661 research outputs found
Thermodynamics of the harmonic oscillator using coherent states
The ongoing discussion whether thermodynamic properties can be extracted from
a (possibly approximate) quantum mechanical time evolution using time averages
is fed with an instructive example. It is shown for the harmonic oscillator how
the Hilbert space or an appropriately defined phase space must be populated in
terms of coherent states in order to obtain the quantum result respectively the
classical one.Comment: 6 pages, 2 postscript figures, uses 'epsfig.sty'. Submitted to
Europhysics Letters. Introduction changed and references added for the
revised version. More information available at
http://obelix.physik.uni-osnabrueck.de/~schnack
Sampling the two-dimensional density of states g(E,M) of a giant magnetic molecule using the Wang-Landau method
The Wang-Landau method is used to study the magnetic properties of the giant
paramagnetic molecule Mo_72Fe_30 in which 30 Fe3+ ions are coupled via
antiferromagnetic exchange. The two-dimensional density of states g(E,M) in
energy and magnetization space is calculated using a self-adaptive version of
the Wang-Landau method. From g(E,M) the magnetization and magnetic
susceptibility can be calculated for any temperature and external field.Comment: 5 pages, 9 figures, submitted to Phys. Rev.
Thermodynamics of the one-dimensional frustrated Heisenberg ferromagnet with arbitrary spin
The thermodynamic quantities (spin-spin correlation functions <{\bf S}_0{\bf
S}_n>, correlation length {\xi}, spin susceptibility {\chi}, and specific heat
C_V) of the frustrated one-dimensional J1-J2 Heisenberg ferromagnet with
arbitrary spin quantum number S below the quantum critical point, i.e. for J2<
|J1|/4, are calculated using a rotation-invariant Green-function formalism and
full diagonalization as well as a finite-temperature Lanczos technique for
finite chains of up to N=18 sites. The low-temperature behavior of the
susceptibility {\chi} and the correlation length {\xi} is well described by
\chi = (2/3)S^4 (|J1|-4J2) T^{-2} + A S^{5/2} (|J1|-4J2)^{1/2} T^{-3/2} and \xi
= S^2 (|J1|-4J2) T^{-1} + B S^{1/2} (|J1|-4J2)^{1/2} T^{-1/2} with A \approx
1.1 ... 1.2 and B \approx 0.84 ... 0.89. The vanishing of the factors in front
of the temperature at J2=|J1|/4 indicates a change of the critical behavior of
{\chi} and {\xi} at T \to 0. The specific heat may exhibit an additional
frustration-induced low-temperature maximum when approaching the quantum
critical point. This maximum appears for S=1/2 and S=1, but was not found for
S>1.Comment: 8 pages, 7 figure
Application of the finite-temperature Lanczos method for the evaluation of magnetocaloric properties of large magnetic molecules
We discuss the magnetocaloric properties of gadolinium containing magnetic
molecules which potentially could be used for sub-Kelvin cooling. We show that
a degeneracy of a singlet ground state could be advantageous in order to
support adiabatic processes to low temperatures and simultaneously minimize
disturbing dipolar interactions. Since the Hilbert spaces of such spin systems
assume very large dimensions we evaluate the necessary thermodynamic
observables by means of the Finite-Temperature Lanczos Method.Comment: 7 pages, 10 figures, invited for the special issue of EPJB on "New
trends in magnetism and magnetic materials
Quantum rotational band model for the Heisenberg molecular magnet Mo72Fe30
We derive the low temperature properties of the molecular magnet Mo72Fe30,
where 30 Fe(3+) paramagnetic ions occupy the sites of an icosidodecahedron and
interact via isotropic nearest-neighbour antiferromagnetic Heisenberg exchange.
The key idea of our model (J.S. & M.L.) is that the low-lying excitations form
a sequence of rotational bands, i.e., for each such band the excitation
energies depend quadratically on the total spin quantum number. For
temperatures below 50 mK we predict that the magnetisation is described by a
staircase with 75 equidistant steps as the magnetic field is increased up to a
critical value and saturated for higher fields. For higher temperatures thermal
broadening effects wash out the staircase and yield a linear ramp below the
critical field, and this has been confirmed by our measurements (R.M.). We
demonstrate that the lowest two rotational bands are separated by an energy gap
of 0.7 meV, and this could be tested by EPR and inelastic neutron scattering
measurements. We also predict the occurrence of resonances at temperatures
below 0.1 K in the proton NMR spin-lattice relaxation rate associated with
level crossings. As rotational bands characterize the spectra of many magnetic
molecules our method opens a new road towards a description of their
low-temperature behaviour which is not otherwise accessible.Comment: 7 pages, 6 figures, accepted for Europhysics Letter
Partial replacement of fishmeal by lyophylized powder of the microalgae Spirulina platensis in Pacific white shrimp diets
An eleven-week feeding trial was conducted to evaluate the growth effect of partial replacement of fishmeal by the microalga Spirulina platensis in the diet of juvenile Pacific white shrimp Litopenaeus vannamei (Boone). The results clearly indicate that Spirulina platensis constitutes an effective food ingredient for shrimp. Growth rates of a Spirulina-fed group differed highly significantly (p < 0,001) compared to two groups fed to less suitable diets and were slightly, even though not significantly superior to that based on an optimal commercial reference fish diet. As a side effect, Spirulinafed shrimps showed measurable differences in pigmentation
Spin-wave spectra of a kagome stripe
We study ground state degeneracy and spin-wave excitations in a 1D version of
a Kagome antiferromagnet -- a Heisenberg antiferromagnet on a Kagome stripe. We
show that for nearest-neighbor interaction, the classical ground state is
infinitely degenerate. For any spin configuration from the degenerate set, the
classical spin-wave spectrum contains, in addition to Goldstone modes, a branch
of zero energy excitations, and a zero mode in another branch. We demonstrate
that the interactions beyond nearest neighbors lift the degeneracy, eliminate a
zero mode, and give a finite dispersion to formerly zero-energy branch, leaving
only Goldstone modes as zero-energy excitations.Comment: 6 pages, 9 figures, submitted to Europhys. Let
Taxonomic diversity and identification problems of oncaeid microcopepods in the Mediterranean Sea
The species diversity of the pelagic microcopepod
family Oncaeidae collected with nets of 0.1-mm mesh
size was studied at 6 stations along a west-to-east transect
in the Mediterranean Sea down to a maximum depth of
1,000 m. A total of 27 species and two form variants have
been identified, including three new records for the
Mediterranean. In addition, about 20, as yet undescribed,
new morphospecies were found (mainly from the genera
Epicalymma and Triconia) which need to be examined
further. The total number of identified oncaeid species was
similar in the Western and Eastern Basins, but for some cooccurring
sibling species, the estimated numerical dominance
changed. The deep-sea fauna of Oncaeidae, studied
at selected depth layers between 400 m and the near-bottom
layer at >4,200 m depth in the eastern Mediterranean
(Levantine Sea), showed rather constant species numbers
down to ∼3,000 m depth. In the near-bottom layers, the
diversity of oncaeids declined and species of Epicalymma
strongly increased in numerical importance. The taxonomic
status of all oncaeid species recorded earlier in the
Mediterranean Sea is evaluated: 19 out of the 46 known
valid oncaeid species are insufficiently described, and most
of the taxonomically unresolved species (13 species) have
originally been described from this area (type locality). The
deficiencies in the species identification of oncaeids cast
into doubt the allegedly cosmopolitan distribution of some
species, in particular those of Mediterranean origin. The
existing identification problems even of well-described
oncaeid species are exemplified for the Oncaea mediacomplex,
including O. media Giesbrecht, O. scottodicarloi
Heron & Bradford-Grieve, and O. waldemari Bersano &
Boxshall, which are often erroneously identified as a single
species (O. media). The inadequacy in the species identification
of Oncaeidae, in particular those from the Atlantic
and Mediterranean, is mainly due to the lack of reliable
identification keys for Oncaeidae in warm-temperate and/or
tropical seas. Future efforts should be directed to the
construction of identification keys that can be updated
according to the latest taxonomic findings, which can be
used by the non-expert as well as by the specialist. The
adequate consideration of the numerous, as yet undescribed,
microcopepod species in the world oceans, in
particular the Oncaeidae, is a challenge for the study of the
structure and function of plankton communities as well as
for global biodiversity estimates
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