16,980 research outputs found
Detectability of f-mode Unstable Neutron Stars by the Schenberg Spherical Antenna
The Brazilian spherical antenna (Schenberg) is planned to detect high
frequency gravitational waves (GWs) ranging from 3.0 kHz to 3.4 kHz. There is a
host of astrophysical sources capable of being detected by the Brazilian
antenna, namely: core collapse in supernova events; (proto)neutron stars
undergoing hydrodynamical instability; f-mode unstable neutron stars, caused by
quakes and oscillations; excitation of the first quadrupole normal mode of 4-9
solar mass black holes; coalescence of neutron stars and/or black holes; exotic
sources such as bosonic or strange matter stars rotating at 1.6 kHz; and
inspiralling of mini black hole binaries. We here address our study in
particular to the neutron stars, which could well become f-mode unstable
producing therefore GWs. We estimate, for this particular source of GWs, the
event rates that in principle can be detected by Schenberg and by the Dutch
Mini-Grail antenna.Comment: 7 pages, 3 figures; Classical and Quantum Gravity (in press
A formulation of a (q+1,8)-cage
Let be a prime power. In this note we present a formulation for
obtaining the known -cages which has allowed us to construct small
--graphs for and . Furthermore, we also obtain smaller
-graphs for even prime power .Comment: 14 pages, 2 figure
A construction of small (q-1)-regular graphs of girth 8
In this note we construct a new infinite family of -regular graphs of
girth and order for all prime powers , which are the
smallest known so far whenever is not a prime power or a prime power plus
one itself.Comment: 8 pages, 2 figure
Emergent Nesting of the Fermi Surface from Local-Moment Description of Iron-Pnictide High-Tc Superconductors
We uncover the low-energy spectrum of a t-J model for electrons on a square
lattice of spin-1 iron atoms with 3dxz and 3dyz orbital character by applying
Schwinger-boson-slave-fermion mean-field theory and by exact diagonalization of
one hole roaming over a 4 x 4 x 2 lattice. Hopping matrix elements are set to
produce hole bands centered at zero two-dimensional (2D) momentum in the
free-electron limit. Holes can propagate coherently in the t-J model below a
threshold Hund coupling when long-range antiferromagnetic order across the d+ =
3d(x+iy)z and d- = 3d(x-iy)z orbitals is established by magnetic frustration
that is off-diagonal in the orbital indices. This leads to two hole-pocket
Fermi surfaces centered at zero 2D momentum. Proximity to a commensurate
spin-density wave (cSDW) that exists above the threshold Hund coupling results
in emergent Fermi surface pockets about cSDW momenta at a quantum critical
point (QCP). This motivates the introduction of a new Gutzwiller wavefunction
for a cSDW metal state. Study of the spin-fluctuation spectrum at cSDW momenta
indicates that the dispersion of the nested band of one-particle states that
emerges is electron-type. Increasing Hund coupling past the QCP can push the
hole-pocket Fermi surfaces centered at zero 2D momentum below the Fermi energy
level, in agreement with recent determinations of the electronic structure of
mono-layer iron-selenide superconductors.Comment: 41 pages, 12 figures, published versio
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