261 research outputs found
Magnetotransport in Sr3PbO antiperovskite with three-dimensional massive Dirac electrons
Novel topological phenomena are anticipated for three-dimensional (3D) Dirac
electrons. The magnetotransport properties of cubic
antiperovskite, theoretically proposed to be a 3D massive Dirac electron
system, are studied. The measurements of Shubnikov-de Haas oscillations and
Hall resistivity indicate the presence of a low density ( ) of holes with an extremely small cyclotron mass of
0.01-0.06. The magnetoresistance is linear in
magnetic field with the magnitude independent of temperature. These results
are fully consistent with the presence of 3D massive Dirac electrons in . The chemical flexibility of the antiperovskites and our findings
in the family member, , point to their potential as a model
system in which to explore exotic topological phases
Self-organized emergence of folded protein-like network structures from geometric constraints
The intricate three-dimensional geometries of protein tertiary structures
underlie protein function and emerge through a folding process from
one-dimensional chains of amino acids. The exact spatial sequence and
configuration of amino acids, the biochemical environment and the temporal
sequence of distinct interactions yield a complex folding process that cannot
yet be easily tracked for all proteins. To gain qualitative insights into the
fundamental mechanisms behind the folding dynamics and generic features of the
folded structure, we propose a simple model of structure formation that takes
into account only fundamental geometric constraints and otherwise assumes
randomly paired connections. We find that despite its simplicity, the model
results in a network ensemble consistent with key overall features of the
ensemble of Protein Residue Networks we obtained from more than 1000 biological
protein geometries as available through the Protein Data Base. Specifically,
the distribution of the number of interaction neighbors a unit (amino acid)
has, the scaling of the structure's spatial extent with chain length, the
eigenvalue spectrum and the scaling of the smallest relaxation time with chain
length are all consistent between model and real proteins. These results
indicate that geometric constraints alone may already account for a number of
generic features of protein tertiary structures
Exotic magnetism in the alkali sesquoxides Rb4O6 and Cs4O6
Among the various alkali oxides the sesquioxides Rb4O6 and Cs4O6 are of
special interest. Electronic structure calculations using the local
spin-density approximation predicted that Rb4O6 should be a half-metallic
ferromagnet, which was later contradicted when an experimental investigation of
the temperature dependent magnetization of Rb4O6 showed a low-temperature
magnetic transition and differences between zero-field-cooled (ZFC) and
field-cooled (FC) measurements. Such behavior is known from spin glasses and
frustrated systems. Rb4O6 and Cs4O6 comprise two different types of dioxygen
anions, the hyperoxide and the peroxide anions. The nonmagnetic peroxide anions
do not contain unpaired electrons while the hyperoxide anions contain unpaired
electrons in antibonding pi*-orbitals. High electron localization (narrow
bands) suggests that electronic correlations are of major importance in these
open shell p-electron systems. Correlations and charge ordering due to the
mixed valency render p-electron-based anionogenic magnetic order possible in
the sesquioxides. In this work we present an experimental comparison of Rb4O6
and the related Cs4O6. The crystal structures are verified using powder x-ray
diffraction. The mixed valency of both compounds is confirmed using Raman
spectroscopy, and time-dependent magnetization experiments indicate that both
compounds show magnetic frustration, a feature only previously known from d-
and f-electron systems
Tracing high density gas in M 82 and NGC 4038
We present the first detection of CS in the Antennae galaxies towards the NGC
4038 nucleus, as well as the first detections of two high-J (5-4 and 7-6) CS
lines in the center of M 82. The CS(7-6) line in M 82 shows a profile that is
surprisingly different to those of other low-J CS transitions we observed. This
implies the presence of a separate, denser and warmer molecular gas component.
The derived physical properties and the likely location of the CS(7-6) emission
suggests an association with the supershell in the centre of M 82.Comment: 10 pages, 3 figures, ApJ Letter - ACCEPTE
Probing highly obscured, self-absorbed galaxy nuclei with vibrationally excited HCN
We present high resolution (0."4) IRAM PdBI and ALMA mm and submm
observations of the (ultra) luminous infrared galaxies ((U)LIRGs)
IRAS17208-0014, Arp220, IC860 and Zw049.057 that reveal intense line emission
from vibrationally excited (=1) J=3-2 and 4-3 HCN. The emission is
emerging from buried, compact (r<17-70 pc) nuclei that have very high implied
mid-infrared surface brightness L kpc.
These nuclei are likely powered by accreting supermassive black holes (SMBHs)
and/or hot (>200 K) extreme starbursts. Vibrational, =1, lines of HCN
are excited by intense 14 micron mid-infrared emission and are excellent probes
of the dynamics, masses, and physical conditions of (U)LIRG nuclei when H
column densities exceed cm. It is clear that these lines open
up a new interesting avenue to gain access to the most obscured AGNs and
starbursts. Vibrationally excited HCN acts as a proxy for the absorbed
mid-infrared emission from the embedded nuclei, which allows for reconstruction
of the intrinsic, hotter dust SED. In contrast, we show strong evidence that
the ground vibrational state (=0), J=3-2 and 4-3 rotational lines of HCN
and HCO fail to probe the highly enshrouded, compact nuclear regions owing
to strong self- and continuum absorption. The HCN and HCO line profiles are
double-peaked because of the absorption and show evidence of non-circular
motions - possibly in the form of in- or outflows. Detections of vibrationally
excited HCN in external galaxies are so far limited to ULIRGs and early-type
spiral LIRGs, and we discuss possible causes for this. We tentatively suggest
that the peak of vibrationally excited HCN emission is connected to a rapid
stage of nuclear growth, before the phase of strong feedback.Comment: 13 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
The effect of clouds in a galactic wind on the evolution of gas-rich dwarf galaxies
(Abridged)
We study the effects of interstellar clouds on the dynamical and chemical
evolution of gas-rich dwarf galaxies. In particular, we focus on two model
galaxies similar to IZw18 and NGC1569 in comparison to models in which a smooth
initial distribution of gas is assumed. We use a 2-D hydrodynamical code
coupled with a series of routines able to trace the chemical products of SNeII,
SNeIa and intermediate-mass stars. Clouds are simulated by adding overdense
regions in the computational grid, whose locations are chosen randomly and
whose density profiles match observed ones. The clouds are inherently
dynamically coupled to the diffuse gas, and they experience heat conduction
from a hot surrounding gas. Due to dynamical processes and thermal evaporation,
the clouds survive only a few tens of Myr. Due to the additional cooling agent,
the internal energy of cloudy models is typically reduced by 20 - 40% compared
with models of diffuse gas alone. The clouds delay the development of
large-scale outflows by mass loading, therefore helping to retain a larger
amount of gas inside the galaxy. However, their bullet effect can pierce the
expanding supershell and create holes through which the superbubble can vent
freshly produced metals. Moreover, assuming a pristine chemical composition for
the clouds, their interaction with the superbubble dilutes the gas, reducing
the metallicity. The resulting final metallicity is therefore generally lower
(by ~ 0.2 - 0.4 dex) than the one attained by diffuse models.Comment: 13 pages, 11 figures, A&A accepte
Ammonia (J,K) = (1,1) to (4,4) and (6,6) inversion lines detected in the Seyfert 2 galaxy NGC 1068
We present the detection of the ammonia (NH3) (J,K) = (1,1) to (4,4) and
(6,6) inversion lines toward the prototypical Seyfert 2 galaxy NGC 1068, made
with the Green Bank Telescope (GBT). This is the first detection of ammonia in
a Seyfert galaxy. The ortho-to-para-NH3 abundance ratio suggests that the
molecule was formed in a warm medium of at least 20 K. For the NH3 column
density and fractional abundance, we find (1.09\pm0.23)\times10^14 cm^-2 and
(2.9\pm0.6)\times10^-8, respectively, from the inner 1.2 kpc of NGC 1068. The
kinetic temperature can be constrained to 80\pm20 K for the bulk of the
molecular gas, while some fraction has an even higher temperature of 140\pm30
K.Comment: 5 pages, 3 figures, accepted by A&
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