly
with the planned next-generation 30m-class telescopes.Comment: 22 pages, 17 figures, 2 tables; ApJ in press; The code BRAINS
available at https://github.com/LiyrAstroph/BRAIN
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