1,860 research outputs found
Quantum tunneling of the Neel vector in antiferromagnetic [3 x 3] grid molecules
Based on numerical calculations it is shown that the antiferromagnetic grid
molecule Mn-[3 x 3] is a very promising candidate to experimentally detect the
phenomenon of quantum tunneling of the Neel vector.Comment: 4 pages, 3 figures, REVTEX 4, to appear in PR
Field dependent anisotropy change in a supramolecular Mn(II)-[3x3] grid
The magnetic anisotropy of a novel Mn(II)-[3x3] grid complex was investigated
by means of high-field torque magnetometry. Torque vs. field curves at low
temperatures demonstrate a ground state with S > 0 and exhibit a torque step
due to a field induced level-crossing at B* \approx 7.5 T, accompanied by an
abrupt change of magnetic anisotropy from easy-axis to hard-axis type. These
observations are discussed in terms of a spin Hamiltonian formalism.Comment: 4 pages, 4 figures, to be published in Phys. Rev. Let
Comment on "Bounding and approximating parabolas for the spectrum of Heisenberg spin systems" by Schmidt, Schnack and Luban
Recently, Schmidt et al. proved that the energy spectrum of a Heisenberg spin
system (HSS) is bounded by two parabolas, i.e. lines which depend on the total
spin quantum number S as S(S+1). The prove holds for homonuclear HSSs which
fulfill a weak homogenity condition. Moreover, the extremal values of the exact
spectrum of various HSS which were studied numerically were found to lie on
approximate parabolas, named rotational bands, which could be obtained by a
shift of the boundary parabolas. In view of this, it has been claimed that the
rotational band structure (RBS) of the energy spectrum is a general behavior of
HSSs. Furthermore, since the approximate parabolas are very close to the true
boundaries of the spectrum for the examples discussed, it has been claimed that
the methods allow to predict the detailed shape of the spectrum and related
properties for a general HSS. In this comment I will show by means of examples
that the RBS hypothesis is not valid for general HSSs. In particular, weak
homogenity is neither a necessary nor a sufficient condition for a HSS to
exhibit a spectrum with RBS.Comment: Comments on the work of Schmidt et al, Europhys. Lett. 55, 105
(2001), cond-mat/0101228 (for the reply see cond-mat/0111581). To be
published in Europhys. Let
Blind extraction of an exoplanetary spectrum through Independent Component Analysis
Blind-source separation techniques are used to extract the transmission
spectrum of the hot-Jupiter HD189733b recorded by the Hubble/NICMOS instrument.
Such a 'blind' analysis of the data is based on the concept of independent
component analysis. The de-trending of Hubble/NICMOS data using the sole
assumption that nongaussian systematic noise is statistically independent from
the desired light-curve signals is presented. By not assuming any prior, nor
auxiliary information but the data themselves, it is shown that spectroscopic
errors only about 10 - 30% larger than parametric methods can be obtained for
11 spectral bins with bin sizes of ~0.09 microns. This represents a reasonable
trade-off between a higher degree of objectivity for the non-parametric methods
and smaller standard errors for the parametric de-trending. Results are
discussed in the light of previous analyses published in the literature. The
fact that three very different analysis techniques yield comparable spectra is
a strong indication of the stability of these results.Comment: ApJ accepte
Field-induced level crossings in spin clusters: Thermodynamics and magneto-elastic instability
Quantum spin clusters with dominant antiferromagnetic Heisenberg exchange
interactions typically exhibit a sequence of field-induced level crossings in
the ground state as function of magnetic field. For fields near a level
crossing, the cluster can be approximated by a two-level Hamiltonian at low
temperatures. Perturbations, such as magnetic anisotropy or spin-phonon
coupling, sensitively affect the behavior at the level-crossing points. The
general two-level Hamiltonian of the spin system is derived in first-order
perturbation theory, and the thermodynamic functions magnetization, magnetic
torque, and magnetic specific heat are calculated. Then a magneto-elastic
coupling is introduced and the effective two-level Hamilitonian for the
spin-lattice system derived in the adiabatic approximation of the phonons. At
the level crossings the system becomes unconditionally unstable against lattice
distortions due to the effects of magnetic anisotropy. The resultant
magneto-elastic instabilities at the level crossings are discussed, as well as
the magnetic behavior.Comment: 13 pages, 8 figures, REVTEX
Non-Local thermal equilibrium spectra of atmospheric molecules for exoplanets
Here we present a study of non-LTE effects on the exoplanetary spectra of a
collection of molecules which are key in the investigation of exoplanet
atmospheres: water, methane, carbon monoxide and titanium oxide. These
molecules are chosen as examples of different spectral ranges (IR and UV),
molecular types (diatomics and polyatomics) and spectral types (electronic and
ro-vibrational); the importance of different vibrational bands in forming
distinct non-LTE spectral features are investigated. Most notably, such key
spectral signatures for distinguishing between the LTE and non-LTE cases
include: for CH4 the 3.15 m band region; for H2O the 2.0 m and 2.7
m band regions; for TiO, a strong variation in intensity in the bands
between 0.5 and 0.75 m; and a sole CO signature between 5 and 6 m.
The analysis is based on the ExoMol cross sections and takes advantage of the
extensive vibrational assignment of these molecular line lists in the ExoMol
database. We examine LTE and non-LTE cross sections under conditions consistent
with those on WASP-12b and WASP-76b using the empirically motivated
bi-temperature Treanor model. In addition, we make a simplistic forward model
simulation of transmission spectra for H2O in the atmosphere of WASP-12b using
the TauREx 3 atmospheric modelling code
Don't Pay Attention to the Noise: Learning Self-supervised Representations of Light Curves with a Denoising Time Series Transformer
Astrophysical light curves are particularly challenging data objects due to the intensity and variety of noise contaminating them. Yet, despite
the astronomical volumes of light curves available, the majority of algorithms used to process them are still operating on a per-sample
basis. To remedy this, we propose a simple
Transformer model –called Denoising Time Series Transformer (DTST)– and show that it excels
at removing the noise and outliers in datasets of
time series when trained with a masked objective, even when no clean targets are available.
Moreover, the use of self-attention enables rich
and illustrative queries into the learned representations. We present experiments on real stellar light
curves from the Transiting Exoplanet Space Satellite (TESS), showing advantages of our approach
compared to traditional denoising techniques1
Magnetic relaxation studies on a single-molecule magnet by time-resolved inelastic neutron scattering
Time-resolved inelastic neutron scattering measurements on an array of
single-crystals of the single-molecule magnet Mn12ac are presented. The data
facilitate a spectroscopic investigation of the slow relaxation of the
magnetization in this compound in the time domain.Comment: 3 pages, 4 figures, REVTEX4, to appear in Appl. Phys. Lett., for an
animation see also
http://www.dcb.unibe.ch/groups/guedel/members/ow2/trins.ht
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