29,934 research outputs found
Zeeman-Induced Gapless Superconductivity with Partial Fermi Surface
We show that an in-plane magnetic field can drive two-dimensional
spin-orbit-coupled systems under superconducting proximity effect into a
gapless phase where parts of the normal state Fermi surface are gapped, and the
ungapped parts are reconstructed into a small Fermi surface of Bogoliubov
quasiparticles at zero energy. Charge distribution, spin texture, and density
of states of such "partial Fermi surface" are discussed. Material platforms for
its physical realization are proposed.Comment: 5 pages, 2 figure
A magnetohydrodynamic model for multi-wavelength flares from Sagittarius~A (I): model and the near-infrared and X-ray flares
Flares from the supermassive black hole in our Galaxy, Sagittarius~A
(Sgr A), are routinely observed over the last decade or so. Despite
numerous observational and theoretical efforts, the nature of such flares still
remains poorly understood, although a few phenomenological scenarios have been
proposed. In this work, we develop the Yuan et al. (2009) scenario into a
magnetohydrodynamic (MHD) model for Sgr A flares. This model is
analogous with the theory of solar flares and coronal mass ejection in solar
physics. In the model, magnetic field loops emerge from the accretion flow onto
Sgr A and are twisted to form flux ropes because of shear and
turbulence. The magnetic energy is also accumulated in this process until a
threshold is reached. This then results in a catastrophic evolution of a flux
rope with the help of magnetic reconnection in the current sheet. In this
catastrophic process, the magnetic energy is partially converted into the
energy of non-thermal electrons. We have quantitatively calculated the
dynamical evolution of the height, size, and velocity of the flux rope, as well
as the magnetic field in the flare regions, and the energy distribution of
relativistic electrons in this process. We further calculate the synchrotron
radiation from these electrons and compare the obtained light curves with the
observed ones. We find that the model can reasonably explain the main
observations of near-infrared (NIR) and X-ray flares including their light
curves and spectra. It can also potentially explain the frequency-dependent
time delay seen in radio flare light curves.Comment: 17 pages, 13 figures, accepted by MNRA
Unconventional Superconductivity and Density Waves in Twisted Bilayer Graphene
We study electronic ordering instabilities of twisted bilayer graphene with
electrons per supercell, where correlated insulator state and
superconductivity are recently observed. Motivated by the Fermi surface nesting
and the proximity to Van Hove singularity, we introduce a hot-spot model to
study the effect of various electron interactions systematically. Using
renormalization group method, we find /-wave superconductivity and
charge/spin density wave emerge as the two types of leading instabilities
driven by Coulomb repulsion. The density wave state has a gapped energy
spectrum at and yields a single doubly-degenerate pocket upon doping to
. The intertwinement of density wave and superconductivity and the
quasiparticle spectrum in the density wave state are consistent with
experimental observations.Comment: 15 pages, 12 figures; updated discussion and analysis on density wave
state
- β¦