Blockwise SVD with error in the operator and application to blind deconvolution

We consider linear inverse problems in a nonparametric statistical framework. Both the signal and the operator are unknown and subject to error measurements. We establish minimax rates of convergence under squared error loss when the operator admits a blockwise singular value decomposition (blockwise SVD) and the smoothness of the signal is measured in a Sobolev sense. We construct a nonlinear procedure adapting simultaneously to the unknown smoothness of both the signal and the operator and achieving the optimal rate of convergence to within logarithmic terms. When the noise level in the operator is dominant, by taking full advantage of the blockwise SVD property, we demonstrate that the block SVD procedure overperforms classical methods based on Galerkin projection or nonlinear wavelet thresholding. We subsequently apply our abstract framework to the specific case of blind deconvolution on the torus and on the sphere

Scanning probe microscopy imaging of metallic nanocontacts

We show scanning probe microscopy measurements of metallic nanocontacts between controlled electromigration cycles. The nanowires used for the thinning process are fabricated by shadow evaporation. The highest resolution obtained using scanning force microscopy is about 3 nm. During the first few electromigration cycles the overall slit structure of the nanocontact is formed. The slit first passes along grain boundaries and then at a later stage vertically splits grains in the course of consuming them. We find that first the whole wire is heated and later during the thinning process as the slit forms the current runs over several smaller contacts which needs less power.Comment: 4 pages, 4 figure

Suppression of spin-pumping by a MgO tunnel-barrier

Spin-pumping generates pure spin currents in normal metals at the ferromagnet (F)/normal metal (N) interface. The efficiency of spin-pumping is given by the spin mixing conductance, which depends on N and the F/N interface. We directly study the spin-pumping through an MgO tunnel-barrier using the inverse spin Hall effect, which couples spin and charge currents and provides a direct electrical detection of spin currents in the normal metal. We find that spin-pumping is suppressed by the tunnel-barrier, which is contrary to recent studies that suggest that the spin mixing conductance can be enhanced by a tunnel-barrier inserted at the interface

CENP-A Is Dispensable for Mitotic Centromere Function after Initial Centromere/Kinetochore Assembly

Human centromeres are defined by chromatin containing the histone H3 variant CENP-A assembled onto repetitive alphoid DNA sequences. By inducing rapid, complete degradation of endogenous CENP-A, we now demonstrate that once the first steps of centromere assembly have been completed in G1/S, continued CENP-A binding is not required for maintaining kinetochore attachment to centromeres or for centromere function in the next mitosis. Degradation of CENP-A prior to kinetochore assembly is found to block deposition of CENP-C and CENP-N, but not CENP-T, thereby producing defective kinetochores and failure of chromosome segregation. Without the continuing presence of CENP-A, CENP-B binding to alphoid DNA sequences becomes essential to preserve anchoring of CENP-C and the kinetochore to each centromere. Thus, there is a reciprocal interdependency of CENP-A chromatin and the underlying repetitive centromere DNA sequences bound by CENP-B in the maintenance of human chromosome segregation

Multi-excitonic complexes in single InGaN quantum dots

Cathodoluminescence spectra employing a shadow mask technique of InGaN layers grown by metal organic chemical vapor deposition on Si(111) substrates are reported. Sharp lines originating from InGaN quantum dots are observed. Temperature dependent measurements reveal thermally induced carrier redistribution between the quantum dots. Spectral diffusion is observed and was used as a tool to correlate up to three lines that originate from the same quantum dot. Variation of excitation density leads to identification of exciton and biexciton. Binding and anti-binding complexes are discovered.Comment: 3 pages, 4 figure

Surface spin flip probability of mesoscopic Ag wires

Spin relaxation in mesoscopic Ag wires in the diffusive transport regime is studied via nonlocal spin valve and Hanle effect measurements performed on permalloy/Ag lateral spin valves. The ratio between momentum and spin relaxation times is not constant at low temperatures. This can be explained with the Elliott-Yafet spin relaxation mechanism by considering the momentum surface relaxation time as being temperature dependent. We present a model to separately determine spin flip probabilities for phonon, impurity and surface scattering and find that the spin flip probability is highest for surface scattering.Comment: 5 pages, 4 figure

Methanol fractionations of Catha edulis frosk (Celastraceae) contracted lewis rat aorta in vitro: a comparison between crimson and green leaves

The study investigated the effect of methanol extract and its fractionations obtained from Yemeni khat on the smooth muscle isometric tension in Lewis rat aortal ring preparations and compared the effects of the crimson and green leaves. Khat leaves were sorted into green (khat Light; KL) and crimson (khat Dark; KD) leaves, extracted with methanol, followed with solvent-solvent extraction (benzene, chloroform and ethylacetate). The contractile activity of the fractions was tested using aortal ring preparations. The control (phenylepherine contraction) methanol extracts contracted aortas at concentrations 250, 125 and 67.5 μg /1 ml buffer by 80.2% , 57.3%, 26.4% and 81.5%, 65.6% , 24.6% for KL and KD, respectively. Fractions of benzene (BF) and ethylacetate (EaF) contracted the aorta with 2μgm, whereas, chloroform (ChF) with 1 μgm / 1 ml buffer was less potent. The shape of contraction curve produced by EaF differed from that of ChF and BF of both (KL and KD). The EaF induced-contraction peaked after 3.3 ± 0.94 mins, whereas those of BF and CHF peaked after 18.0 ± 2.2, 19.7 ± 0.94 mins, respectively. Pre-incubation with nifedipine (10-6 M) insignificantly reduced the contraction induced by all fractionations, but prazosin (10-6 M) reduced the contraction by 81.9%, 63.1%, 71.8% with p= 0.23, 0.09, 0.15 for BF, ChF and EaF of KL, respectively. It significantly reduced contraction of ChF, 64.1%; p= 0.02, and of EaF, 73.5%; p= 0.04 of KD, while the reduction in contraction of BF was 63.1%; p= 0.06. In conclusion, fractions of green and crimson Yemeni khat leaves contracted aortas of Lewis rats. Both leaves behave almost similarly. Contraction induced by chloroform fraction produced alphasympathetic activity.Key words: Catha edulis, aorta contraction, rat, cathinone