9 research outputs found
Raman cooling and heating of two trapped Ba+ ions
We study cooling of the collective vibrational motion of two 138Ba+ ions
confined in an electrodynamic trap and irradiated with laser light close to the
resonances S_1/2-P_1/2 (493 nm) and P_1/2-D_3/2 (650 nm). The motional state of
the ions is monitored by a spatially resolving photo multiplier. Depending on
detuning and intensity of the cooling lasers, macroscopically different
motional states corresponding to different ion temperatures are observed. We
also derive the ions' temperature from detailed analytical calculations of
laser cooling taking into account the Zeeman structure of the energy levels
involved. The observed motional states perfectly match the calculated
temperatures. Significant heating is observed in the vicinity of the dark
resonances of the Zeeman-split S_1/2-D_3/2 Raman transitions. Here two-photon
processes dominate the interaction between lasers and ions. Parameter regimes
of laser light are identified that imply most efficient laser cooling.Comment: 8 pages, 5 figure
Scale‐dependent patterns and drivers of plant diversity in steppe grasslands of the Central Alborz Mts., Iran
Questions: Which are the main environmental drivers of plot scale alpha-diversity and beta-diversity in arid steppes? Do they vary between functional groups and across grain sizes?
Location: Central Alborz Mts., N Iran.
Methods: We sampled vascular plants in 23 nested-plot series with nine grain sizes from 0.0001 m2 to 1,000 m2 and 334 25-m2 vegetation plots in different vegetation types of natural dry steppe grasslands. To assess beta-diversity, we calculated overall and local z-values of species–area relationships modelled with the power function. As potential drivers of species richness and z-values, we used topographic, edaphic and climatic variables as well as management types. Generalised linear models (GLMs), and generalised linear mixed-effect models (GLMMs) if spatial autocorrelation occurred, were used in a multi-model inference framework to build statistical models.
Results: Mean annual temperature was the most important predictor for total species richness and richness of functional groups across grain sizes, with a unimodal relationship for grains of 25–100 m2, but mostly increasing for finer grain sizes. Precipitation of the driest month and cover of gravel were influential drivers at the smallest grains. The explanatory power of regression models increased towards larger grain sizes. The overall z-values showed a high positive relationship with precipitation of the driest month, mean annual temperature and mean soil depth.
Conclusions: Related to our more than 3,000-m elevational gradient, mean annual temperature (highly negatively correlated with elevation) was the most influential and consistent driver across functional groups and grain sizes with mostly unimodal relationships for alpha-diversity and a positive effect on beta-diversity. Findings for other drivers were less consistent, and overall the explained variance of our models was relatively low, calling for additional studies to determine whether in the arid grasslands of Iran stochasticity is just higher or there are additional important variables