1,125 research outputs found
Spin-Wave Spectrum in `Single-Domain' Magnetic Ground State of Triangular Lattice Antiferromagnet CuFeO2
By means of neutron scattering measurements, we have investigated spin-wave
excitation in a collinear four-sublattice (4SL) magnetic ground state of a
triangular lattice antiferromagnet CuFeO2, which has been of recent interest as
a strongly frustrated magnet, a spin-lattice coupled system and a multiferroic.
To avoid mixing of spin-wave spectrum from magnetic domains having three
different orientations reflecting trigonal symmetry of the crystal structure,
we have applied uniaxial pressure on [1-10] direction of a single crystal
CuFeO2. By elastic neutron scattering measurements, we have found that only 10
MPa of the uniaxial pressure results in almost 'single domain' state in the 4SL
phase. We have thus performed inelastic neutron scattering measurements using
the single domain sample, and have identified two distinct spin- wave branches.
The dispersion relation of the upper spin-wave branch cannot be explained by
the previous theoretical model [R. S. Fishman: J. Appl. Phys. 103 (2008)
07B109]. This implies the importance of the lattice degree of freedom in the
spin-wave excitation in this system, because the previous calculation neglected
the effect of the spin-driven lattice distortion in the 4SL phase. We have also
discussed relationship between the present results and the recently discovered
"electromagnon" excitation.Comment: 5 pages, 3 figures, accepted for publication in J. Phys. Soc. Jp
Electromagnetic radiation due to naked singularity formation in self-similar gravitational collapse
Dynamical evolution of test fields in background geometry with a naked
singularity is an important problem relevant to the Cauchy horizon instability
and the observational signatures different from black hole formation. In this
paper we study electromagnetic perturbations generated by a given current
distribution in collapsing matter under a spherically symmetric self-similar
background. Using the Green's function method, we construct the formula to
evaluate the outgoing energy flux observed at the future null infinity. The
contributions from "quasi-normal" modes of the self-similar system as well as
"high-frequency" waves are clarified. We find a characteristic power-law time
evolution of the outgoing energy flux which appears just before naked
singularity formation, and give the criteria as to whether or not the outgoing
energy flux diverges at the future Cauchy horizon.Comment: 20 pages, 7 figures, references added to match the published versio
Molecular Dynamics Study of Rotating Nanodroplets: Finite-size Effects and Nonequilibrium Deformation
Noneqiuilibrium dynamics of rotating droplets are studied by molecular
dynamics simulations. Small deviations from the theoretical prediction are
observed when the size of a droplet is small, and the deviations become smaller
as the size of the droplet increases. The characteristic timescale of the
deformation is observed, and we find (i) the deformation timescale is almost
independent of the rotating velocity with for small frequency and (ii) the
deformation timescale becomes shorter as temperature increases. A simple model
is proposed to explain the deformation dynamics of droplets.Comment: 14 pages, 8 figure, added references, changed titl
High Magnetic Field Behaviour of the Triangular Lattice Antiferromagnet, CuFeO_2
The high magnetic field behaviour of the triangular lattice antiferromagnet
CuFeO_2 is studied using single crystal neutron diffraction measurements in a
field of up to 14.5 T and also by magnetisation measurements in a field of up
to 12 T. At low temperature, two well-defined first order magnetic phase
transitions are found in this range of applied magnetic field (H // c): at
H_c1=7.6(3)/7.1(3) T and H_c2=13.2(1)/12.7(1) T when ramping the field up/down.
In a field above H_c2 the magnetic Bragg peaks show unusual history dependence.
In zero field T_N1=14.2(1) K separates a high temperature paramagnetic and an
intermediate incommensurate structure, while T_N2=11.1(3) K divides an
incommensurate phase from the low-temperature 4-sublattice ground state. The
ordering temperature T_N1 is found to be almost field independent, while T_N2
decreases noticeably in applied field. The magnetic phase diagram is discussed
in terms of the interactions between an applied magnetic field and the highly
frustrated magnetic structure of CuFeO_2Comment: 7 pages, 8 figures in ReVTeX. To appear in PR
Kilohertz QPO Frequency and Flux Decrease in AQL X-1 and Effect of Soft X-ray Spectral Components
We report on an RXTE/PCA observation of Aql X-1 during its outburst in March
1997 in which, immediately following a Type-I burst, the broad-band 2-10 keV
flux decreased by about 10% and the kilohertz QPO frequency decreased from
813+-3 Hz to 776+-4 Hz. This change in kHz QPO frequency is much larger than
expected from a simple extrapolation of a frequency-flux correlation
established using data before the burst. Meanwhile a very low frequency noise
(VLFN) component in the broad-band FFT power spectra with a fractional
root-mean-square (rms) amplitude of 1.2% before the burst ceased to exist after
the burst. All these changes were accompanied by a change in the energy
spectral shape. If we characterize the energy spectra with a model composed of
two blackbody (BB) components and a power law component, almost all the
decrease in flux was in the two BB components. We attribute the two BB
components to the contributions from a region very near the neutron star or
even the neutron star itself and from the accretion disk, respectively.Comment: 12 pages with 4 figures, accepted for publication in ApJ Letters,
typos corrected and references update
Detection of highly ionized O and Ne absorption lines in the X-ray spectrum of 4U1820-303 in the globular cluster, NGC 6624
We searched for absorption lines of highly ionized O and Ne in the energy
spectra of two Low-mass X-ray binaries, 4U1820-303 in the globular cluster
NGC6624 and Cyg X-2, observed with the Chandra LETG, and detected O VII, O VIII
and Ne IX absorption lines for 4U1820-303. The equivalent width of the O VII K
alpha line was 1.19 +0.47/-0.30 eV (90 % errors) and the significance was 6.5
sigma. Absorption lines were not detected for Cyg X-2 with a 90 % upper limit
on the equivalent width of 1.06 eV for O VII K alpha. The absorption lines
observed in 4U1820-303 are likely due to hot interstellar medium, because O
will be fully photo-ionized if the absorbing column is located close to the
binary system. The velocity dispersion is restricted to b = 200 - 420 km/s from
consistency between O VII K alpha and K beta lines, Ne/O abundance ratio, and H
column density. The average temperature and the O VII density are respectively
estimated to be log(T[K]) = 6.2 - 6.3 and n(OVII) = (0.7 - 2.3) x 10^{-6}
cm^{-3}. The difference of O VII column densities for the two sources may be
connected to the enhancement of the soft X-ray background (SXB) towards the
Galactic bulge region. Using the polytrope model of hot gas to account for the
SXB we corrected for the density gradient and estimated the midplane O VII
density at the solar neighborhood. The scale height of hot gas is then
estimated using the AGN absorption lines. It is suggested that a significant
portion of both the AGN absorption lines and the high-latitude SXB emission
lines can be explained by the hot gas in our Galaxy.Comment: Accepted for publication in ApJ. 7 pages, 9 eps figure
Electric Polarization Induced by a Proper Helical Magnetic Ordering in a Delafossite Multiferroic CuFe1-xAlxO2
Multiferroic CuFe1-xAlxO2 (x=0.02) exhibits a ferroelectric ordering
accompanied by a proper helical magnetic ordering below T=7K under zero
magnetic field. By polarized neutron diffraction and pyroelectric measurements,
we have revealed a one-to-one correspondence between the spin helicity and the
direction of the spontaneous electric polarization. This result indicates that
the spin helicity of the proper helical magnetic ordering is essential for the
ferroelectricity in CuFe1-xAlxO2. The induction of the electric polarization by
the proper helical magnetic ordering is, however, cannot be explained by the
Katsura-Nagaosa-Balatsky model, which successfully explains the
ferroelectricity in the recently explored ferroelectric helimagnets, such as
TbMnO3. We thus conclude that CuFe1-xAlxO2 is a new class of magnetic
ferroelectrics.Comment: 4 pages, 4 figure
- …