3,831 research outputs found
Quantum dynamics of the Neel vector in the antiferromagnetic molecular wheel CsFe8
The inelastic neutron scattering (INS) spectrum is studied for the
antiferromagnetic molecular wheel CsFe8, in the temperature range 2 - 60 K, and
for transfer energies up 3.6 meV. A qualitative analysis shows that the
observed peaks correspond to the transitions between the L-band states, from
the ground state up to the S = 5 multiplet. For a quantitative analysis, the
wheel is described by a microscopic spin Hamiltonian (SH), which includes the
nearest-neighbor Heisenberg exchange interactions and uniaxial easy-axis
single-ion anisotropy, characterized by the constants J and D, respectively.
For a best-fit determination of J and D, the L band is modeled by an effective
SH, and the effective SH concept extended such as to facilitate an accurate
calculation of INS scattering intensities, overcoming difficulties with the
dimension of the Hilbert space. The low-energy magnetism in CsFe8 is
excellently described by the generic SH used. The two lowest states are
characterized by a tunneling of the Neel vector, as found previously, while the
higher-lying states are well described as rotational modes of the Neel vector.Comment: 12 pages, 10 figures, REVTEX4, to appear in PR
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
Classification of Invariant Star Products up to Equivariant Morita Equivalence on Symplectic Manifolds
In this paper we investigate equivariant Morita theory for algebras with
momentum maps and compute the equivariant Picard groupoid in terms of the
Picard groupoid explicitly. We consider three types of Morita theory:
ring-theoretic equivalence, *-equivalence and strong equivalence. Then we apply
these general considerations to star product algebras over symplectic manifolds
with a Lie algebra symmetry. We obtain the full classification up to
equivariant Morita equivalence.Comment: 28 pages. Minor update, fixed typos
Improving ocean-glider's payload with a new generation of spectrophotometric PH sensor
Ocean gliders have clearly become nowadays useful autonomous
platforms addressed to measure a wide range of seawater parameters in a more
sustainable and efficient way. This new ocean monitoring approach has implied
the need to develop smaller, faster and more efficient sensors without reducing key
features like accuracy, resolution, time-response, among others, in order to fit the
glider operational capabilities. This work is aiming to present the latest development
stages of a new spectrophotometric pH sensor, its integration process into a Wave
Glider SV3 platform and the preliminary results derived from an offshore mission
performed in subtropical waters between the Canary Islands and Cape Verde
archipelagos.Peer Reviewe
Q-dependence of the inelastic neutron scattering cross section for molecular spin clusters with high molecular symmetry
For powder samples of polynuclear metal complexes the dependence of the
inelastic neutron scattering intensity on the momentum transfer Q is known to
be described by a combination of so called interference terms. They reflect the
interplay between the geometrical structure of the compound and the spatial
properties of the wave functions involved in the transition. In this work, it
is shown that the Q-dependence is strongly interrelated with the molecular
symmetry of molecular nanomagnets, and, if the molecular symmetry is high
enough, is actually completely determined by it. A general formalism connecting
spatial symmetry and interference terms is developed. The arguments are
detailed for cyclic spin clusters, as experimentally realized by e.g. the
octanuclear molecular wheel Cr8, and the star like tetranuclear cluster Fe4.Comment: 8 pages, 1 figures, REVTEX
Deformation Quantization of a Certain Type of Open Systems
We give an approach to open quantum systems based on formal deformation
quantization. It is shown that classical open systems of a certain type can be
systematically quantized into quantum open systems preserving the complete
positivity of the open time evolution. The usual example of linearly coupled
harmonic oscillators is discussed.Comment: Major update. Improved main statements. 21 page
Inelastic neutron scattering study and Hubbard model description of the antiferromagnetic tetrahedral molecule Ni4Mo12
The tetrameric Ni(II) spin cluster Ni4Mo12 has been studied by INS. The data
were analyzed extensively in terms of a very general spin Hamiltonian, which
includes antiferromagnetic Heisenberg interactions, biquadratic 2-spin and
3-spin interactions, a single-ion magnetic anisotropy, and Dzyaloshinsky-Moriya
interactions. Some of the experimentally observed features in the INS spectra
could be reproduced, however, one feature at 1.65 meV resisted all efforts.
This supports the conclusion that the spin Hamiltonian approach is not adequate
to describe the magnetism in Ni4Mo12. The isotropic terms in the spin
Hamiltonian can be obtained in a strong-coupling expansion of the Hubbard model
at half-filling. Therefore detailed theoretical studies of the Hubbard model
were undertaken, using analytical as well as numerical techniques. We carefully
analyzed its abilities and restrictions in applications to molecular spin
clusters. As a main result it was found that the Hubbard model is also unable
to appropriately explain the magnetism in Ni4Mo12. Extensions of the model are
also discussed.Comment: 12 pages, 12 figure
Approximating parabolas as natural bounds of Heisenberg spectra: Reply on the comment of O. Waldmann
O. Waldmann has shown that some spin systems, which fulfill the condition of
a weakly homogeneous coupling matrix, have a spectrum whose minimal or maximal
energies are rather poorly approximated by a quadratic dependence on the total
spin quantum number. We comment on this observation and provide the new
argument that, under certain conditions, the approximating parabolas appear as
natural bounds of the spectrum generated by spin coherent states.Comment: 2 pages, accepted for Europhysics Letter
Exchange-coupling constants, spin density map, and Q dependence of the inelastic neutron scattering intensity in single-molecule magnets
The Q dependence of the inelastic neutron scattering (INS) intensity of
transitions within the ground-state spin multiplet of single-molecule magnets
(SMMs) is considered. For these transitions, the Q dependence is related to the
spin density map in the ground state, which in turn is governed by the
Heisenberg exchange interactions in the cluster. This provides the possibility
to infer the exchange-coupling constants from the Q dependence of the INS
transitions within the spin ground state. The potential of this strategy is
explored for the M = +-10 -> +- 9 transition within the S = 10 multiplet of the
molecule Mn12 as an example. The Q dependence is calculated for powder as well
as single-crystal Mn12 samples for various exchange-coupling situations
discussed in the literature. The results are compared to literature data on a
powder sample of Mn12 and to measurements on an oriented array of about 500
single-crystals of Mn12. The calculated Q dependence exhibits significant
variation with the exchange-coupling constants, in particular for a
single-crystal sample, but the experimental findings did not permit an
unambiguous determination. However, although challenging, suitable experiments
are within the reach of today's instruments.Comment: 11 pages, 6 figures, REVTEX4, to appear in PR
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