208 research outputs found
Magnetic susceptibility of alkali-TCNQ salts and extended Hubbard models with bond order and charge density wave phases
The molar spin susceptibilities of Na-TCNQ, K-TCNQ and Rb-TCNQ(II)
are fit quantitatively to 450 K in terms of half-filled bands of three
one-dimensional Hubbard models with extended interactions using exact results
for finite systems. All three models have bond order wave (BOW) and charge
density wave (CDW) phases with boundary for nearest-neighbor
interaction and on-site repulsion . At high , all three salts have
regular stacks of anion radicals. The fits place Na and
K in the CDW phase and Rb(II) in the BOW phase with . The Na and
K salts have dimerized stacks at while Rb(II) has regular stacks at
100K. The analysis extends to dimerized stacks and to dimerization
fluctuations in Rb(II). The three models yield consistent values of ,
and transfer integrals for closely related stacks. Model
parameters based on are smaller than those from optical data that in
turn are considerably reduced by electronic polarization from quantum chemical
calculation of , and on adjacent ions. The
analysis shows that fully relaxed states have reduced model parameters compared
to optical or vibration spectra of dimerized or regular stacks.Comment: 9 pages and 5 figure
Electromagnetically induced transparency in cold 85Rb atoms trapped in the ground hyperfine F = 2 state
We report electromagnetically induced transparency (EIT) in cold 85Rb atoms,
trapped in the lower hyperfine level F = 2, of the ground state 5
(Tiwari V B \textit{et al} 2008 {\it Phys. Rev.} A {\bf 78} 063421). Two steady
state -type systems of hyperfine energy levels are investigated using
probe transitions into the levels F = 2 and F = 3 of the
excited state 5 in the presence of coupling transitions F = 3
F = 2 and F = 3 F = 3, respectively. The
effects of uncoupled magnetic sublevel transitions and coupling field's Rabi
frequency on the EIT signal from these systems are studied using a simple
theoretical model.Comment: 12 pages, 7 figure
A Density Matrix Renormalization Group Method Study of Optical Properties of Porphines and Metalloporphines
The symmetrized Density-Matrix-Renormalization-Group (DMRG) method is used to
study linear and nonlinear optical properties of Free base porphine and
metallo-porphine. Long-range interacting model, namely, Pariser-Parr-Pople
(PPP) model is employed to capture the quantum many body effect in these
systems. The non-linear optical coefficients are computed within correction
vector method. The computed singlet and triplet low-lying excited state
energies and their charge densities are in excellent agreement with
experimental as well as many other theoretical results. The rearrangement of
the charge density at carbon and nitrogen sites, on excitation, is discussed.
From our bond order calculation, we conclude that porphine is well described by
the 18-annulenic structure in the ground state and the molecule expands upon
excitation. We have modelled the regular metalloporphine by taking an effective
electric field due to the metal ion and computed the excitation spectrum.
Metalloporphines have symmetry and hence have more degenerate excited
states. The ground state of Metalloporphines show 20-annulenic structure, as
the charge on the metal ion increases. The linear polarizability seems to
increase with the charge initially and then saturates. The same trend is
observed in third order polarizability coefficients.Comment: 13 pages, 6 figure
DMRG study of scaling exponents in spin-1/2 Heisenberg chains with dimerization and frustration
In conformal field theory, key properties of spin-1/2 chains, such as the
ground state energy per site and the excitation gap scale with dimerization
delta as delta^alpha with known exponents alpha and logarithmic corrections.
The logarithmic corrections vanish in a spin chain with nearest (J=1) and next
nearest neighbor interactions (J_2), for J_2c=0.2411. DMRG analysis of a
frustrated spin chain with no logarithmic corrections yields the field
theoretic values of alpha, and the scaling relation is valid up to the
physically realized range, delta ~ 0.1. However, chains with logarithmic
corrections (J_2<0.2411 J) are more accurately fit by simple power laws with
different exponents for physically realized dimerizations. We show the
exponents decreasing from approximately 3/4 to 2/3 for the spin gap and from
approximately 3/2 to 4/3 for the energy per site and error bars in the exponent
also decrease as J_2 approaches to J_2c.Comment: 9 pages including two figures; added standard deviations of various
fitting parameters such as exponents, and several references to earlier wor
Like Sign Dilepton Signature for Gluino Production at LHC with or without R Conservation
The isolated like sign dilepton signature for gluino production is
investigated at the LHC energy for the conserving as well as the and
violating SUSY models over a wide range of the parameter space. One gets
viable signals for gluino masses of 300 and 600 GeV for both conserving and
violating models, while it is less promising for the violating case.
For a 1000 GeV gluino, the violating signal should still be viable; but the
conserving signal becomes too small at least for the low luminosity option
of LHC.Comment: (e-mail: [email protected]) Latex: No. of pages 21, no. of figures
6 - available on reques
COVID-19 Detection Through Transfer Learning Using Multimodal Imaging Data
Detecting COVID-19 early may help in devising an appropriate treatment plan and disease containment decisions. In this study, we demonstrate how transfer learning from deep learning models can be used to perform COVID-19 detection using images from three most commonly used medical imaging modes X-Ray, Ultrasound, and CT scan. The aim is to provide over-stressed medical professionals a second pair of eyes through intelligent deep learning image classification models. We identify a suitable Convolutional Neural Network (CNN) model through initial comparative study of several popular CNN models. We then optimize the selected VGG19 model for the image modalities to show how the models can be used for the highly scarce and challenging COVID-19 datasets. We highlight the challenges (including dataset size and quality) in utilizing current publicly available COVID-19 datasets for developing useful deep learning models and how it adversely impacts the trainability of complex models. We also propose an image pre-processing stage to create a trustworthy image dataset for developing and testing the deep learning models. The new approach is aimed to reduce unwanted noise from the images so that deep learning models can focus on detecting diseases with specific features from them. Our results indicate that Ultrasound images provide superior detection accuracy compared to X-Ray and CT scans. The experimental results highlight that with limited data, most of the deeper networks struggle to train well and provides less consistency over the three imaging modes we are using. The selected VGG19 model, which is then extensively tuned with appropriate parameters, performs in considerable levels of COVID-19 detection against pneumonia or normal for all three lung image modes with the precision of up to 86% for X-Ray, 100% for Ultrasound and 84% for CT scans
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