One-dimensional Topological Edge States of Bismuth Bilayers

The hallmark of a time-reversal symmetry protected topologically insulating state of matter in two-dimensions (2D) is the existence of chiral edge modes propagating along the perimeter of the system. To date, evidence for such electronic modes has come from experiments on semiconducting heterostructures in the topological phase which showed approximately quantized values of the overall conductance as well as edge-dominated current flow. However, there have not been any spectroscopic measurements to demonstrate the one-dimensional (1D) nature of the edge modes. Among the first systems predicted to be a 2D topological insulator are bilayers of bismuth (Bi) and there have been recent experimental indications of possible topological boundary states at their edges. However, the experiments on such bilayers suffered from irregular structure of their edges or the coupling of the edge states to substrate's bulk states. Here we report scanning tunneling microscopy (STM) experiments which show that a subset of the predicted Bi-bilayers' edge states are decoupled from states of Bi substrate and provide direct spectroscopic evidence of their 1D nature. Moreover, by visualizing the quantum interference of edge mode quasi-particles in confined geometries, we demonstrate their remarkable coherent propagation along the edge with scattering properties that are consistent with strong suppression of backscattering as predicted for the propagating topological edge states.Comment: 15 pages, 5 figures, and supplementary materia

Ultraviolet Spectrophotometry of Variable Early-Type Be and B stars Derived from High-Resolution IUE Data

High-dispersion IUE data encode significant information about aggregate line absorptions that cannot be conveniently extracted from individual spectra. We apply a new technique in which fluxes from each echelle order of a short wavelength IUE spectrum are binned together to construct low-resolution spectra of a rapidly varying B or Be star. The ratio of binned spectra obtained bright- star and faint-star phases contains information about the mechanism responsible for a star's variability, such as from pulsations or occultations of the star by ejected matter. We model the variations caused by these mechanism by means of model atmosphere and absorbing-slab codes. Line absorptions strength changes are sensitive to conditions in circumstellar clouds with T = 8,000--13,000K. To demonstrate proofs of concept, we construct spectral ratios for circumstellar structures associated with flux variability in various Be stars: (1) Vela X1 has bow-shock wind trailing its neutron star companion and shows signatures of gas at 13,000K or 26,000K medium in different sectors, (2) 88 Her undergoes episodic outbursts as its UV flux fades, followed a year later by a dimming in visible wavelengths, a result of a gray opacity that dominates as the shell expands and cools, and (3) zeta Tau and 60 Cyg exhibit periodic spectrum and flux changes, which match model absorptions for occulting clouds. Also, ratioed UV spectra of strongly pulsating stars show unique spectrophotometric signatures which can be simulated with models. An analysis of ratioed spectra obtained for a typical sample of 18 classical Be stars known to have rapid periodic flux variations indicates that 13 of them have ratioed spectra which are relatively featureless or have signatures of pulsation. Ratioed spectra of 3 others in the sample are consistent with the presence of co-rotating clouds.Comment: Latex 49 dbl-spaced pages plus 9 figures. Accepted by ApJ. Files available at ftp://nobel.stsci.edu/pub/uv

Drug-Initiated Synthesis of Cladribine-Based Polymer Prodrug Nanoparticles: Biological Evaluation and Structure Activity Relationships

International audienceBy using two reversible deactivation radical polymerization techniques, either nitroxide-mediated polymerization or reversible addition-fragmentation chain transfer polymerization, the "drug-initiated" approach was applied to cladribine (CdA) as an anticancer drug to synthesize small libraries of well-defined and self-stabilized CdA-based polymer prodrug nanoparticles, differing from the nature and the molar mass of the grown polymer, and the nature of the linker between CdA and the polymer, thus allowing structure-cytotoxicity relationships to be determined. Their biological evaluation was investigated in vitro on L1210 cancer cells. The preparation of fluorescent CdA-based nanoparticles with excellent imaging ability was also reported by applying the "drug-initiated" approach to an aggregation-induced emission-active dye

Topologically protected localised states in spin chains

We consider spin chain families inspired by the Su, Schrieffer and Hegger (SSH) model. We demonstrate explicitly the topologically induced spatial localisation of quantum states in our systems. We present detailed investigations of the effects of random noise, showing that these topologically protected states are very robust against this type of perturbation. Systems with such topological robustness are clearly good candidates for quantum information tasks and we discuss some potential applications. Thus, we present interesting spin chain models which show promising applications for quantum devices

Birefringent solid-core photonic bandgap fibers assisted by interstitial air holes

International audienceWe report on the fabrication and the characterization of a design of highly birefringent solid-core photonic bandgap fibers. A form birefringence is obtained by keeping open all interstitial air holes but two diametrically opposite and close to the fiber core. Group birefringence as high as 0.57×10-3 and 0.47×10-3 at 1.1 and 1.55 μm, respectively, are demonstrated in the first bandgap

A new boronate ester-based crosslinking strategy allows the design of nonswelling and long-term stable dynamic covalent hydrogels

International audienceTesting libraries of phenylboronic acid derivatives and diols revealed a new crosslinking couple for the formation of viscoelastic hydrogels with tunable properties and long-term stability