5,709 research outputs found
The outer filament of Centaurus A as seen by MUSE
We investigate signatures of a jet-interstellar medium (ISM) interaction
using optical integral-field observations of the so-called outer filament near
Centaurus A, expanding on previous results obtained on a more limited area.
Using the Multi Unit Spectroscopic Explorer (MUSE) on the VLT during science
verification, we observed a significant fraction of the brighter emitting gas
across the outer filament. The ionized gas shows complex morphology with
compact blobs, arc-like structures and diffuse emission. Based on the
kinematics, we identified three main components. The more collimated component
is oriented along the direction of the radio jet. The other two components
exhibit diffuse morphology together with arc-like structures also oriented
along the radio jet direction. Furthermore, the ionization level of the gas is
found to decrease from the more collimated component to the more diffuse
components. The morphology and velocities of the more collimated component
confirm our earlier results that the outer filament and the nearby HI cloud are
likely partially shaped by the lateral expansion of the jet. The arc-like
structures embedded within the two remaining components are the clearest
evidence of a smooth jet-ISM interaction along the jet direction. This suggests
that, although poorly collimated, the radio jet is still active and has an
impact on the surrounding gas. This result indicates that the effect on the ISM
of even low-power radio jets should be considered when studying the influence
Active Galactic Nuclei can have on their host galaxy.Comment: 5 pages, 3 figures, Accepted for publication by A&
Breakdown of Fermi liquid behavior at the (\pi,\pi)=2k_F spin-density wave quantum-critical point: the case of electron-doped cuprates
Many correlated materials display a quantum critical point between a
paramagnetic and a SDW state. The SDW wave vector connects points (hot spots)
on opposite sides of the Fermi surface. The Fermi velocities at these pairs of
points are in general not parallel. Here we consider the case where pairs of
hot spots coalesce, and the wave vector (\pi,\pi) of the SDW connects hot spots
with parallel Fermi velocities. Using the specific example of electron-doped
cuprates, we first show that Kanamori screening and generic features of the
Lindhard function make this case experimentally relevant. The temperature
dependence of the correlation length, the spin susceptibility and the
self-energy at the hot spots are found using the Two-Particle-Self-Consistent
theory and specific numerical examples worked out for parameters characteristic
of the electron-doped cuprates. While the curvature of the Fermi surface at the
hot spots leads to deviations from perfect nesting, the pseudo-nesting
conditions lead to drastic modifications to the temperature dependence of these
physical observables: Neglecting logarithmic corrections, the correlation
length \xi scales like 1/T, i.e. z=1 instead of the naive z=2, the (\pi,\pi)
static spin susceptibility \chi like , and the imaginary part of the
self-energy at the hot spots like . The correction to the Korringa NMR relaxation rate is subdominant. We also consider
this problem at zero temperature, or for frequencies larger than temperature,
using a field-theoretical model of gapless SDW fluctuations interacting with
fermions. The imaginary part of the fermionic self-energy close to the hot
spots scales as . This is less singular than earlier
predictions of the form . The difference arises from the
effects of umklapp terms that were not included in previous studies.Comment: 23 pages, 12 figures; (v2) minor changes; (v3) Final published
versio
Breakup of the Fermi surface near the Mott transition in low-dimensional systems
We investigate the Mott transition in weakly-coupled one-dimensional (1d)
fermionic chains. Using a generalization of Dynamic Mean Field Theory, we show
that the Mott gap is suppressed at some critical hopping . The
transition from the 1d insulator to a 2d metal proceeds through an intermediate
phase where the Fermi surface is broken into electron and hole pockets. The
quasiparticle spectral weight is strongly anisotropic along the Fermi surface,
both in the intermediate and metallic phases. We argue that such pockets would
look like `arcs' in photoemission experiments.Comment: REVTeX 4, 5 pages, 4 EPS figures. References added; problem with
figure 4 fixed; typos correcte
Spin Susceptibility Representation of the Pairing Interaction for the two-dimensional Hubbard Model
Using numerical dynamic cluster quantum Monte Carlo results, we study a
simple approximation for the pairing interaction of a two-dimensional Hubbard
model with an on-site Coulomb interaction equal to the bandwidth. We find
that with an effective temperature dependent coupling \Ub(T) and the
numerically calculated spin susceptibility , the d-wave pairing
interaction is well approximated by \frac{3}{2} \Ub^2\chi(K-K').Comment: 5 pages, 7 figure
Spiral Magnets as Gapless Mott Insulators
In the large limit, the ground state of the half-filled, nearest-neighbor
Hubbard model on the triangular lattice is the three-sublattice
antiferromagnet. In sharp contrast with the square-lattice case, where
transverse spin-waves and charge excitations remain decoupled to all orders in
, it is shown that beyond leading order in the three Goldstone modes
on the triangular lattice are a linear combination of spin and charge. This
leads to non-vanishing conductivity at any finite frequency, even though the
magnet remains insulating at zero frequency. More generally, non-collinear spin
order should lead to such gapless insulating behavior.Comment: 10 pages, REVTEX 3.0, 3 uuencoded postscript figures, CRPS-94-0
The Field White Dwarf Mass Distribution
We revisit the properties and astrophysical implications of the field white
dwarf mass distribution in preparation of Gaia applications. Our study is based
on the two samples with the best established completeness and most precise
atmospheric parameters, the volume-complete survey within 20 pc and the Sloan
Digital Sky Survey (SDSS) magnitude-limited sample. We explore the modelling of
the observed mass distributions with Monte Carlo simulations, but find that it
is difficult to constrain independently the initial mass function (IMF), the
initial-to-final-mass relation (IFMR), the stellar formation history (SFH), the
variation of the Galactic disk vertical scale height as a function of stellar
age, and binary evolution. Each of these input ingredients has a moderate
effect on the predicted mass distributions, and we must also take into account
biases owing to unidentified faint objects (20 pc sample), as well as unknown
masses for magnetic white dwarfs and spectroscopic calibration issues (SDSS
sample). Nevertheless, we find that fixed standard assumptions for the above
parameters result in predicted mean masses that are in good qualitative
agreement with the observed values. It suggests that derived masses for both
studied samples are consistent with our current knowledge of stellar and
Galactic evolution. Our simulations overpredict by 40-50% the number of massive
white dwarfs (M > 0.75 Msun) for both surveys, although we can not exclude a
Salpeter IMF when we account for all biases. Furthermore, we find no evidence
of a population of double white dwarf mergers in the observed mass
distributions.Comment: 15 pages, 16 figures, accepted for publication in MNRA
Can magnetic fields suppress convection in the atmosphere of cool white dwarfs? A case study on WD2105-820
Around 10% of white dwarfs exhibit global magnetic structures with fields
ranging from 1 kG to hundreds of MG. Recently, the first radiation
magnetohydrodynamics simulations of the atmosphere of white dwarfs showed that
convection should be suppressed in their photospheres for magnetic fields with
strengths B 50 kG. These predictions are in agreement with our
knowledge of stellar physics (e.g. energy transfer in strong magnetic field
regions of the solar photosphere), but have yet to be directly confirmed from
white dwarf observations. We obtained COS far-UV spectroscopy of the weakly
magnetic, hydrogen-atmosphere, white dwarf WD2105-820 and of three additional
non-magnetic, convective remnants (all in the range
9000-11,000 K). We fitted both the COS and the already available optical
spectra with convective and radiative atmospheric models. As expected, we find
that for two of the non-magnetic comparison stars only convective model fits
predict consistent values from both the optical and the FUV
spectra. In contrast, for WD2105-820 only the best fitting radiative model
produced consistent results.Comment: 8 pages, 7 figures, 1 table, accepted for publication in MNRA
Characteristics of oxygen isotope substitutions in the quasiparticle spectrum of BiSrCaCuO
There is an ongoing debate about the nature of the bosonic excitations
responsible for the quasiparticle self energy in high temperature
superconductors -- are they phonons or spin fluctuations? We present a careful
analysis of the bosonic excitations as revealed by the `kink' feature at 70 meV
in angle resolved photoemission data using Eliashberg theory for a d-wave
superconductor. Starting from the assumption that nodal quasiparticles are not
coupled to the magnetic resonance, the sharp structure at meV
can be assigned to phonons. We find that not only can we account for the shifts
of the kink energy seen on oxygen isotope substitution but also get a
quantitative estimate of the fraction of the area under the electron-boson
spectral density which is due to phonons. We conclude that for optimally doped
BiSrCaCuO phonons contribute % and
non-phononic excitations %.Comment: 6 pages, 3 figure
Reply to `A comment on `The Cauchy problem of f(R) gravity''
We reply to a comment by Capozziello and Vignolo about the Cauchy problem of
Palatini f(R) gravity.Comment: 3 pages, late
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