] was a real surprise and has generated tremendous interest. Although superconductivity exists in alloy that contains the element Fe, LaOMPn (with M ‫؍‬ Fe, Ni; and Pn ‫؍‬ P and As) is the first system where Fe plays the key role to the occurrence of superconductivity. LaOMPn has a layered crystal structure with an Fe-based plane. It is quite natural to search whether there exists other Fe based planar compounds that exhibit superconductivity. Here, we report the observation of superconductivity with zero-resistance transition temperature at 8 K in the PbO-type ␣-FeSe compound. A key observation is that the clean superconducting phase exists only in those samples prepared with intentional Se deficiency. FeSe, compared with LaOFeAs, is less toxic and much easier to handle. What is truly striking is that this compound has the same, perhaps simpler, planar crystal sublattice as the layered oxypnictides. Therefore, this result provides an opportunity to better understand the underlying mechanism of superconductivity in this class of unconventional superconductors.electronic properties ͉ Fe-oxypnictide A lthough superconductivity exists in alloy (1) that contains the element Fe, LaOMPn (2-9) (with M ϭ Fe, Ni; and Pn ϭ P and As) is the first system where Fe plays the key role in the occurrence of superconductivity. LaOMPn has a layered crystal structure with an Fe-based plane. It is quite natural to ask whether other Fe-based planar compounds exist that exhibit superconductivity. Here, we report the observation of superconductivity with zero resistance transition temperature at 8 K in the PbO-type ␣-FeSe compound. Although FeSe has been studied quite extensively (10, 11), a key observation is that the clean superconducting phase exists only in those samples prepared with intentional Se deficiency.FeSe comes in several phases: (i) a tetragonal phase ␣-FeSe with PbO-structure, (ii) a NiAs-type ␤-phase with a wide range of homogeneity showing a transformation from hexagonal to monoclinic symmetry, and (iii) an FeSe 2 phase that has the orthorhombic marcasite structure. The most studied of these compounds are the hexagonal Fe 7 Se 8 , which is a ferrimagnet with Curie temperature at Ϸ125 K, and monoclinic Fe 3 Se 4 .Unlike the high-temperature (high-Tc) superconductors (12) discovered Ͼ20 years ago that have a CuO 2 plane that is essential for the observed superconductivity, the tetragonal phase ␣-FeSe with PbO structure has an Fe-based planar sublattice equivalent to the layered iron-based quaternary oxypnictides, which have a layered crystal structure belonging to the P4/nmm space group (2). The crystal of ␣-FeSe is composed of a stack of edge-sharing FeSe 4 -tetrahedra layer by layer, as shown schematically in Fig. 1. Polycrystalline samples with nominal concentration FeSe 1Ϫx (x ϭ 0.03 and 0.18) were synthesized and studied. X-ray diffraction analysis of the samples in Fig. 2 shows that ␣-FeSe is dominant, and ␤-FeSe phases exist in trace amounts. This result is reasonable because in the Fe-Se binary alloy system, the...

We have carried out a systematic study of the PbO-type compound FeSe 1-x Te x (x = 0~1), where Te substitution effect on superconductivity is investigated. It is found that superconducting transition temperature reaches a maximum of Tc=15.2K at about 50% Te substitution. The pressure-enhanced Tc of FeSe 0.5 Te 0.5 is more than 10 times larger than that of FeSe. Interestingly, FeTe is no longer superconducting. A low temperature structural distortion changes FeTe from triclinic symmetry to orthorhombic symmetry. We believe that this structural change breaks the magnetic symmetry and suppresses superconductivity in FeTe.

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Mycobacteriophages are viruses that infect mycobacterial hosts such as Mycobacterium smegmatis and Mycobacterium tuberculosis. All mycobacteriophages characterized to date are dsDNA tailed phages, and have either siphoviral or myoviral morphotypes. However, their genetic diversity is considerable, and although sixty-two genomes have been sequenced and comparatively analyzed, these likely represent only a small portion of the diversity of the mycobacteriophage population at large. Here we report the isolation, sequencing and comparative genomic analysis of 18 new mycobacteriophages isolated from geographically distinct locations within the United States. Although no clear correlation between location and genome type can be discerned, these genomes expand our knowledge of mycobacteriophage diversity and enhance our understanding of the roles of mobile elements in viral evolution. Expansion of the number of mycobacteriophages grouped within Cluster A provides insights into the basis of immune specificity in these temperate phages, and we also describe a novel example of apparent immunity theft. The isolation and genomic analysis of bacteriophages by freshman college students provides an example of an authentic research experience for novice scientists.

Efficient light-emitting diodes and photovoltaic energy-harvesting devices are expected to play an important role in the continued efforts towards sustainable global power consumption. Semiconductor nanowires are promising candidates as the active components of both light-emitting diodes1, 2, 3, 4, 5, 6 and photovoltaic cells7, 8, 9, 10, primarily due to the added freedom in device design offered by the nanowire geometry. However, for nanowire-based components to move past the proof-of-concept stage and be implemented in production-grade devices, it is necessary to precisely quantify and control fundamental material properties such as doping and carrier mobility. Unfortunately, the nanoscale geometry that makes nanowires interesting for applications also makes them inherently difficult to characterize. Here, we report a method to carry out Hall measurements on single core–shell nanowires. Our technique allows spatially resolved and quantitative determination of the carrier concentration and mobility of the nanowire shell. As Hall measurements have previously been completely unavailable for nanowires, the experimental platform presented here should facilitate the implementation of nanowires in advanced practical devices