4,411 research outputs found
Orbital ordering promotes weakly-interacting S=1/2 dimers in the triangular lattice compound Sr3Cr2O8
The weakly interacting S=1/2 dimers system Sr3Cr2O8 has been investigated by
powder neutron diffraction and inelastic neutron scattering. Our data reveal a
structural phase transition below room temperature corresponding to an
antiferro-orbital ordering with nearly 90 degrees arrangement of the occupied
3z^2-r^2 d-orbital. This configuration leads to a drastic reduction of the
inter-dimer exchange energies with respect to the high temperature
orbital-disorder state, as shown by a spin-dimer analysis of the
super-superexchange interactions performed using the Extended Huckel Tight
Binding method. Inelastic neutron scattering reveals the presence of a quasi
non-dispersive magnetic excitation at 5.4 meV, in agreement with the picture of
weakly-interacting dimers
A possible chiral spin-liquid phase in non-centrosymmetric BaCoO
Based on a symmetry approach, we propose a possible explanation of the weak
ferromagnetic component recently observed in YBaCoFeO (Valldor et al.
Phys Rev B, 224426 (2011)) and other isostructural compounds in the
high-temperature spin-liquid phase. Due to the polar nature of their crystal
structure, a coupling between time-odd scalar spin chirality which we suggest
as the primary order parameter and macroscopic magnetization is possible as
follows from the general form of the appropriate free-energy invariant. The
deduced pseudoproper coupling between both physical quantities provides a
unique possibility to study the critical behaviour of the chiral order
parameter
Anomalous quartic WWgamma gamma, ZZgamma gamma, and trilinear WWgamma couplings in two-photon processes at high luminosity at the LHC
We study the W/Z pair production via two-photon exchange at the LHC and give
the sensitivities on trilinear and quartic gauge anomalous couplings between
photons and W/Z bosons for an integrated luminosity of 30 and 200 fb^{-1}. For
simplicity and to obtain lower backgrounds, only the leptonic decays of the
electroweak bosons are considered.Comment: 22 pages, 17 figures, sumitted to Phys. Rev.
Anomalous trilinear and quartic , , and couplings in photon induced processes at the LHC
We first report on possible measurements at the LHC using the first data and
a luminosity of 10 pb of and pair production via two-photon
exchange. This measurement allows in particular to increase the present
sensitivity on and quartic anomalous
couplings from the LEP experiments by almost three orders of magnitude. We also
discuss the possible improvements on quartic and trilinear anomalous couplings
at high luminosity at the LHC using new forward proton taggers to be installed
at 220 and 420 m from the CMS or ATLAS detectors.Comment: Proceedings of the EPS 2009 conference, Cracow, July 200
Spin-stripe phase in a frustrated zigzag spin-1/2 chain
Motifs of periodic modulations are encountered in a variety of natural
systems, where at least two rival states are present. In strongly correlated
electron systems such behaviour has typically been associated with competition
between short- and long-range interactions, e.g., between exchange and
dipole-dipole interactions in the case of ferromagnetic thin films. Here we
show that spin-stripe textures may develop also in antiferromagnets, where
long-range dipole-dipole magnetic interactions are absent. A comprehensive
analysis of magnetic susceptibility, high-field magnetization, specific heat,
and neutron diffraction measurements unveils -TeVO as a nearly
perfect realization of a frustrated (zigzag) ferromagnetic spin-1/2 chain.
Strikingly, a narrow spin stripe phase develops at elevated magnetic fields due
to weak frustrated short-range interchain exchange interactions possibly
assisted by the symmetry allowed electric polarization. This concept provides
an alternative route for the stripe formation in strongly correlated electron
systems and may help understanding other widespread, yet still elusive,
stripe-related phenomena.Comment: accapted in Nature Communication
Enhanced ferroelectric polarization by induced Dy spin-order in multiferroic DyMnO3
Neutron powder diffraction and single crystal x-ray resonant magnetic
scattering measurements suggest that Dy plays an active role in enhancing the
ferroelectric polarization in multiferroic DyMnO3 above TNDy = 6.5 K. We
observe the evolution of an incommensurate ordering of Dy moments with the same
periodicity as the Mn spiral ordering. It closely tracks the evolution of the
ferroelectric polarization which reaches a maximum value of 0.2 muC/m^2. Below
TNDy, where Dy spins order commensurately, the polarization decreases to values
similar for those of TbMnO3
Galactic star formation in parsec-scale resolution simulations
The interstellar medium (ISM) in galaxies is multiphase and cloudy, with
stars forming in the very dense, cold gas found in Giant Molecular Clouds
(GMCs). Simulating the evolution of an entire galaxy, however, is a
computational problem which covers many orders of magnitude, so many
simulations cannot reach densities high enough or temperatures low enough to
resolve this multiphase nature. Therefore, the formation of GMCs is not
captured and the resulting gas distribution is smooth, contrary to
observations. We investigate how star formation (SF) proceeds in simulated
galaxies when we obtain parsec-scale resolution and more successfully capture
the multiphase ISM. Both major mergers and the accretion of cold gas via
filaments are dominant contributors to a galaxy's total stellar budget and we
examine SF at high resolution in both of these contexts.Comment: 4 pages, 4 figures. To appear in the proceedings for IAU Symposium
270: Computational Star Formation (eds. Alves, Elmegreen, Girart, Trimble
Non-collinear long-range magnetic ordering in HgCr2S4
The low-temperature magnetic structure of \HG has been studied by
high-resolution powder neutron diffraction. Long-range incommensurate magnetic
order sets in at T22K with propagation vector
\textbf{k}=(0,0,0.18). On cooling below T, the propagation vector
increases and saturates at the commensurate value \textbf{k}=(0,0,0.25). The
magnetic structure below T consists of ferromagnetic layers in the
\textit{ab}-plane stacked in a spiral arrangement along the \textit{c}-axis.
Symmetry analysis using corepresentations theory reveals a point group symmetry
in the ordered magnetic phase of 422 (D), which is incompatible with
macroscopic ferroelectricity. This finding indicates that the spontaneous
electric polarization observed experimentally cannot be coupled to the magnetic
order parameter
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