570 research outputs found
Thermal and electromagnetic properties of 166-Er and 167-Er
The primary gamma-ray spectra of 166-Er and 167-Er are deduced from the
(3-He,alpha gamma) and (3-He,3-He' gamma) reaction, respectively, enabling a
simultaneous extraction of the level density and the gamma-ray strength
function. Entropy, temperature and heat capacity are deduced from the level
density within the micro-canonical and the canonical ensemble, displaying
signals of a phase-like transition from the pair-correlated ground state to an
uncorrelated state at Tc=0.5 MeV. The gamma-ray strength function displays a
bump around E-gamma=3 MeV, interpreted as the pygmy resonance.Comment: 21 pages including 2 tables and 11 figure
Level densities and -strength functions in Sm
The level densities and -strength functions of the weakly deformed
Sm and Sm nuclei have been extracted. The temperature versus
excitation energy curve, derived within the framework of the micro canonical
ensemble, shows structures, which we associate with the break up of Cooper
pairs. The nuclear heat capacity is deduced within the framework of both the
micro canonical and the canonical ensemble. We observe negative heat capacity
in the micro canonical ensemble whereas the canonical heat capacity exhibits an
S-shape as function of temperature, both signals of a phase transition. The
structures in the -strength functions are discussed in terms of the
pygmy resonance and the scissors mode built on exited states. The samarium
results are compared with data for the well deformed Dy,
Er and Yb isotopes and with data from
(n,)-experiments and giant dipole resonance studies.Comment: 12 figure
Critical temperature for quenching of pair correlations
The level density at low spin in the 161,162-Dy and 171,172-Yb nuclei has
been extracted from primary gamma rays. The nuclear heat capacity is deduced
within the framework of the canonical ensemble. The heat capacity exhibits an
S-formed shape as a function of temperature, which is interpreted as a
fingerprint of the phase transition from a strongly correlated to an
uncorrelated phase. The critical temperature for the quenching of pair
correlations is found at Tc=0.50(4) MeV.Comment: 8 pages including 4 figures, different method to extract Tc,
different figures, text partly rewritte
Preconditioning effects of intermittent stream flow on leaf litter decomposition
Autumnal input of leaf litter is a pivotal energy source in most headwater streams. In temporary streams, however, water stress may lead to a seasonal shift in leaf abscission. Leaves accumulate at the surface of the dry streambed or in residual pools and are subject to physicochemical preconditioning before decomposition starts after flow recovery. In this study, we experimentally tested the effect of photodegradation on sunlit streambeds and anaerobic fermentation in anoxic pools on leaf decomposition during the subsequent flowing phase. To mimic field preconditioning, we exposed Populus tremula leaves to UV-VIS irradiation and wet-anoxic conditions in the laboratory. Subsequently, we quantified leaf mass loss of preconditioned leaves and the associated decomposer community in five low-order temporary streams using coarse and fine mesh litter bags. On average, mass loss after approximately 45 days was 4 and 7% lower when leaves were preconditioned by irradiation and anoxic conditions, respectively. We found a lower chemical quality and lower ergosterol content (a proxy for living fungal biomass) in leaves from the anoxic preconditioning, but no effects on macroinvertebrate assemblages were detected for any preconditioning treatment. Overall, results from this study suggest a reduced processing efficiency of organic matter in temporary streams due to preconditioning during intermittence of flow leading to reduced substrate quality and repressed decomposer activity. These preconditioning effects may become more relevant in the future given the expected worldwide increase in the geographical extent of intermittent flow as a consequence of global change. © 2011 Springer Basel AG
Level density and thermal properties in rare earth nuclei
A convergent method to extract the nuclear level density and the gamma-ray
strength function from primary gamma-ray spectra has been established.
Thermodynamical quantities have been obtained within the microcanonical and
canonical ensemble theory. Structures in the caloric curve and in the heat
capacity curve are interpreted as fingerprints of breaking of Cooper pairs and
quenching of pairing correlations. The strength function can be described using
models and common parameterizations for the E1, M1 and pygmy resonance
strength. However, a significant decrease of the pygmy resonance strength at
finite temperatures has been observed.Comment: 15 pages including 8 figures. Proceedings article for the conference
Nuclear Structure and Related Topics, Dubna, Russia, June 6-10, 200
A global synthesis of human impacts on the multifunctionality of streams and rivers
Human impacts, particularly nutrient pollution and land-use change, have caused significant declines in the quality and quantity of freshwater resources. Most global assessments have concentrated on species diversity and composition, but effects on the multifunctionality of streams and rivers remain unclear. Here, we analyse the most comprehensive compilation of stream ecosystem functions to date to provide an overview of the responses of nutrient uptake, leaf litter decomposition, ecosystem productivity, and food web complexity to six globally pervasive human stressors. We show that human stressors inhibited ecosystem functioning for most stressor-function pairs. Nitrate uptake efficiency was most affected and was inhibited by 347% due to agriculture. However, concomitant negative and positive effects were common even within a given stressor-function pair. Some part of this variability in effect direction could be explained by the structural heterogeneity of the landscape and latitudinal position of the streams. Ranking human stressors by their absolute effects on ecosystem multifunctionality revealed significant effects for all studied stressors, with wastewater effluents (194%), agriculture (148%), and urban land use (137%) having the strongest effects. Our results demonstrate that we are at risk of losing the functional backbone of streams and rivers if human stressors persist in contemporary intensity, and that freshwaters are losing critical ecosystem services that humans rely on. We advocate for more studies on the effects of multiple stressors on ecosystem multifunctionality to improve the functional understanding of human impacts. Finally, freshwater management must shift its focus toward an ecological function-based approach and needs to develop strategies for maintaining or restoring ecosystem functioning of streams and rivers
Transport in Quantum Dots from the Integrability of the Anderson Model
In this work we exploit the integrability of the two-lead Anderson model to
compute transport properties of a quantum dot, in and out of equilibrium. Our
method combines the properties of integrable scattering together with a
Landauer-Buttiker formalism. Although we use integrability, the nature of the
problem is such that our results are not generically exact, but must only be
considered as excellent approximations which nonetheless are valid all the way
through crossover regimes.
The key to our approach is to identify the excitations that correspond to
scattering states and then to compute their associated scattering amplitudes.
We are able to do so both in and out of equilibrium. In equilibrium and at zero
temperature, we reproduce the Friedel sum rule for an arbitrary magnetic field.
At finite temperature, we study the linear response conductance at the
symmetric point of the Anderson model, and reproduce Costi et al.'s numerical
renormalization group computation of this quantity. We then explore the
out-of-equilibrium conductance for a near-symmetric Anderson model, and arrive
at quantitative expressions for the differential conductance, both in and out
of a magnetic field. We find the expected splitting of the differential
conductance peak into two in a finite magnetic field, . We determine the
width, height, and position of these peaks. In particular we find for H >> T_k,
the Kondo temperature, the differential conductance has maxima of e^2/h
occuring for a bias V close to but smaller than H. The nature of our
construction of scattering states suggests that our results for the
differential magneto-conductance are not merely approximate but become exact in
the large field limit.Comment: 88 pages, 16 figures, uses harvmac.te
Excitation mapping of whispering gallery modes in silica microcavities
We report the direct observation of the electromagnetic-field distribution of
whispering?gallery modes in silica microcavities (spheres and toroids). It is
revealed by their excitation efficiency with a tapered fiber coupler swept
along the meridian. The originality of this method lies in the use of the
coupler itself for the near field mapping, eliminating the need of additional
tools used in previous work. This method is successfully applied to
microspheres and microtoroid
Supersymmetry breaking in two dimensions: the lattice N=1 Wess-Zumino model
We study dynamical supersymmetry breaking by non perturbative lattice
techniques in a class of two-dimensional N=1 Wess-Zumino models. We work in the
Hamiltonian formalism and analyze the phase diagram by analytical
strong-coupling expansions and explicit numerical simulations with Green
Function Monte Carlo methods.Comment: 53 pages, 17 figures, revtex
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