12,230 research outputs found
Coexistence of coupled magnetic phases in epitaxial TbMnO3 films revealed by ultrafast optical spectroscopy
Ultrafast optical pump-probe spectroscopy is used to reveal the coexistence
of coupled antiferromagnetic/ferroelectric and ferromagnetic orders in
multiferroic TbMnO3 films through their time domain signatures. Our
observations are explained by a theoretical model describing the coupling
between reservoirs with different magnetic properties. These results can guide
researchers in creating new kinds of multiferroic materials that combine
coupled ferromagnetic, antiferromagnetic and ferroelectric properties in one
compound.Comment: Accepted by Appl. Phys. let
Non-Hermitian coherent coupling of nanomagnets by exchange spin waves
Non-Hermitian physics has recently attracted much attention in optics and
photonics. Less explored is non-Hermitian magnonics that provides opportunities
to take advantage of the inevitable dissipation of magnons or spin waves in
magnetic systems. Here we demonstrate non-Hermitian coherent coupling of two
distant nanomagnets by fast spin waves with sub-50 nm wavelengths. Magnons in
two nanomagnets are unidirectionally phase-locked with phase shifts controlled
by magnon spin torque and spin-wave propagation. Our results are attractive for
analog neuromorphic computing that requires unidirectional information
transmission
On the Convergence of Ritz Pairs and Refined Ritz Vectors for Quadratic Eigenvalue Problems
For a given subspace, the Rayleigh-Ritz method projects the large quadratic
eigenvalue problem (QEP) onto it and produces a small sized dense QEP. Similar
to the Rayleigh-Ritz method for the linear eigenvalue problem, the
Rayleigh-Ritz method defines the Ritz values and the Ritz vectors of the QEP
with respect to the projection subspace. We analyze the convergence of the
method when the angle between the subspace and the desired eigenvector
converges to zero. We prove that there is a Ritz value that converges to the
desired eigenvalue unconditionally but the Ritz vector converges conditionally
and may fail to converge. To remedy the drawback of possible non-convergence of
the Ritz vector, we propose a refined Ritz vector that is mathematically
different from the Ritz vector and is proved to converge unconditionally. We
construct examples to illustrate our theory.Comment: 20 page
A systematic TMRT observational study of Galactic C/C ratios from Formaldehyde
We present observations of the C-band (4.8 GHz) and Ku-band
(14.5 GHz) K-doublet lines of HCO and the C-band
(4.6 GHz) line of HCO toward a large sample of
Galactic molecular clouds, through the Shanghai Tianma 65-m radio telescope
(TMRT). Our sample with 112 sources includes strong HCO sources from the
TMRT molecular line survey at C-band and other known HCO sources. All three
lines are detected toward 38 objects (43 radial velocity components) yielding a
detection rate of 34\%. Complementary observations of their continuum emission
at both C- and Ku-bands were performed. Combining spectral line parameters and
continuum data, we calculate the column densities, the optical depths and the
isotope ratio HCO/HCO for each source. To evaluate photon
trapping caused by sometimes significant opacities in the main isotopologue's
rotational mm-wave lines connecting our measured K-doublets, and to obtain
C/C abundance ratios, we used the RADEX non-LTE model accounting
for radiative transfer effects. This implied the use of the new collision rates
from \citet{Wiesenfeld2013}. Also implementing distance values from
trigonometric parallax measurements for our sources, we obtain a linear fit of
C/C = (5.081.10)D + (11.866.60), with a
correlation coefficient of 0.58. D refers to Galactocentric distances.
Our C/C ratios agree very well with the ones deduced from CN and
CO but are lower than those previously reported on the basis of HCO,
tending to suggest that the bulk of the HCO in our sources was formed on
dust grain mantles and not in the gas phase.Comment: 27 pages, 8 figures, 7 tables. Accepted for publication in The
Astrophysical Journa
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