125 research outputs found
Study of state-of-the-art static inverter design Final report, 6 Jan. - 6 Jun. 1966
Multiple purpose inverter design based on phase demodulated inverter circuit selected from state-of-the-art assessment of ten inverter circuit
Rydberg trimers and excited dimers bound by internal quantum reflection
Quantum reflection is a pure wave phenomena that predicts reflection of a
particle at a changing potential for cases where complete transmission occurs
classically. For a chemical bond, we find that this effect can lead to
non-classical vibrational turning points and bound states at extremely large
interatomic distances. Only recently has the existence of such ultralong-range
Rydberg molecules been demonstrated experimentally. Here, we identify a broad
range of molecular lines, most of which are shown to originate from two
different novel sources: a single-photon associated triatomic molecule formed
by a Rydberg atom and two ground state atoms and a series of excited dimer
states that are bound by a so far unexplored mechanism based on internal
quantum reflection at a steep potential drop. The properties of the Rydberg
molecules identified in this work qualify them as prototypes for a new type of
chemistry at ultracold temperatures.Comment: 6 pages, 3 figures, 1 tabl
Thermalization of coupled atom-light states in the presence of optical collisions
The interaction of a two-level atomic ensemble with a quantized single mode
electromagnetic field in the presence of optical collisions (OC) is
investigated both theoretically and experimentally. The main accent is made on
achieving thermal equilibrium for coupled atom-light states (in particular
dressed states). We propose a model of atomic dressed state thermalization that
accounts for the evolution of the pseudo-spin Bloch vector components and
characterize the essential role of the spontaneous emission rate in the
thermalization process. Our model shows that the time of thermalization of the
coupled atom-light states strictly depends on the ratio of the detuning and the
resonant Rabi frequency. The predicted time of thermalization is in the
nanosecond domain and about ten times shorter than the natural lifetime at full
optical power in our experiment. Experimentally we are investigating the
interaction of the optical field with rubidium atoms in an ultra-high pressure
buffer gas cell under the condition of large atom-field detuning comparable to
the thermal energy in frequency units. In particular, an observed detuning
dependence of the saturated lineshape is interpreted as evidence for thermal
equilibrium of coupled atom-light states. A significant modification of
sideband intensity weights is predicted and obtained in this case as well.Comment: 14 pages, 12 figures; the content was edite
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Adaptive divergence in shoot gravitropism creates hybrid sterility in an Australian wildflower.
Natural selection is responsible for much of the diversity we see in nature. Just as it drives the evolution of new traits, it can also lead to new species. However, it is unclear whether natural selection conferring adaptation to local environments can drive speciation through the evolution of hybrid sterility between populations. Here, we show that adaptive divergence in shoot gravitropism, the ability of a plant's shoot to bend upwards in response to the downward pull of gravity, contributes to the evolution of hybrid sterility in an Australian wildflower, Senecio lautus We find that shoot gravitropism has evolved multiple times in association with plant height between adjacent populations inhabiting contrasting environments, suggesting that these traits have evolved by natural selection. We directly tested this prediction using a hybrid population subjected to eight rounds of recombination and three rounds of selection in the field. Our experiments revealed that shoot gravitropism responds to natural selection in the expected direction of the locally adapted population. Using the advanced hybrid population, we discovered that individuals with extreme differences in gravitropism had more sterile crosses than individuals with similar gravitropic responses, which were largely fertile, indicating that this adaptive trait is genetically correlated with hybrid sterility. Our results suggest that natural selection can drive the evolution of locally adaptive traits that also create hybrid sterility, thus revealing an evolutionary connection between local adaptation and the origin of new species
Assessment and Student Learning – a fundamental relationship and the role of information and communication technologies
This paper reviews the role of assessment in student learning and its relationship with the use of information and communication technologies (ICT). There is ample evidence of technology-led innovations failing to achieve the transformations expected by educators. We draw upon existing research to illustrate the links between aspects of student learning, assessment practices and the use of ICT. Assessment influences not only what parts of a course get studied, but also how those parts are studied. While the adoption of ICT does not, in itself, change student behaviours, appropriately designed assessment that exploits the potential of ICT can change students’ approaches to learning. We argue that ICT can enable important learning outcomes to be achieved, but these must be underpinned by an assessment strategy that cues students to adopt a suitable approach to learning
Formation and interactions of cold and ultracold molecules: new challenges for interdisciplinary physics
Progress on researches in the field of molecules at cold and ultracold
temperatures is reported in this review. It covers extensively the experimental
methods to produce, detect and characterize cold and ultracold molecules
including association of ultracold atoms, deceleration by external fields and
kinematic cooling. Confinement of molecules in different kinds of traps is also
discussed. The basic theoretical issues related to the knowledge of the
molecular structure, the atom-molecule and molecule-molecule mutual
interactions, and to their possible manipulation and control with external
fields, are reviewed. A short discussion on the broad area of applications
completes the review.Comment: to appear in Reports on Progress in Physic
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