724 research outputs found
Heat Transport as a Probe of Electron Scattering by Spin Fluctuations: the Case of Antiferromagnetic CeRhIn5
Heat and charge conduction were measured in the heavy-fermion metal CeRhIn5,
an antiferromagnet with T_N=3.8 K. The thermal resistivity is found to be
proportional to the magnetic entropy, revealing that spin fluctuations are as
effective in scattering electrons as they are in disordering local moments. The
electrical resistivity, governed by a q^2 weighting of fluctuations, increases
monotonically with temperature. In contrast, the difference between thermal and
electrical resistivities, characterized by an omega^2 weighting, peaks sharply
at T_N and eventually goes to zero at a temperature T^* ~ 8 K. T^* thus emerges
as a measure of the characteristic energy of magnetic fluctuations.Comment: 4 pages, 4 figure
Field-Induced Quantum Critical Point in CeCoIn5
The resistivity of the heavy-fermion superconductor CeCoIn5 was measured as a
function of temperature, down to 25 mK and in magnetic fields of up to 16 T
applied perpendicular to the basal plane. With increasing field, we observe a
suppression of the non-Fermi liquid behavior, rho ~ T, and the development of a
Fermi liquid state, with its characteristic rho = rho_0 + AT^2 dependence. The
field dependence of the T^2 coefficient shows critical behavior with an
exponent of 1.37. This is evidence for a field-induced quantum critical point
(QCP), occuring at a critical field which coincides, within experimental
accuracy, with the superconducting critical field H_c2. We discuss the relation
of this field-tuned QCP to a change in the magnetic state, seen as a change in
magnetoresistance from positive to negative, at a crossover line that has a
common border with the superconducting region below ~ 1 K.Comment: 4 pages, 3 figures (published version
Nonvanishing Energy Scales at the Quantum Critical Point of CeCoIn5
Heat and charge transport were used to probe the magnetic field-tuned quantum
critical point in the heavy-fermion metal CeCoIn. A comparison of
electrical and thermal resistivities reveals three characteristic energy
scales. A Fermi-liquid regime is observed below , with both transport
coefficients diverging in parallel and as , the
critical field. The characteristic temperature of antiferromagnetic spin
fluctuations, , is tuned to a minimum but {\it finite} value at ,
which coincides with the end of the -linear regime in the electrical
resistivity. A third temperature scale, , signals the formation of
quasiparticles, as fermions of charge obeying the Wiedemann-Franz law.
Unlike , it remains finite at , so that the integrity of
quasiparticles is preserved, even though the standard signature of Fermi-liquid
theory fails.Comment: 4 pages, 4 figures (published version
Exploring the Dust Content of Galactic Winds with Herschel. I. NGC 4631
We present a detailed analysis of deep far-infrared observations of the
nearby edge-on star-forming galaxy NGC 4631 obtained with the Herschel Space
Observatory. Our PACS images at 70 and 160 um show a rich complex of filaments
and chimney-like features that extends up to a projected distance of 6 kpc
above the plane of the galaxy. The PACS features often match extraplanar
Halpha, radio-continuum, and soft X-ray features observed in this galaxy,
pointing to a tight disk-halo connection regulated by star formation. On the
other hand, the morphology of the colder dust component detected on larger
scale in the SPIRE 250, 350, and 500 um data matches the extraplanar H~I
streams previously reported in NGC 4631 and suggests a tidal origin. The PACS
70/160 ratios are elevated in the central ~3.0 kpc region above the nucleus of
this galaxy (the "superbubble"). A pixel-by-pixel analysis shows that dust in
this region has a higher temperature and/or an emissivity with a steeper
spectral index (beta > 2) than the dust in the disk, possibly the result of the
harsher environment in the superbubble. Star formation in the disk seems
energetically insufficient to lift the material out of the disk, unless it was
more active in the past or the dust-to-gas ratio in the superbubble region is
higher than the Galactic value. Some of the dust in the halo may also have been
tidally stripped from nearby companions or lifted from the disk by galaxy
interactions.Comment: Accepted for publication in The Astrophysical Journa
The Vela Cloud: A Giant HI Anomaly in the NGC 3256 Group
We present Australia Telescope Compact Array (ATCA) observations of a
galaxy-sized intergalactic HI cloud (the Vela Cloud) in the NGC 3256 galaxy
group. The group contains the prominent merging galaxy NGC 3256, which is
surrounded by a number of HI fragments, the tidally disturbed galaxy NGC 3263,
and several other peculiar galaxies. The Vela Cloud, with an HI mass of 3-5 *
10**9 solar masses, resides southeast of NGC 3256 and west of NGC 3263, within
an area of 9' x 16' (100 kpc x 175 kpc for an adopted distance of 38 Mpc). In
our ATCA data the Vela Cloud appears as 3 diffuse components and contains 4
density enhancements. The Vela Cloud's properties, together with its group
environment, suggest that it has a tidal origin. Each density enhancement
contains ~10**8 solar masses of HI gas which is sufficient material for the
formation of globular cluster progenitors. However, if we represent the
enhancements as Bonnor-Ebert spheres, then the pressure of the surrounding HI
would need to increase by at least a factor of 6 in order to cause the collapse
of an enhancement. Thus we do not expect them to form massive bound stellar
systems like super star clusters or tidal dwarf galaxies. Since the HI density
enhancements have some properties in common with High Velocity Clouds, we
explore whether they may evolve to be identified with these starless clouds
instead.Comment: 47 pages, 13 figures (incl. a & b), accepted by AJ, changes are minor
additions, rearranging, and clarifications esp. in sections 6 &
Unpaired Electrons in the Heavy-Fermion Superconductor CeCoIn_{5}
Thermal conductivity and specific heat were measured in the superconducting
state of the heavy fermion material Ce_{1-x}La_{x}CoIn_{5}. With increasing
impurity concentration x, the suppression of T_{c} is accompanied by the
increase in the residual electronic specific heat expected of a d-wave
superconductor, but it occurs in parallel with a decrease in residual
electronic thermal conductivity. This contrasting behavior reveals the presence
of uncondensed electrons coexisting with nodal quasiparticles. An extreme
multiband scenario is proposed, with a d-wave superconducting gap on the
heavy-electron sheets of the Fermi surface and a negligible gap on the light,
three-dimensional pockets.Comment: 4 pages, 3 figure
Discovery of a 500 pc shell in the nucleus of Centaurus A
Spitzer Space Telescope mid-infrared images of the radio galaxy Centaurus A
reveal a shell-like, bipolar, structure 500 pc to the north and south of the
nucleus. This shell is seen in 5.8, 8.0 and 24 micron broad-band images. Such a
remarkable shell has not been previously detected in a radio galaxy and is the
first extragalactic nuclear shell detected at mid-infrared wavelengths. We
estimate that the shell is a few million years old and has a mass of order
million solar masses. A conservative estimate for the mechanical energy in the
wind driven bubble is 10^53 erg. The shell could have created by a small few
thousand solar mass nuclear burst of star formation. Alternatively, the
bolometric luminosity of the active nucleus is sufficiently large that it could
power the shell. Constraints on the shell's velocity are lacking. However, if
the shell is moving at 1000 km/s then the required mechanical energy would be
100 times larger.Comment: submitted to ApJ Letter
Pre-galactic metal enrichment - The chemical signatures of the first stars
The emergence of the first sources of light at redshifts of z ~ 10-30
signaled the transition from the simple initial state of the Universe to one of
increasing complexity. We review recent progress in our understanding of the
formation of the first stars and galaxies, starting with cosmological initial
conditions, primordial gas cooling, and subsequent collapse and fragmentation.
We emphasize the important open question of how the pristine gas was enriched
with heavy chemical elements in the wake of the first supernovae. We conclude
by discussing how the chemical abundance patterns conceivably allow us to probe
the properties of the first stars and subsequent stellar generations, and allow
us to test models of early metal enrichment.Comment: 52 pages, 20 figures, clarifications, references added, accepted for
publication in the Reviews of Modern Physic
Chromospherically Active Stars in the RAVE Survey. I. The Catalogue
RAVE, the unbiased magnitude limited survey of the southern sky stars,
contained 456,676 medium-resolution spectra at the time of our analysis.
Spectra cover the CaII IRT range which is a known indicator of chromospheric
activity. Our previous work (Matijevi\v{c} et al. 2012) classified all spectra
using locally linear embedding. It identified 53,347 cases with a suggested
emission component in calcium lines. Here we use a spectral subtraction
technique to measure the properties of this emission. Synthetic templates are
replaced by the observed spectra of non-active stars to bypass the difficult
computations of non-LTE profiles of the line cores and stellar parameter
dependence. We derive both the equivalent width of the excess emission for each
calcium line on a 5\AA\ wide interval and their sum EW_IRT for ~44,000
candidate active dwarf stars with S/N>20 and with no respect to the source of
their emission flux. From these ~14,000 show a detectable chromospheric flux
with at least 2\sigma\ confidence level. Our set of active stars vastly
enlarges previously known samples. Atmospheric parameters and in some cases
radial velocities of active stars derived from automatic pipeline suffer from
systematic shifts due to their shallower calcium lines. We re-estimate the
effective temperature, metallicity and radial velocities for candidate active
stars. The overall distribution of activity levels shows a bimodal shape, with
the first peak coinciding with non-active stars and the second with the pre
main-sequence cases. The catalogue will be publicly available with the next
RAVE public data releases.Comment: 13 pages, 9 figure
- …