58 research outputs found
Superconductivity of Bi-III phase of elemental Bismuth: insights from Muon-Spin Rotation and Density Functional Theory
Using muon-spin rotation the pressure-induced superconductivity in the Bi-III
phase of elemental Bismuth (transition temperature K) was
investigated. The Ginzburg-Landau parameter
( is the magnetic penetration depth, is the coherence length)
was estimated which is the highest among single element superconductors. The
temperature dependence of the superconducting energy gap []
reconstructed from deviates from the weak-coupled BCS
prediction. The coupling strength was
estimated thus implying that Bi-III stays within the strong coupling regime.
The Density Functional Theory calculations suggest that superconductivity in
Bi-III could be described within the Eliashberg approach with the
characteristic phonon frequency meV. An alternative
pairing mechanism to the electron-phonon coupling involves the possibility of
Cooper pairing induced by the Fermi surface nesting.Comment: 5 pages, 4 figure
On the superconducting nature of the Bi-II phase of elemental Bismuth
The superconductivity in the Bi-II phase of elemental Bismuth (transition
temperature K at pressure GPa) was studied
experimentally by means of the muon-spin rotation as well as theoretically by
using the Eliashberg theory in combination with Density Functional Theory
calculations. Experiments reveal that Bi-II is a type-I superconductor with a
zero temperature value of the thermodynamic critical field ~mT. The Eliashberg theory approach provides a good agreement
with the experimental and the temperature evolution of .
The estimated value for the retardation (coupling) parameter ( is the logarithmically
averaged phonon frequency) suggests that Bi-II is an intermediately-coupled
superconductor.Comment: 6 pages, 2 figure
Dynamical mean-field theory of the Anderson-Hubbard model with local and nonlocal disorder in tensor formulation
To explore correlated electrons in the presence of local and non-local
disorder, the Blackman-Esterling-Berk method for averaging over off-diagonal
disorder is implemented into dynamical mean-field theory using tensor notation.
The impurity model combining disorder and correlations is solved using the
recently developed fork tensor-product state solver, which allows one to
calculate the single particle spectral functions on the real-frequency axis. In
the absence of off-diagonal hopping, we establish exact bounds of the spectral
function of the non-interacting Bethe lattice with coordination number . In
the presence of interaction, the Mott insulating paramagnetic phase of the
one-band Hubbard model is computed at zero temperature in alloys with site- and
off-diagonal disorder. When the Hubbard parameter is increased, transitions
from an alloy band-insulator through a correlated metal into a Mott insulating
phase are found to take place.Comment: 15 pages, 5 figures. See ancillary folder for numerical data and
computational detail
The Lyman alpha Reference Sample: Extended Lyman alpha Halos Produced at Low Dust Content
We report on new imaging observations of the Lyman alpha emission line (Lya),
performed with the Hubble Space Telescope, that comprise the backbone of the
Lyman alpha Reference Sample (LARS). We present images of 14 starburst galaxies
at redshifts 0.028 < z < 0.18 in continuum-subtracted Lya, Halpha, and the far
ultraviolet continuum. We show that Lya is emitted on scales that
systematically exceed those of the massive stellar population and recombination
nebulae: as measured by the Petrosian 20 percent radius, RP20, Lya radii are
larger than those of Halpha by factors ranging from 1 to 3.6, with an average
of 2.4. The average ratio of Lya-to-FUV radii is 2.9. This suggests that much
of the Lya light is pushed to large radii by resonance scattering. Defining the
"Relative Petrosian Extension" of Lya compared to Halpha, \xi_ext = RP20_Lya /
RP20_Ha, we find \xi_ext to be uncorrelated with total Lya luminosity. However
\xi_ext is strongly correlated with quantities that scale with dust content, in
the sense that a low dust abundance is a necessary requirement (although not
the only one) in order to spread Lya photons throughout the interstellar medium
and drive a large extended Lya halo.Comment: Published in ApJ Letters ~~ 6 pages using emulateapj, 4 figures ~~
Higher-resolution, larger, nicer jpeg versions of Figures 1 and 2 can be
found here: http://xayes.org/pub/press_lars.htm
The Lyman Alpha Reference Sample IX: Revelations from deep surface photometry
The Lyman Alpha Reference Sample (LARS) of 14 star-forming galaxies offers a
wealth of insight into the workings of these local analogs to high-redshift
star-forming galaxies. The sample has been well-studied in terms of LyA and
other emission line properties, such as HI mass, gas kinematics, and
morphology. We analyze deep surface photometry of the LARS sample in UBIK
broadband imaging obtained at the Nordic Optical Telescope and the
Canada-France-Hawaii Telescope, and juxtaposition their derived properties with
a sample of local high-redshift galaxy analogs, namely, with blue compact
galaxies (BCGs). We construct radial surface brightness and color profiles with
both elliptical and isophotal integration, as well as RGB images, deep
contours, color maps, a burst fraction estimate, and a radial mass-to-light
ratio profile for each LARS galaxy. Standard morphological parameters like
asymmetry, clumpiness, the Gini and M20 coefficients are [...] analyzed, as
well as isophotal asymmetry profiles for each galaxy. [...] We compare the LARS
to the properties of the BCG sample and highlight the differences. Several
diagnostics indicate that the LARS galaxies have highly disturbed morphologies
even at the level of the faintest isophotes [...]. The ground-based photometry
[...] reveals previously unexplored isophotes [...]. The burst fraction
estimate suggests a spatially more extended burst region in LARS than in the
BCGs. [...] The galaxies in the LARS sample appear to be in earlier stages of a
merger event compared to the BCGs. Standard morphological diagnostics like
asymmetry, clumpiness, Gini and M20 coefficients cannot separate the two
samples, although an isophotal asymmetry profile successfully captures the
average difference in morphology. These morphological diagnostics do not show
any correlation with the equivalent width or the escape fraction of Lyman
Alpha. [abridged]Comment: 27 pages, 15 figures, accepted for publication in A&
The Lyman Alpha Reference Sample: V. The impact of neutral ISM kinematics and geometry on Lyman Alpha escape
We present high-resolution far-UV spectroscopy of the 14 galaxies of the
Lyman Alpha Reference Sample; a sample of strongly star-forming galaxies at low
redshifts (). We compare the derived properties to global
properties derived from multi band imaging and 21 cm HI interferometry and
single dish observations, as well as archival optical SDSS spectra. Besides the
Lyman line, the spectra contain a number of metal absorption features
allowing us to probe the kinematics of the neutral ISM and evaluate the optical
depth and and covering fraction of the neutral medium as a function of
line-of-sight velocity. Furthermore, we show how this, in combination with
precise determination of systemic velocity and good Ly spectra, can be
used to distinguish a model in which separate clumps together fully cover the
background source, from the "picket fence" model named by Heckman et al.
(2011). We find that no one single effect dominates in governing Ly
radiative transfer and escape. Ly escape in our sample coincides with a
maximum velocity-binned covering fraction of and bulk outflow
velocities of km s, although a number of galaxies show
these characteristics and yet little or no Ly escape. We find that
Ly peak velocities, where available, are not consistent with a strong
backscattered component, but rather with a simpler model of an intrinsic
emission line overlaid by a blueshifted absorption profile from the outflowing
wind. Finally, we find a strong anticorrelation between H equivalent
width and maximum velocity-binned covering factor, and propose a heuristic
explanatory model.Comment: 28 pages, 19 figures, 5 table
The Lyman Alpha Reference Sample: III. Properties of the Neutral ISM from GBT and VLA Observations
We present new H I imaging and spectroscopy of the 14 UV-selected
star-forming galaxies in the Lyman Alpha Reference Sample (LARS), aimed for a
detailed study of the processes governing the production, propagation, and
escape of Ly photons. New H I spectroscopy, obtained with the 100m
Green Bank Telescope (GBT), robustly detects the H I spectral line in 11 of the
14 observed LARS galaxies (although the profiles of two of the galaxies are
likely confused by other sources within the GBT beam); the three highest
redshift galaxies are not detected at our current sensitivity limits. The GBT
profiles are used to derive fundamental H I line properties of the LARS
galaxies. We also present new pilot H I spectral line imaging of 5 of the LARS
galaxies obtained with the Karl G. Jansky Very Large Array (VLA). This imaging
localizes the H I gas and provides a measurement of the total H I mass in each
galaxy. In one system, LARS 03 (UGC 8335 or Arp 238), VLA observations reveal
an enormous tidal structure that extends over 160 kpc from the main interacting
systems and that contains 10 M of H I. We compare various H I
properties with global Ly quantities derived from HST measurements. The
measurements of the Ly escape fraction are coupled with the new direct
measurements of H I mass and significantly disturbed H I velocities. Our
robustly detected sample reveals that both total H I mass and linewidth are
tentatively correlated with key Ly tracers. Further, on global scales,
these data support a complex coupling between Ly propagation and the H
I properties of the surrounding medium.Comment: Preprint form, 16 figures, accepted in Ap
Large ordered moment with strong easy-plane anisotropy and vortex-domain pattern in the kagome ferromagnet FeSn
We report the structural and magnetic properties of high-quality bulk single
crystals of the kagome ferromagnet FeSn. The dependence of magnetisation on
the magnitude and orientation of the external field reveals strong easy-plane
type uniaxial magnetic anisotropy, which shows a monotonous increase from
at 300\,K to at 2\,K. Our
\textit{ab initio} electronic structure calculations yield the value of total
magnetic moment of about 6.9 /f.u. and a magnetocrystalline anisotropy
energy density of 0.406\,meV/f.u. () both being in good
agreement with the experimental values. The self-consistent DFT computations
for the components of the spin/orbital moments indicate that the small
difference between the saturation magnetisations measured along and
perpendicular to the kagome layers results from the subtle balance between the
Fe and Sn spin/orbital moments on the different sites. In zero field, magnetic
force microscopy reveals micrometer-scale magnetic vortices with weakly pinned
cores that vanish at 3\,T applied perpendicular to the kagome plane. Our
micromagnetic simulations, using the experimentally determined value of
anisotropy, well reproduce the observed vortex-domain structure. The present
study, in comparison with the easy-axis ferromagnet FeSn, shows that
varying the stacking of kagome layers provides an efficient control over
magnetic anisotropy in this family of Fe-based kagome magnets.Comment: 10 pages, 5 figure
The Lyman alpha reference sample. II. Hubble space telescope imaging results, integrated properties, and trends
We report new results regarding the Lyα output of galaxies, derived from the Lyman Alpha Reference Sample, and focused on Hubble Space Telescope imaging. For 14 galaxies we present intensity images in Lyα, Hα, and UV, and maps of Hα/HÎČ, Lyα equivalent width (EW), and Lyα/Hα. We present Lyα and UV radial light profiles and show they are well-fitted by SĂ©rsic profiles, but Lyα profiles show indices systematically lower than those of the UV (n â 1-2 instead of gsim 4). This reveals a general lack of the central concentration in Lyα that is ubiquitous in the UV. Photometric growth curves increase more slowly for Lyα than the far ultraviolet, showing that small apertures may underestimate the EW. For most galaxies, however, flux and EW curves flatten by radii â10 kpc, suggesting that if placed at high-z only a few of our galaxies would suffer from large flux losses. We compute global properties of the sample in large apertures, and show total Lyα luminosities to be independent of all other quantities. Normalized Lyα throughput, however, shows significant correlations: escape is found to be higher in galaxies of lower star formation rate, dust content, mass, and nebular quantities that suggest harder ionizing continuum and lower metallicity. Six galaxies would be selected as high-z Lyα emitters, based upon their luminosity and EW. We discuss the results in the context of high-z Lyα and UV samples. A few galaxies have EWs above 50 Ă
, and one shows of 80%; such objects have not previously been reported at low-z
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