Doublet structures in quantum well absorption spectra due to Fano-related interference

In this theoretical investigation we predict an unusual interaction between a discrete state and a continuum of states, which is closely related to the case of Fano-interference. It occurs in a GaAs/AlxGa1-xAs quantum well between the lowest light-hole exciton and the continuum of the second heavy-hole exciton. Unlike the typical case for Fano-resonance, the discrete state here is outside the continuum; we use uniaxial stress to tune its position with respect to the onset of the continuum. State-of-the art calculations of absorption spectra show that as the discrete state approaches the continuum, a doublet structure forms which reveals anticrossing behaviour. The minimum separation energy of the anticrossing depends characteristically on the well width and is unusually large for narrow wells. This offers striking evidence for the strong underlying valence-band mixing. Moreover, it proves that previous explanations of similar doublets in experimental data, employing simple two-state models, are incomplete.Comment: 21 pages, 5 figures and 5 equations. Accepted for publication in Physical Review

The Laser Astrometric Test of Relativity Mission

This paper discusses new fundamental physics experiment to test relativistic gravity at the accuracy better than the effects of the 2nd order in the gravitational field strength. The Laser Astrometric Test Of Relativity (LATOR) mission uses laser interferometry between two micro-spacecraft whose lines of sight pass close by the Sun to accurately measure deflection of light in the solar gravity. The key element of the experimental design is a redundant geometry optical truss provided by a long-baseline (100 m) multi-channel stellar optical interferometer placed on the International Space Station. The geometric redundancy enables LATOR to measure the departure from Euclidean geometry caused by the solar gravity field to a very high accuracy. LATOR will not only improve the value of the parameterized post-Newtonian (PPN) parameter gamma to unprecedented levels of accuracy of 1 part in 1e8, it will also reach ability to measure effects of the next post-Newtonian order (1/c^4) of light deflection resulting from gravity's intrinsic non-linearity. The solar quadrupole moment parameter, J2, will be measured with high precision, as well as a variety of other relativistic. LATOR will lead to very robust advances in the tests of fundamental physics: this mission could discover a violation or extension of general relativity, or reveal the presence of an additional long range interaction in the physical law. There are no analogs to the LATOR experiment; it is unique and is a natural culmination of solar system gravity experiments.Comment: 8 pages, 2 figures, invited talk given at the Second International Conference on Particle and Fundamental Physics in Space (SpacePart'03), 10-12 December 2003, Washington, D

Development of multiple calcium channel types in cultured mouse hippocampal neurons

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Differential Modulation of Ca V 2.3 Ca 2ϩ Channels by G␣q/11-Coupled Muscarinic Receptors

ABSTRACT Ca V 2.3 subunits are expressed in neuronal and neuroendocrine cells where they are believed to form native R-type Ca 2ϩ channels. Although R-type currents are involved in triggering neurotransmitter and hormone secretion, little is known about their modulation. Previous studies have shown that muscarinic acetylcholine receptors evoke both inhibition and stimulation of Ca V 2.3. Muscarinic inhibition of Ca V 2.3 is mediated by G␤␥ subunits, whereas stimulation is mediated by pertussis toxininsensitive G␣ subunits. In the present study, we compared modulation of Ca V 2.3 by the three G␣q/11-coupled muscarinic receptors (M1, M3, and M5). Our data indicate that these receptors trigger comparable stimulation of Ca V 2.3. The signaling pathway that mediates stimulation was meticulously analyzed for M1 receptors. Stimulation is blocked by neutralizing antibodies directed against G␣q/11, coexpression of the regulatory domain of protein kinase C␦ (PKC␦), preactivating PKC with phorbol ester, or pharmacological suppression of PKC with bisindolylmaleimide I. Stimulation of Ca V 2.3 is Ca 2ϩ -independent and insensitive to 12-(2-cyanoethyl)-6,7,12,13-tetrahydro-13-methyl-5-oxo-5H-indolo(2,3-a)pyrrolo(3,4-c)-carbazole (Gö 6976), a specific inhibitor of Ca 2ϩ -dependent PKC isozymes. These results indicate that muscarinic stimulation of Ca V 2.3 involves signaling by G␣q/11, diacylglycerol, and a Ca 2ϩ -independent PKC. In contrast to stimulation, the magnitude of Ca V 2.3 inhibition depended on receptor subtype, with M3 and M5 receptors producing much larger Ca V 2.3 inhibition than M1 receptors. Interestingly, muscarinic inhibition of Ca V 2.3 was notably enhanced during pharmacological suppression of PKC, suggesting the presence of cross-talk between G␤␥-mediated inhibition and PKC-mediated stimulation of R-type channels similar to that described previously for N-type channels