1,664 research outputs found
Polar and magnetic order in GaV4Se8
In the present work, we provide results from specific heat, magnetic
susceptibility, dielectric constant, ac conductivity, and electrical
polarization measurements performed on the lacunar spinel GaV4Se8. With
decreasing temperature, we observe a transition from the paraelectric and
paramagnetic cubic state into a polar, probably ferroelectric state at 42 K
followed by magnetic ordering at 18 K. The polar transition is likely driven by
the Jahn-Teller effect due to the degeneracy of the V4 cluster orbitals. The
excess polarization arising in the magnetic phase indicates considerable
magnetoelectric coupling. Overall, the behavior of GaV4Se8 in many respects is
similar to that of the skyrmion host GaV4S8, exhibiting a complex interplay of
orbital, spin, lattice, and polar degrees of freedom. However, its dielectric
behavior at the polar transition markedly differs from that of the Jahn-Teller
driven ferroelectric GeV4S8, which can be ascribed to the dissimilar electronic
structure of the Ge compound.Comment: 7 pages, 6 figures. Revised version according to suggestions of
referee
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Precision measurement of the lifetime of the 1s2s3S1 metastable level in heliumlike O6+
The lifetime of the 1s2s3S1 level of the He-like O6+ ion has been measured using the Electron Beam Ion Trap in the magnetic trapping mode. A value of 956-4+5 μs is found, which corresponds to a radiative transition rate of 1046-5+4 s-1 for the magnetic dipole transition to the 1s21S0 ground state. This value is in excellent agreement with recent theoretical predictions and distinguishes among different treatments of negative energy states and correlation in multiconfiguration Dirac-Fock calculations
Sub-gap optical response across the structural phase transition in van der Waals layered \alpha-RuCl
We report magnetic, thermodynamic, thermal expansion, and on detailed optical
experiments on the layered compound -RuCl focusing on the THz and
sub-gap optical response across the structural phase transition from the
monoclinic high-temperature to the rhombohedral low-temperature structure,
where the stacking sequence of the molecular layers is changed. This type of
phase transition is characteristic for a variety of tri-halides crystallizing
in a layered honeycomb-type structure and so far is unique, as the
low-temperature phase exhibits the higher symmetry. One motivation is to
unravel the microscopic nature of spin-orbital excitations via a study of
temperature and symmetry-induced changes. We document a number of highly
unusual findings: A characteristic two-step hysteresis of the structural phase
transition, accompanied by a dramatic change of the reflectivity. An electronic
excitation, which appears in a narrow temperature range just across the
structural phase transition, and a complex dielectric loss spectrum in the THz
regime, which could indicate remnants of Kitaev physics. Despite significant
symmetry changes across the monoclinic to rhombohedral phase transition, phonon
eigenfrequencies and the majority of spin-orbital excitations are not strongly
influenced. Obviously, the symmetry of the single molecular layers determine
the eigenfrequencies of most of these excitations. Finally, from this combined
terahertz, far- and mid-infrared study we try to shed some light on the so far
unsolved low energy (< 1eV) electronic structure of the ruthenium
electrons in -RuCl.Comment: 22 pages, 9 figure
Measurement of the hyperfine structure of antihydrogen in a beam
A measurement of the hyperfine structure of antihydrogen promises one of the
best tests of CPT symmetry. We describe an experiment planned at the Antiproton
Decelerator of CERN to measure this quantity in a beam of slow antihydrogen
atoms.Comment: 5th International Symposium on Symmetries in Subatomic Physics
(SSP2012), Groningen (The Netherlands), June 18 to 22, 201
First observation of two hyperfine transitions in antiprotonic He-3
We report on the first experimental results for microwave spectroscopy of the
hyperfine structure of antiprotonic He-3. Due to the helium nuclear spin,
antiprotonic He-3 has a more complex hyperfine structure than antiprotonic He-4
which has already been studied before. Thus a comparison between theoretical
calculations and the experimental results will provide a more stringent test of
the three-body quantum electrodynamics (QED) theory. Two out of four
super-super-hyperfine (SSHF) transition lines of the (n,L)=(36,34) state were
observed. The measured frequencies of the individual transitions are
11.12559(14) GHz and 11.15839(18) GHz, less than 1 MHz higher than the current
theoretical values, but still within their estimated errors. Although the
experimental uncertainty for the difference of these frequencies is still very
large as compared to that of theory, its measured value agrees with theoretical
calculations. This difference is crucial to be determined because it is
proportional to the magnetic moment of the antiproton.Comment: 8 pages, 6 figures, just published (online so far) in Physics Letters
RasGAP Shields Akt from Deactivating Phosphatases in Fibroblast Growth Factor Signaling but Loses This Ability Once Cleaved by Caspase-3.
Fibroblast growth factor receptors (FGFRs) are involved in proliferative and differentiation physiological responses. Deregulation of FGFR-mediated signaling involving the Ras/PI3K/Akt and the Ras/Raf/ERK MAPK pathways is causally involved in the development of several cancers. The caspase-3/p120 RasGAP module is a stress sensor switch. Under mild stress conditions, RasGAP is cleaved by caspase-3 at position 455. The resulting N-terminal fragment, called fragment N, stimulates anti-death signaling. When caspase-3 activity further increases, fragment N is cleaved at position 157. This generates a fragment, called N2, that no longer protects cells. Here, we investigated in Xenopus oocytes the impact of RasGAP and its fragments on FGF1-mediated signaling during G2/M cell cycle transition. RasGAP used its N-terminal Src homology 2 domain to bind FGFR once stimulated by FGF1, and this was necessary for the recruitment of Akt to the FGFR complex. Fragment N, which did not associate with the FGFR complex, favored FGF1-induced ERK stimulation, leading to accelerated G2/M transition. In contrast, fragment N2 bound the FGFR, and this inhibited mTORC2-dependent Akt Ser-473 phosphorylation and ERK2 phosphorylation but not phosphorylation of Akt on Thr-308. This also blocked cell cycle progression. Inhibition of Akt Ser-473 phosphorylation and entry into G2/M was relieved by PHLPP phosphatase inhibition. Hence, full-length RasGAP favors Akt activity by shielding it from deactivating phosphatases. This shielding was abrogated by fragment N2. These results highlight the role played by RasGAP in FGFR signaling and how graded stress intensities, by generating different RasGAP fragments, can positively or negatively impact this signaling
GHRS and ORFEUS-II Observations of the Highly Ionized Interstellar Medium Toward ESO141-055
We present Goddard High Resolution Spectrograph and ORFEUS-II measurements of
Si IV, CIV, N V, and O VI absorption in the interstellar medium of the Galactic
disk and halo toward the nucleus of the Seyfert galaxy ESO141-055. The high
ionization absorption is strong, with line strengths consistent with the
spectral signature expected for hot (log T = 5-6) collisionally ionized gas in
either a ``Galactic fountain'' or an inhomogeneous medium containing a mixture
of conductive interfaces and turbulent mixing layers. The total O VI column
density of log N ~ 15 suggests that the scale height of O VI is large (>3 kpc)
in this direction. Comparison of the high ion column densities with
measurements for other sight lines indicates that the highly ionized gas
distribution is patchy. The amount of O VI perpendicular to the Galactic plane
varies by at least a factor of ~4 among the complete halo sight lines thus far
studied. In addition to the high ion absorption, lines of low ionization
species are also present in the spectra. With the possible exception of Ar I,
which may have a lower than expected abundance resulting from partial
photoionization of gas along the sight line, the absorption strengths are
typical of those expected for the warm, neutral interstellar medium. The sight
line intercepts a cold molecular cloud with log N(H2) ~ 19. The cloud has an
identifiable counterpart in IRAS 100-micron emission maps of this region of the
sky. We detect a Ly-alpha absorber associated with ESO141-055 at z = 0.03492.
This study presents an enticing glimpse into the interstellar and intergalactic
absorption patterns that will be observed at high spectral resolution by the
Far Ultraviolet Spectroscopic Explorer.Comment: 24 pages + 8 figures, uses aaspp4.sty. Accepted for publication in
Ap
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