The newly discovered iron-chalcogenide superconductorKxFe2ySe2exhibits a distinct electronic structure from other iron-based superconductors.Exploiting polarization-dependent angle-resolved photoemission spectroscopy,we have determined the orbital characters of band structure in aKxFe2ySe2superconductor.To a large extent,we find thatKxFe2ySe2superconductor shares similar orbital characters with other iron-based superconductors,but with its own characteristics.For example,we have resolved two highly degenerate electron cylinders around the zone corner in the s and p geometries,respectively,indicating negligible interactions between them.Moreover,in contrast to the band calculation results,the small electron pocket around Z is found to be mainly consisted of the d z 2 orbital.The determined orbital characters would help to construct a realistic model forKxFe2ySe2.
Graphene-doped polymer nanofibers are fabricated by taper drawing of solvated polyvinyl alcohol doped with liquid-phase exfoliated graphene flakes.Nanofibers drawn this way typically have diameters measured in hundreds of nanometers and lengths in tens of millimeters;they show excellent uniformity and surface smoothness for optical waveguiding.Owing to their tightly confined waveguiding behavior,light–matter interaction in these subwavelength-diameter nanofibers is significantly enhanced.Using approximately 1350-nm-wavelength femto-second pulses,we demonstrate saturable absorption behavior in these nanofibers with a saturation threshold down to 0.25 pJ pulse^(-1)(peak power,1.3 W).Additionally,using 1064-nm-wavelength nanosecond pulses as switching light,we show all-optical modulation of a 1550-nm-wavelength signal light guided along a single nanofiber with a switching peak power of,3.2 W.
Angle-resolved photoemission spectroscopy (ARPES) has played an important role in determining the band structure and the superconducting gap structure of iron-based superconductors. In this paper, from the ARPES perspective, we briefly review the main results from our group in recent years on the iron-based superconductors and their parent compounds, and depict our current understanding on the antiferromagnetism and superconductivity in these materials.
