1,536 research outputs found
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
Discrete antiferromagnetic spin-wave excitations in the giant ferric wheel Fe18
The low-temperature elementary spin excitations in the AFM molecular wheel
Fe18 were studied experimentally by inelastic neutron scattering and
theoretically by modern numerical methods, such as dynamical density matrix
renormalization group or quantum Monte Carlo techniques, and analytical
spin-wave theory calculations. Fe18 involves eighteen spin-5/2 Fe(III) ions
with a Hilbert space dimension of 10^14, constituting a physical system that is
situated in a region between microscopic and macroscopic. The combined
experimental and theoretical approach allowed us to characterize and discuss
the magnetic properties of Fe18 in great detail. It is demonstrated that
physical concepts such as the rotational-band or L&E-band concepts developed
for smaller rings are still applicable. In particular, the higher-lying
low-temperature elementary spin excitations in Fe18 or AFM wheels in general
are of discrete antiferromagnetic spin-wave character.Comment: 16 pages, 10 figure
Identification of Radiopure Titanium for the LZ Dark Matter Experiment and Future Rare Event Searches
The LUX-ZEPLIN (LZ) experiment will search for dark matter particle
interactions with a detector containing a total of 10 tonnes of liquid xenon
within a double-vessel cryostat. The large mass and proximity of the cryostat
to the active detector volume demand the use of material with extremely low
intrinsic radioactivity. We report on the radioassay campaign conducted to
identify suitable metals, the determination of factors limiting radiopure
production, and the selection of titanium for construction of the LZ cryostat
and other detector components. This titanium has been measured with activities
of U~1.6~mBq/kg, U~0.09~mBq/kg,
Th~~mBq/kg, Th~~mBq/kg, K~0.54~mBq/kg, and Co~0.02~mBq/kg (68\% CL).
Such low intrinsic activities, which are some of the lowest ever reported for
titanium, enable its use for future dark matter and other rare event searches.
Monte Carlo simulations have been performed to assess the expected background
contribution from the LZ cryostat with this radioactivity. In 1,000 days of
WIMP search exposure of a 5.6-tonne fiducial mass, the cryostat will contribute
only a mean background of (stat)(sys) counts.Comment: 13 pages, 3 figures, accepted for publication in Astroparticle
Physic
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