594 research outputs found
Dirty two-band superconductivity with interband pairing order
We study theoretically the effects of random nonmagnetic impurities on the
superconducting transition temperature in a two-band superconductor
characterized by an equal-time s-wave interband pairing order parameter. The
Fermi-Dirac statistics of electrons allows a spin-triplet s-wave pairing order
as well as a spin-singlet s-wave order parameter due to the two-band degree of
freedom. In a spin-singlet superconductor, is insensitive to the impurity
concentration when we estimate the self-energy due to the random impurity
potential within the Born approximation. On the other hand in a spin-triplet
superconductor, decreases with the increase of the impurity
concentration. We conclude that Cooper pairs belonging to odd-band-parity
symmetry class are fragile under the random impurity potential even though they
have s-wave pairing symmetry.Comment: 7 pages, 2 figures embedde
Orbital Evolution of a Circumbinary Planet in a Gaseous Disk
Sub-Jupiter classed circumbinary planets discovered in close-in binary
systems have orbits just beyond the dynamically unstable region, which is
determined by the eccentricity and mass ratio of the host binary stars. These
planets are assumed to have formed beyond the snow line and migrated to the
current orbits rather than forming in situ. We propose a scenario in which a
planet formed beyond the snow line and migrated to the inner edge of the
circumbinary disk, which was within the unstable area, and then moved to the
current orbit through outward transportation. This outward transportation is
driven by the balance of orbital excitation of the central stars inside the
gravitationally unstable region and damping by the gas-drag force. We carried
out N-body simulations with a dissipating circumbinary protoplanetary disk for
binary systems with different eccentricities and mass ratios. Planets are more
likely to achieve a stable orbit just beyond the unstable region in less
eccentric binary systems. This result is not as sensitive to mass ratio as it
is to eccentricity. These dependencies are consistent with the data from
observed binary systems hosting circumbinary planets. We find CBPs' orbits
close to the instability boundaries are explained by our orbital evolution
scenario.Comment: 26 pages, 8 figures, accepted for publication in Earth, Planets and
Spac
Josephson effect in two-band superconductors
We study theoretically the Josephson effect between two time-reversal
two-band superconductors, where we assume the equal-time spin-singlet -wave
pair potential in each conduction band. %as well as the band asymmetry and the
band hybridization in the normal state. The superconducting phase at the first
band and that at the second band characterize a
two-band superconducting state. We consider a Josephson junction where an
insulating barrier separates two such two-band superconductors. By applying the
tunnel Hamiltonian description, the Josephson current is calculated in terms of
the anomalous Green's function on either side of the junction. We find that the
Josephson current consists of three components which depend on three types of
phase differences across the junction: the phase difference at the first band
, the phase difference at the second band ,
and the difference at the center-of-mass phase .
A Cooper pairs generated by the band hybridization carries the last current
component. In some cases, the current-phase relationship deviates from the
sinusoidal function as a result of time-reversal symmetry breaking down.Comment: 6 page, 2 figure
中性流体およびプラズマにおける亜臨界不安定性について
International audience亜臨界不安定性は,非線形不安定性の一種である.亜臨界不安定な系は,線形安定であっても非線形的に不安定となる.特徴として,不安定性が生じるための初期摂動の大きさに閾値が存在し,閾値以下の摂動は減衰し安定化する.亜臨界不安定性は,流体やプラズマにおいて広くみられる現象である.亜臨界不安定性は,乱流や構造形成,異常抵抗性や乱流輸送に本質的なインパクトを与えるため重要な問題である.この解説では,亜臨界不安定性の概念について解説し,様々な物理的局面における研究について紹介する
Cervical Cancer Treatment using AI
In cervical cancer treatment, radiation therapy is selected based on the degree of tumor progression, and radiation oncologists are required to delineate tumor contours. To reduce the burden on radiation oncologists, an automatic segmentation of the tumor contours would prove useful. To the best of our knowledge, automatic tumor contour segmentation has rarely been applied to cervical cancer treatment. In this study, diffusion-weighted images (DWI) of 98 patients with cervical cancer were acquired. We trained an automatic tumor contour segmentation model using 2D U-Net and 3D U-Net to investigate the possibility of applying such a model to clinical practice. A total of 98 cases were employed for the training, and they were then predicted by swapping the training and test images. To predict tumor contours, six prediction images were obtained after six training sessions for one case. The six images were then summed and binarized to output a final image through automatic contour segmentation. For the evaluation, the Dice similarity coefficient (DSC) and Hausdorff distance (HD) was applied to analyze the difference between tumor contour delineation by radiation oncologists and the output image. The DSC ranged from 0.13 to 0.93 (median 0.83, mean 0.77). The cases with DSC <0.65 included tumors with a maximum diameter < 40 mm and heterogeneous intracavitary concentration due to necrosis. The HD ranged from 2.7 to 9.6 mm (median 4.7 mm). Thus, the study confirmed that the tumor contours of cervical cancer can be automatically segmented with high accuracy
Subcritical Instabilities in Neutral Fluids and Plasmas
International audienceIn neutral fluids and plasmas, the analysis of perturbations often starts with an inventory of linearly unstable modes. Then, the nonlinear steady-state is analyzed or predicted based on these linear modes. A crude analogy would be to base the study of a chair on how it responds to infinitesimaly small perturbations. One would conclude that the chair is stable at all frequencies, and cannot fall down. Of course, a chair falls down if subjected to finite-amplitude perturbations. Similarly, waves and wave-like structures in neutral fluids and plasmas can be triggered even though they are linearly stable. These subcritical instabilities are dormant until an interaction, a drive, a forcing, or random noise pushes their amplitude above some threshold. Investigating their onset conditions requires nonlinear calculations. Subcritical instabilities are ubiquitous in neutral fluids and plasmas. In plasmas, subcritical instabilities have been investigated based on analytical models and numerical simulations since the 1960s. More recently, they have been measured in laboratory and space plasmas, albeit not always directly. The topic could benefit from the much longer and richer history of subcritical instability and transition to subcritical turbulence in neutral fluids. In this tutorial introduction, we describe the fundamental aspects of subcritical instabilities in plasmas, based on systems of increasing complexity, from simple examples of a point-mass in a potential well or a box on a table, to turbulence and instabilities in neutral fluids, and finally, to modern applications in magnetized toroidal fusion plasmas
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