No abstract

Using time-domain terahertz spectroscopy we performed direct studies of the photoinduced suppression and recovery of the superconducting gap in a conventional BCS superconductor NbN. Both processes are found to be strongly temperature and excitation density dependent. The analysis of the data with the established phenomenological Rothwarf-Taylor model enabled us to determine the bare quasiparticle recombination rate, the Cooper pair-breaking rate and the electron-phonon coupling constant, λ=1.1±0.1, which is in excellent agreement with theoretical estimates.

We present a common generalization of counting lattice points in rational polytopes and the enumeration of proper graph colorings, nowhere-zero flows on graphs, magic squares and graphs, antimagic squares and graphs, compositions of an integer whose parts are partially distinct, and generalized latin squares. Our method is to generalize Ehrhart's theory of lattice-point counting to a convex polytope dissected by a hyperplane arrangement. We particularly develop the applications to graph and signed-graph coloring, compositions of an integer, and antimagic labellings.

The photo-Dember effect is a source of impulsive THz emission following femtosecond pulsed optical excitation. This emission results from the ultrafast spatial separation of electron-hole pairs in strong carrier gradients due to their different diffusion coefficients. The associated time dependent polarization is oriented perpendicular to the excited surface which is inaptly for efficient out coupling of THz radiation. We propose a scheme for generating strong carrier gradients parallel to the excited surface. The resulting photo-Dember currents are oriented in the same direction and emit THz radiation into the favorable direction perpendicular to the surface. This effect is demonstrated for GaAs and In(0.53)Ga(0.47)As. Surprisingly the photo-Dember THz emitters provide higher bandwidth than photoconductive emitters. Multiplexing of phase coherent photo-Dember currents by periodically tailoring the photoexcited spatial carrier distribution gives rise to a strongly enhanced THz emission, which reaches electric field amplitudes comparable to a high-efficiency externally biased photoconductive emitter.

Signaling by phosphatidylinositol (PI) 3-kinases is mediated by 3-phosphoinositides, which bind to Pleckstrin homology (PH) domains that are present in a wide spectrum of proteins. PH domains can be classified into three groups based on their different lipid binding specificities. Distinct 3-phosphoinositides can accumulate upon PI 3-kinase activation in cells in response to different stimuli and mediate specific cellular responses. In Swiss 3T3 mouse fibroblasts, oxidative stress induced by 1 mM H 2 O 2 caused almost exclusive accumulation of phosphatidylinositol 3,4-bisphosphate (PtdIns(3,4)P 2 ), whereas osmotic stress increased both phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P 3 ) and PtdIns-(3,4)P 2 levels. The increase in PtdIns(3,4)P 2 levels, caused by oxidative stress, correlated with the activation of protein kinase B, which has a promiscuous PH domain that binds both PtdIns(3,4,5)P 3 and PtdIns(3,4)P 2 . p70 S6 kinase, another signaling component downstream of PI 3-kinase, however, was not activated by this oxidative stress-induced increase in PtdIns(3,4)P 2 levels. Increased PtdIns(3,4,5)P 3 and PtdIns(3,4)P 2 levels in response to osmotic stress did not correlate with protein kinase B activation, because of concomitant activation of an inhibitory pathway, but p70 S6 kinase was activated by osmotic stress. These results demonstrate that PtdIns(3,4)P 2 can accumulate independently of PtdIns(3,4,5)P 3 and exerts a pattern of cellular responses that is distinct from that induced by accumulation of PtdIns(3,4,5)P 3 .The significance of PI 1 3-kinases in the regulation of a wide spectrum of cellular signaling events has been well established (1, 2). Based on structural features and substrate specificity, three classes of catalytic subunits of PI 3-kinases have so far been recognized (2, 3). The Type I class, members of which phosphorylate PtdIns, PtdIns4P, and PtdIns(4,5)P 2 in vitro but utilize PtdIns(4,5)P 2 as the likely substrate in vivo, is subdivided into Type I a and Type 1 b . Type I a PI 3-kinases interact with adaptor proteins containing SH2 domains that bind phosphotyrosine residues, linking this class to tyrosine kinase signaling cascades. Type 1 b PI 3-kinases are stimulated by Gprotein ␤␥ subunits and do not interact with SH2 domain containing adaptor proteins but with p101, a novel adaptor protein that has no homology with known proteins. Type II PI 3-kinases phosphorylate PtdIns and PtdIns4P in vitro (and not PtdIns(4,5)P 2 ) and contain a C2 domain, implicated in lipid binding. Type III PI 3-kinases only phosphorylate PtdIns and are thought to be constitutively active. They can be recruited and regulated by their adaptor, a dual specificity protein kinase, with which they form heterodimers.The principle enzymatic activity of PI 3-kinases involves the phosphorylation of phosphoinositides on the 3-position of the inositol ring, resulting in the formation of 3-phosphoinositides, which can either directly interact with appropriate modules of specific target proteins or first under...