Coexistence of Topological Edge State and Superconductivity in Bismuth Ultrathin Film.

Ultrathin freestanding bismuth film is theoretically predicted to be one kind of two-dimensional topological insulators. Experimentally, the topological nature of bismuth strongly depends on the situations of the Bi films. Film thickness and interaction with the substrate often change the topological properties of Bi films. Using angle-resolved photoemission spectroscopy, scanning tunneling microscopy or spectroscopy and first-principle calculation, the properties of Bi(111) ultrathin film grown on the NbSe2 superconducting substrate have been studied. We find the band structures of the ultrathin film is quasi-freestanding, and one-dimensional edge state exists on Bi(111) film as thin as three bilayers. Superconductivity is also detected on different layers of the film and the pairing potential exhibits an exponential decay with the layer thicknesses. Thus, the topological edge state can coexist with superconductivity, which makes the system a promising platform for exploring Majorana Fermions.

Majorana Zero Mode Detected with Spin Selective Andreev Reflection in the Vortex of a Topological Superconductor.

Recently, theory has predicted a Majorana zero mode (MZM) to induce spin selective Andreev reflection (SSAR), a novel magnetic property which can be used to detect the MZM. Here, spin-polarized scanning tunneling microscopy or spectroscopy has been applied to probe SSAR of MZMs in a topological superconductor of the Bi_{2}Te_{3}/NbSe_{2} heterostructure. The zero-bias peak of the tunneling differential conductance at the vortex center is observed substantially higher when the tip polarization and the external magnetic field are parallel rather than antiparallel to each other. This spin dependent tunneling effect provides direct evidence of MZM and reveals its magnetic property in addition to the zero energy modes. Our work will stimulate MZM research on these novel physical properties and, hence, is a step towards experimental study of their statistics and application in quantum computing.

De novo synthesis and assembly of multiplex riboswitches in vitro.

Pools of short synthetic oligonucleotides (oligos) are required in the multiplex and parallel DNA construction. Microarray technology provides a fast and economical mean for massive parallel synthesis of oligos. The method of oligo synthesis with the programmable microfluidic PicoArray could simultaneously synthesize the designed oligos for multiple riboswitch genes. The synthetic oligos were recovered and purified as a pool of oligo mixture (OligoMix). Three temperature steps were employed to denature, anneal and extend the designed OligoMix until, after multiple rounds of thermocycling, the riboswitches with the desired length are obtained. The OligoMix was amplified using this PCR-based technique and the flanking adapter segments were cleaved for following assembly. Based on these oligos derived from 197 riboswitch sequences, the method of simultaneous assembling multiplex riboswitches (SAMRs) showed high fidelity by sequence identification. The resultant error rate was determined to be 2.78 per thousand. With the templates from SAMRs, in vitro transcription was applied to produce milligram amounts of biologically active riboswitches. With the verification of biological activity based on the high specificity of recognizing small-molecule metabolites as well as the DNA sequence redivivus by RT-PCR, the assembled riboswitches can be used for further gene operation and biological application.