11,694 research outputs found
Recent progress in the identification and determination of freshwater phytoplankton in the natural environment
The biomass of the phytoplankton and its composition is one of the most important factors in water quality control. Determination of the phytoplankton assemblage is usually done by microscopic analysis (Utermöhl's method). Quantitative estimations of the biovolume, by cell counting and cell size measurements, are time-consuming and normally are not done in routine water quality control. Several alternatives have been tried: computer-based image analysis, spectral fluorescence signatures, flow cytometry and pigment fingerprinting aided by high performance liquid chromatography (HPLC). The latter method is based on the fact that each major algal group of taxa contains a specific carotenoid which can be used for identification and relative quantification of the taxa in the total assemblage. This article gives a brief comparative introduction to the different techniques available and presents some recent results obtained by HPLC-based pigment fingerprinting, applied to three lakes of different trophic status. The results show that this technique yields reliable results from different lake types and is a powerful tool for studying the distribution pattern of the phytoplankton community in relation to water depth. However, some restrictions should be taken into account for the interpretation of routine data
Theoretical analysis of continuously driven dissipative solid-state qubits
We study a realistic model for driven qubits using the numerical solution of
the Bloch-Redfield equation as well as analytical approximations using a
high-frequency scheme. Unlike in idealized rotating-wave models suitable for
NMR or quantum optics, we study a driving term which neither is orthogonal to
the static term nor leaves the adiabatic energy value constant. We investigate
the underlying dynamics and analyze the spectroscopy peaks obtained in recent
experiments. We show, that unlike in the rotating-wave case, this system
exhibits nonlinear driving effects.We study the width of spectroscopy peaks and
show, how a full analysis of the parameters of the system can be performed by
comparing the first and second resonance. We outline the limitations of the NMR
linewidth formula at low temperature and show, that spectrocopic peaks
experience a strong shift which goes much beyond the Bloch-Siegert shift of the
Eigenfrequency.Comment: Accepted for publication in Phys. Rev.
New Numerical Results Indicate a Half-Filling SU(4) Kondo State in Carbon Nanotubes
Numerical calculations simulate transport experiments in carbon nanotube
quantum dots (P. Jarillo-Herrero et al., Nature 434, 484 (2005)), where a
strongly enhanced Kondo temperature T_K ~ 8K was associated with the SU(4)
symmetry of the Hamiltonian at quarter-filling for an orbitally
double-degenerate single-occupied electronic shell. Our results clearly suggest
that the Kondo conductance measured for an adjacent shell with T_K ~ 16K,
interpreted as a singlet-triplet Kondo effect, can be associated instead to an
SU(4) Kondo effect at half-filling. Besides presenting spin-charge Kondo
screening similar to the quarter-filling SU(4), the half-filling SU(4) has been
recently associated to very rich physical behavior, including a
non-Fermi-liquid state (M. R. Galpin et al., Phys. Rev. Lett. 94, 186406
(2005)).Comment: 7 pages, 7 figure
Fast recognition of alternating groups of unknown degree
We present a constructive recognition algorithm to decide whether a given
black-box group is isomorphic to an alternating or a symmetric group without
prior knowledge of the degree. This eliminates the major gap in known
algorithms, as they require the degree as additional input.
Our methods are probabilistic and rely on results about proportions of
elements with certain properties in alternating and symmetric groups. These
results are of independent interest; for instance, we establish a lower bound
for the proportion of involutions with small support.Comment: 31 pages, submitted to Journal of Algebr
Proton recoil polarization in exclusive (e,e'pp) reactions
The general formalism of nucleon recoil polarization in the () reaction is given. Numerical predictions are presented for the
components of the outgoing proton polarization and of the polarization transfer
coefficient in the specific case of the exclusive O()C knockout reaction leading to discrete states in the residual
nucleus. Reaction calculations are performed in a direct knockout framework
where final-state interactions and one-body and two-body currents are included.
The two-nucleon overlap integrals are obtained from a calculation of the
two-proton spectral function of O where long-range and short-range
correlations are consistently included. The comparison of results obtained in
different kinematics confirms that resolution of different final states in the
O()C reaction may act as a filter to
disentangle and separately investigate the reaction processes due to
short-range correlations and two-body currents and indicates that measurements
of the components of the outgoing proton polarization may offer good
opportunities to study short-range correlations.Comment: 12 pages, 6 figure
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