2,212 research outputs found
Orbital and Spin Excitations in Cobalt Oxide
By means of neutron scattering we have determined new branches of magnetic
excitations in orbitally active CoO (TN=290 K) up to 15 THz and for
temperatures from 6 K to 450 K. Data were taken in the (111) direction in six
single-crystal zones. From the dependence on temperature and Q we have
identified several branches of magnetic excitation. We describe a model for the
coupled orbital and spin states of Co2+ subject to a crystal field and
tetragonal distortion.Comment: To be published in Physica B (Proceedings of SCES07 conference in
Houston
Magnetic excitations of spin and orbital moments in cobalt oxide
Magnetic and phonon excitations in the antiferromagnet CoO with an unquenched
orbital angular momentum are studied by neutron scattering. Results of energy
scans in several Brillouin zones in the (HHL) plane for energy transfers up to
16 THz are presented. The measurements were performed in the antiferromagnetic
ordered state at 6 K (well below TN~290 K) as well as in the paramagnetic state
at 450 K. Several magnetic excitation modes are identified from the dependence
of their intensity on wavevector and temperature. Within a Hund's rule model
the excitations correspond to fluctuations of coupled orbital and spin degrees
of freedom whose bandwidth is controlled by interionic superexchange. The
different ordering domains give rise to several magnetic peaks at each
wavevector transfer.Comment: Accepted for publication in Canadian Journal of Physic
The Quasi-1D S=1/2 Antiferromagnet Cs2CuCl4 in a Magnetic Field
Magnetic excitations of the quasi-1D S=1/2 Heisenberg antiferromagnet (HAF)
Cs2CuCl4 have been measured as a function of magnetic field using neutron
scattering. For T<0.62 K and B=0 T the weak inter-chain coupling produces 3D
incommensurate ordering. Fields greater than Bc =1.66 T, but less than the
field (~8 T) required to fully align the spins, are observed to decouple the
chains, and the system enters a disordered intermediate-field phase (IFP). The
IFP excitations are in agreement with the predictions of Muller et al. for the
1D S=1/2 HAF, and Talstra and Haldane for the related 1/r^2 chain (the
Haldane-Shastry model). This behaviour is inconsistent with linear spin-wave
theory.Comment: 10 pages, 4 encapsulated postscript figures, LaTeX, to be published
in PRL, e-mail comments to [email protected]
Two and Three Dimensional Incommensurate Modulation in Optimally-Doped BiSrCaCuO
X-ray scattering measurements on optimally-doped single crystal samples of
the high temperature superconductor BiSrCaCuO reveal
the presence of three distinct incommensurate charge modulations, each
involving a roughly fivefold increase in the unit cell dimension along the {\bf
b}-direction. The strongest scattering comes from the well known (H, K
0.21, L) modulation and its harmonics. However, we also observe broad
diffraction which peak up at the L values complementary to those which
characterize the known modulated structure. These diffraction features
correspond to correlation lengths of roughly a unit cell dimension,
20 in the {\bf c} direction, and of 185
parallel to the incommensurate wavevector. We interpret these features as
arising from three dimensional incommensurate domains and the interfaces
between them, respectively. In addition we investigate the recently discovered
incommensuate modulations which peak up at (1/2, K 0.21, L) and related
wavevectors. Here we explicitly study the L-dependence of this scattering and
see that these charge modulations are two dimensional in nature with weak
correlations on the scale of a bilayer thickness, and that they correspond to
short range, isotropic correlation lengths within the basal plane. We relate
these new incommensurate modulations to the electronic nanostructure observed
in BiSrCaCuO using STM topography.Comment: 8 pages, 8 figure
Green's function of fully anharmonic lattice vibration
Motivated by the discovery of superconductivity in beta-pyrochlore oxides, we
study property of rattling motion coupled with conduction electrons. We derive
the general expression of the Green's function of fully anharmonic lattice
vibration within the accuracy of the second order perturbation of electron-ion
interaction by introducing self-energy, vertex-correction, and normalization
factor for each transition. Using the expression, we discuss the characteristic
properties of the spectral function in the entire range from weakly anharmonic
potential to double-well case, and calculate NMR relaxation rate due to the two
phonon Raman process
Scaling properties of the critical behavior in the dilute antiferromagnet Fe(0.93)Zn(0.07)F2
Critical scattering analyses for dilute antiferromagnets are made difficult
by the lack of predicted theoretical line shapes beyond mean-field models.
Nevertheless, with the use of some general scaling assumptions we have
developed a procedure by which we can analyze the equilibrium critical
scattering in these systems for H=0, the random-exchange Ising model, and, more
importantly, for H>0, the random-field Ising model. Our new fitting approach,
as opposed to the more conventional techniques, allows us to obtain the
universal critical behavior exponents and amplitude ratios as well as the
critical line shapes. We discuss the technique as applied to
Fe(0.93)Zn(0.07)F2. The general technique, however, should be applicable to
other problems where the scattering line shapes are not well understood but
scaling is expected to hold.Comment: 17 pages, 5 figure
Correlational Origin of the Roton Minimum
We present compelling evidence supporting the conjecture that the origin of
the roton in Bose-condensed systems arises from strong correlations between the
constituent particles. By studying the two dimensional bosonic dipole systems a
paradigm, we find that classical molecular dynamics (MD) simulations provide a
faithful representation of the dispersion relation for a low- temperature
quantum system. The MD simulations allow one to examine the effect of coupling
strength on the formation of the roton minimum and to demonstrate that it is
always generated at a sufficiently high enough coupling. Moreover, the
classical images of the roton-roton, roton-maxon, etc. states also appear in
the MD simulation spectra as a consequence of the strong coupling.Comment: 7 pages, 4 figure
Discovery of the progenitor of the type Ia supernova 2007on
Type Ia supernovae are exploding stars that are used to measure the
accelerated expansion of the Universe and are responsible for most of the iron
ever produced. Although there is general agreement that the exploding star is a
white dwarf in a binary system, the exact configuration and trigger of the
explosion is unclear, which could hamper their use for precision cosmology. Two
families of progenitor models have been proposed. In the first, a white dwarf
accretes material from a companion until it exceeds the Chandrasekhar mass,
collapses and explodes. Alternatively, two white dwarfs merge, again causing
catastrophic collapse and an explosion. It has hitherto been impossible to
determine if either model is correct. Here we report the discovery of an object
in pre-supernova archival X-ray images at the position of the recent type Ia
supernova (2007on) in the elliptical galaxy NGC 1404. Deep optical images (also
archival) show no sign of this object. From this we conclude that the X-ray
source is the progenitor of the supernova, which favours the accretion model
for this supernova, although the host galaxy is older (6-9 Gyr) than the age at
which the explosions are predicted in the accreting models.Comment: Published in Nature See also the two follow-up papers: Roelofs,
Bassa, Voss, Nelemans Nelemans, Voss, Roelofs, Bassa both on astro-ph
02/15/0
Short time-scale frequency and amplitude variations in the pulsations of an roAp star: HD 217522
Photometric observations of HD 217522 in 1981 revealed only one pulsation frequency ν1 = 1.215 29 mHz. Subsequent observations in 1989 showed the presence of an additional frequency ν2 = 2.0174 mHz. New observations in 2008 confirm the presence of the mode with ν2 = 2.0174 mHz. Examination of the 1989 data shows amplitude modulation over a time-scale of the order of a day, much shorter than what has been observed in other rapidly oscillating Ap (roAp) stars. High spectral and time resolution data obtained using the Very Large Telescope in 2008 confirm the presence of ν2 and short-term modulations in the radial velocity amplitudes of rare earth elements. This suggests growth and decay times shorter than a day, more typical of solar-like oscillations. The driving mechanism of roAp stars and the Sun are different, and the growth and decay seen in the Sun are due to stochastic nature of the driving mechanism. The driving mechanism in roAp stars usually leads to mode stability on a longer time-scale than in the Sun. We interpret the reported change in ν1 between the 1982 and 1989 data as part of the general frequency variability observed in this star on many time-scales
Evidence for anisotropic polar nanoregions in relaxor PMN: A neutron study of the elastic constants and anomalous TA phonon damping
We use neutron scattering to characterize the acoustic phonons in the relaxor
PMN and demonstrate the presence of an anisotropic damping mechanism directly
related to short-range, polar correlations. For a large range of temperatures
above Tc ~ 210, K, where dynamic polar correlations exist, acoustic phonons
propagating along [1\bar{1}0] and polarized along [110] (TA2 phonons) are
overdamped and softened across most of the Brillouin zone. By contrast,
acoustic phonons propagating along [100] and polarized along [001] (TA1
phonons) are overdamped and softened for only a limited range of wavevectors.
The anisotropy and temperature dependence of the acoustic phonon energy
linewidth are directly correlated with the elastic diffuse scattering,
indicating that polar nanoregions are the cause of the anomalous behavior. The
damping and softening vanish for q -> 0, i.e. for long-wavelength acoustic
phonons, which supports the notion that the anomalous damping is a result of
the coupling between the relaxational component of the diffuse scattering and
the harmonic TA phonons. Therefore, these effects are not due to large changes
in the elastic constants with temperature because the elastic constants
correspond to the long-wavelength limit. We compare the elastic constants we
measure to those from Brillouin scattering and to values reported for pure PT.
We show that while the values of C44 are quite similar, those for C11 and C12
are significantly less in PMN and result in a softening of (C11-C12) over PT.
There is also an increased elastic anisotropy (2C44/(C11-C12)) versus that in
PT. These results suggest an instability to TA2 acoustic fluctuations in
relaxors. We discuss our results in the context of the debate over the
"waterfall" effect and show that they are inconsistent with TA-TO phonon
coupling or other models that invoke the presence of a second optic mode.Comment: (21 pages, 16 figures, to be published in Physical Review B
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