2,410 research outputs found
Low-temperature lattice effects in the spin-liquid candidate -(BEDT-TTF)Cu(CN)
The quasi-two-dimensional organic charge-transfer salt
-(BEDT-TTF)Cu(CN) is one of the prime candidates for a
quantum spin-liquid due the strong spin frustration of its anisotropic
triangular lattice in combination with its proximity to the Mott transition.
Despite intensive investigations of the material's low-temperature properties,
several important questions remain to be answered. Particularly puzzling are
the 6\,K anomaly and the enigmatic effects observed in magnetic fields. Here we
report on low-temperature measurements of lattice effects which were shown to
be particularly strongly pronounced in this material (R. S. Manna \emph{et
al.}, Phys. Rev. Lett. \textbf{104}, 016403 (2010)). A special focus of our
study lies on sample-to-sample variations of these effects and their
implications on the interpretation of experimental data. By investigating
overall nine single crystals from two different batches, we can state that
there are considerable differences in the size of the second-order phase
transition anomaly around 6\,K, varying within a factor of 3. In addition, we
find field-induced anomalies giving rise to pronounced features in the sample
length for two out of these nine crystals for temperatures 9 K. We
tentatively assign the latter effects to -induced magnetic clusters
suspected to nucleate around crystal imperfections. These -induced effects
are absent for the crystals where the 6\,K anomaly is most strongly pronounced.
The large lattice effects observed at 6\,K are consistent with proposed pairing
instabilities of fermionic excitations breaking the lattice symmetry. The
strong sample-to-sample variation in the size of the phase transition anomaly
suggests that the conversion of the fermions to bosons at the instability is
only partial and to some extent influenced by not yet identified
sample-specific parameters
Far-Ultraviolet H_2 Emission from Circumstellar Disks
We analyze the far-ultraviolet (FUV) spectra of 33 classical T Tauri stars (CTTS), including 20 new spectra obtained with the Advanced Camera for Surveys Solar Blind Channel (ACS/SBC) on the Hubble Space Telescope. Of the sources, 28 are in the ~1 Myr old Taurus-Auriga complex or Orion Molecular Cloud, 4 in the 8-10 Myr old Orion OB1a complex, and 1, TW Hya, in the 10 Myr old TW Hydrae Association. We also obtained FUV ACS/SBC spectra of 10 non-accreting sources surrounded by debris disks with ages between 10 and 125 Myr. We use a feature in the FUV spectra due mostly to electron impact excitation of H_2 to study the evolution of the gas in the inner disk. We find that the H_2 feature is absent in non-accreting sources, but is detected in the spectra of CTTS and correlates with accretion luminosity. Since all young stars have active chromospheres which produce strong X-ray and UV emission capable of exciting H_2 in the disk, the fact that the non-accreting sources show no H_2 emission implies that the H_2 gas in the inner disk has dissipated in the non-accreting sources, although dust (and possibly gas) remains at larger radii. Using the flux at 1600 Å, we estimate that the column density of H_2 left in the inner regions of the debris disks in our sample is less than ~3 × 10^(–6) g cm^(-2), 9 orders of magnitude below the surface density of the minimum mass solar nebula at 1 AU
GRB Repetition Limits from Current BATSE Observations
Revised upper limits on gamma-ray burst repetition rates are found using the BATSE 3B and 4B catalogs. A statistical repetition model is assumed in which sources burst at a mean rate but in which BATSE observes bursts randomly from each source
Testing the Dipole and Quadrupole Moments of Galactic Models
If gamma-ray bursts originate in the Galaxy, at some level there should be a
galactic pattern in their distribution on the sky. We test published galactic
models by comparing their dipole and quadrupole moments with the moments of the
BATSE 3B catalog. While many models have moments that are too large, several
models are in acceptable or good agreement with the data.Comment: 5 pages, LaTex using Revtex macro aipbook.sty and psfig. To appear in
the Proceedings of the 3rd Huntsville Symposium on Gamma-Ray Bursts, AIP,
eds. C. Kouveliotou, M. S. Briggs, G. J. Fishma
The Initial Mass Function and Disk Frequency of the Rho Ophiuchi Cloud: An Extinction-Limited Sample
We have completed an optical spectroscopic survey of an unbiased,
extinction-limited sample of candidate young stars covering 1.3 square degrees
of the Rho Ophiuchi star forming region. While infrared, X-ray, and optical
surveys of the cloud have identified many young stellar objects (YSOs), these
surveys are biased towards particular stages of stellar evolution and are not
optimal for studies of the disk frequency and initial mass function.We have
obtained over 300 optical spectra to help identify 135 association members
based on the presence of H-alpha in emission, lithium absorption, X-ray
emission, a mid-infrared excess, a common proper motion, reflection nebulosity,
and/or extinction considerations. Spectral types along with R and I band
photometry were used to derive effective temperatures and bolometric
luminosities for association members to compare with theoretical tracks and
isochrones for pre-main-sequence stars. An average age of 3.1 Myr is derived
for this population which is intermediate between that of objects embedded in
the cloud core of Rho Ophiuchi and low mass stars in the Upper Scorpius
subgroup. Consistent with this age we find a circumstellar disk frequency of
27% plus or minus 5%. We also constructed an initial mass function for an
extinction-limited sample of 123 YSOs (A_v less than or equal to 8 mag), which
is consistent with the field star initial mass function for YSOs with masses >
0.2 M_sun. There may be a deficit of brown dwarfs but this result relies on
completeness corrections and requires confirmation.Comment: 46 pages, 7 figures, 4 table
Effect of the strain Bacillus amyloliquefaciens FZB42 on the microbial community in the rhizosphere of lettuce under field conditions analyzed by whole metagenome sequencing
doi: 10.3389/fmicb.2014.00252 Effect of the strain Bacillus amyloliquefaciens FZB42 on the microbial community in the rhizosphere of lettuce under field conditions analyzed by whole metagenome sequencin
Multiferroicity in an organic charge-transfer salt: Electric-dipole-driven magnetism
Multiferroics, showing simultaneous ordering of electrical and magnetic
degrees of freedom, are remarkable materials as seen from both the academic and
technological points of view. A prominent mechanism of multiferroicity is the
spin-driven ferroelectricity, often found in frustrated antiferromagnets with
helical spin order. There, similar to conventional ferroelectrics, the
electrical dipoles arise from an off-centre displacement of ions. However,
recently a different mechanism, namely purely electronic ferroelectricity,
where charge order breaks inversion symmetry, has attracted considerable
interest. Here we provide evidence for this exotic type of ferroelectricity,
accompanied by antiferromagnetic spin order, in a two-dimensional organic
charge-transfer salt, thus representing a new class of multiferroics. Quite
unexpectedly for electronic ferroelectrics, dipolar and spin order arise nearly
simultaneously. This can be ascribed to the loss of spin frustration induced by
the ferroelectric ordering. Hence, here the spin order is driven by the
ferroelectricity, in marked contrast to the spin-driven ferroelectricity in
helical magnets.Comment: 8 pages, 9 figures (including 4 pages and 6 figures in supplementary
information). Version 2 with minor errors corrected (legend of Fig. 3c and
definition of vectors e and Q
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