9 research outputs found
Optical generation of excitonic valley coherence in monolayer WSe 2
Due to degeneracies arising from crystal symmetries, it is possible for
electron states at band edges ("valleys") to have additional spin-like quantum
numbers. An important question is whether coherent manipulation can be
performed on such valley pseudospins, analogous to that routinely implemented
using true spin, in the quest for quantum technologies. Here we show for the
first time that SU(2) valley coherence can indeed be generated and detected.
Using monolayer semiconductor WSe2 devices, we first establish the circularly
polarized optical selection rules for addressing individual valley excitons and
trions. We then reveal coherence between valley excitons through the
observation of linearly polarized luminescence, whose orientation always
coincides with that of any linearly polarized excitation. Since excitons in a
single valley emit circularly polarized photons, linear polarization can only
be generated through recombination of an exciton in a coherent superposition of
the two valleys. In contrast, the corresponding photoluminescence from trions
is not linearly polarized, consistent with the expectation that the emitted
photon polarization is entangled with valley pseudospin. The ability to address
coherence, in addition to valley polarization, adds a critical dimension to the
quantum manipulation of valley index necessary for coherent valleytronics.Comment: 12 pages Main Text + 12 Supplemental, 4 Main Text Figure
Impaired Representation of Time in Schizophrenia Is Linked to Positive Symptoms and Cognitive Demand
Ammonoid Intraspecific Variability
Because ammonoids have never been observed swimming, there is no alternative to seeking indirect indications of the locomotory abilities of ammonoids. This approach is based on actualistic comparisons with the closest relatives of ammonoids, the Coleoidea and the Nautilida, and on the geometrical and physical properties of the shell. Anatomical comparison yields information on the locomotor muscular systems and organs as well as possible modes of propulsion while the shape and physics of ammonoid shells provide information on buoyancy, shell orientation, drag, added mass, cost of transportation and thus on limits of acceleration and swimming speed. On these grounds, we conclude that ammonoid swimming is comparable to that of Recent nautilids and sepiids in terms of speed and energy consumption, although some ammonoids might have been slower swimmers than nautilids