227 research outputs found
Presunrise Heating of the Ambient Electrons in the Ionosphere Due to Conjugate Point Photoelectrons
Presunrise heating of ambient electrons in ionosphere due to conjugate point photoelectron
Presunrise effects due to conjugate region photoelectrons
Presunrise heating effects due to conjugate region photoelectron
Spin Dynamics of the Magnetoresistive Pyrochlore Tl_2Mn_2O_7
Neutron scattering has been used to study the magnetic order and spin
dynamics of the colossal magnetoresistive pyrochlore Tl_2Mn_2O_7. On cooling
from the paramagnetic state, magnetic correlations develop and appear to
diverge at T_C (123 K). In the ferromagnetic phase well defined spin waves are
observed, with a gapless ( meV) dispersion relation E=Dq^{2} as
expected for an ideal isotropic ferromagnet. As T approaches T_C from low T,
the spin waves renormalize, but no significant central diffusive component to
the fluctuation spectrum is observed in stark contrast to the
La(Ca,Ba,Sr)MnO system. These results argue strongly that the
mechanism responsible for the magnetoresistive effect has a different origin in
these two classes of materials.Comment: 4 pages (RevTex), 4 figures (encapsulated postscript), to be
published in Phys. Rev. Let
Indications for Factorization and from Rare B Decay Data
Surveying known hadronic rare B decays, we find that the factorization
approximation can give a coherent account of , and
data and give predictions for , and modes,
{\it if is taken as negative} (in standard phase convention)
rather than positive. As further confirmation, we expect a lower
value at B Factories as compared to current fits, and mixing close to LEP
bounds at SLD and CDF.Comment: 11 pages, revtex, 4 figures (unchanged and eps files included);
version (including title and abstract change) to appear in Phys. Rev. Let
Local lattice disorder in the geometrically-frustrated spin glass pyrochlore Y2Mo2O7
The geometrically-frustrated spin glass Y2Mo2O7 has been considered widely to
be crystallographically ordered with a unique nearest neighbor magnetic
exchange interaction, J. To test this assertion, we present x-ray-absorption
fine-structure results for the Mo and Y K edges as a function of temperature
and compare them to results from a well-ordered pyrochlore, Tl2Mn2O7. We find
that the Mo-Mo pair distances are significantly disordered at approximately
right angles to the Y-Mo pairs. These results strongly suggest that lattice
disorder nucleates the spin-glass phase in this material.Comment: 9 pages, 2 Postscript figures, Phys. Rev. B: Rapid, in pres
Prospects for Direct CP Violaton in Exclusive and Inclusive Charmless B decays
Within the Standard Model, CP rate asymmetries for could
reach 10%. With strong final state phases, they could go up to 20--30%, even
for mode which would have opposite sign. We can account for
, and rate data with new physics enhanced
color dipole coupling and destructive interference. Asymmetries could reach
40--60% for and modes and are all of the same sign. We are
unable to account for rate. Our inclusive study supports our
exclusive results.Comment: Minor changes, correct a small bug in Fig. 1(b). Version to appear in
Phys. Rev. Let
Analysis of B-> \phi K Decays in QCD Factorization
We analyze the decay within the framework of QCD-improved
factorization. We found that although the twist-3 kaon distribution amplitude
dominates the spectator interactions, it will suppress the decay rates
slightly. The weak annihilation diagrams induced by penguin
operators, which are formally power-suppressed by order , are
chirally and logarithmically enhanced. Therefore, these annihilation
contributions are not subject to helicity suppression and can be sizable. The
predicted branching ratio of is in
the absence of annihilation contributions and it becomes
when annihilation effects are taken into
account. The prediction is consistent with CLEO and BaBar data but smaller than
the BELLE result.Comment: 13 pages, 3 figures. A major change for the presentation of
branching-ratio predictions. Experimental data are update
Born Effective Charges of Barium Titanate: band by band decomposition and sensitivity to structural features
The Born effective charge tensors of Barium Titanate have been calculated for
each of its 4 phases. Large effective charges of Ti and O, also predicted by
shell model calculations and made plausible by a simplified model, reflect the
partial covalent character of the chemical bond. A band by band decomposition
confirms that orbital hybridization is not restricted to Ti and O atoms but
also involves Ba which appears more covalent than generally assumed. Our
calculations reveal a strong dependence of the effective charges on the atomic
positions contrasting with a relative insensitivity on isotropic volume
changes.Comment: 13 page
Structural phase control of (LaNdSr)CuO thin films by epitaxial growth technique
Epitaxial growth of (LaNdSr)CuO thin films was
studied by pulsed-laser deposition technique on three different substrates,
SrTiO (100), LaSrAlO (001), and YAlO (001). The
(Nd,Sr,Ce)CuO-type structure appears at the initial growth stage on
SrTiO (100) when the film is deposited under the growth conditions
optimized for (La,Sr)CuO. This (Nd,Sr,Ce)CuO-type structure can
be eliminated by increasing the substrate temperature and the laser repetition
frequency. Films on LaSrAlO (001) maintain a LaCuO-type structure
as bulk samples, but those on YAlO (001) show phase separation into
LaCuO- and NdCuO-type structures. Such complicated results are
explained in terms of the competition between lattice misfit and thermodynamic
conditions. Interestingly the films with LaCuO-type structure prepared
on SrTiO and LaSrAlO show different surface structures and transport
properties. The results indicate the possibility of controlling charge stripes
of (LaNdSr)CuO as was demonstrated in
(La,Ba)CuO thin films by Sato et al. (Phys. Rev. B {\bf 62}, R799
(2000)).Comment: 5 pages, 6 EPS figure, accepted for publication in Phys. Rev.
Magnetic Raman Scattering in Two-Dimensional Spin-1/2 Heisenberg Antiferromagnets: Spectral Shape Anomaly and Magnetostrictive Effects
We calculate the Raman spectrum of the two-dimensional (2D) spin-1/2
Heisenberg antiferromagnet by exact diagonalization and quantum Monte Carlo
techniques on clusters of up to 144 sites and, on a 16-site cluster, by
considering the phonon-magnon interaction which leads to random fluctuations of
the exchange integral. Results are in good agreement with experiments on
various high-T_c precursors, such as La_2CuO_4 and YBa_2Cu_3O_{6.2}. In
particular, our calculations reproduce the broad lineshape of the two-magnon
peak, the asymmetry about its maximum, the existence of spectral weight at high
energies, and the observation of nominally forbidden A_{1g} scattering.Comment: 12 pages, REVTEX, 1 postscript figur
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