C. T. Chen

Octet-Line Node Structure of Superconducting Order Parameter in KFe2As2

An Eight-Noded Monster In superconductors, electrons are bound into pairs, and the exact form of that pairing and the resulting energy gap can vary, depending on the details of the electron-electron interaction and the band structure of the material. The energy gaps of the recently discovered iron-based superconductors exhibit a variety of pairing functions. KFe2As2 has been suggested to have a d-wave gap, similar to cuprate superconductors. Okazaki et al. (p. 1314) use laser-based angle-resolved photoemission spectroscopy (ARPES) to map out the superconducting gap on three Fermi surfaces (FS) of the compound. They find a different gap structure on each, with the middle FS gap vanishing at eight distinct positions (nodes). It appears that the gap respects the tetragonal symmetry of the crystal, indicating (although the details may vary) the all iron-based superconductors have an extended s-wave–symmetric pairing—a finding that will help understanding of unconventional superconductivity. Laser-based photoemission spectroscopy is used to map out the pairing gap of an iron-based superconductor. In iron-pnictide superconductivity, the interband interaction between the hole and electron Fermi surfaces (FSs) is believed to play an important role. However, KFe2As2 has three zone-centered hole FSs and no electron FS but still exhibits superconductivity. Our ultrahigh-resolution laser angle-resolved photoemission spectroscopy unveils that KFe2As2 is a nodal s-wave superconductor with highly unusual FS-selective multi-gap structure: a nodeless gap on the inner FS, an unconventional gap with “octet-line nodes” on the middle FS, and an almost-zero gap on the outer FS. This gap structure may arise from the frustration between competing pairing interactions on the hole FSs causing the eightfold sign reversal. Our results suggest that the A1g superconducting symmetry is universal in iron-pnictides, in spite of the variety of gap functions.

Ab initio studies on the optical effects in the deep ultraviolet nonlinear optical crystals of the KBe2BO3F2 family

Electronic structures of the deep ultraviolet nonlinear optical crystals of the KBe(2)BO(3)F(2) (KBBF) family, including KBBF, RbBe(2)BO(3)F(2) and CsBe(2)BO(3)F(2), have been investigated based on a plane-wave pseudopotential method. Their linear and nonlinear optical coefficients are also calculated, and are in good agreement with the experimental results. A real-space atom-cutting method is adopted to analyze the respective contributions of the alkali metal cations and anionic groups to optical response. The results show that the contributions of anionic groups to the nonlinear optical anisotropic responses are dominant, but the influence of the A-site alkali metal cations becomes slightly more pronounced with the increase of their radius. Moreover, the birefringence difference among these crystals strongly depends on the volume effect, i.e., the spatial density of the (BO(3))(3-) anionic groups.

Abrupt change in the energy gap of superconducting Ba1−xKxFe2As2single crystals with hole doping

We performed a Laser angle-resolved photoemission spectroscopy (ARPES) study on a wide doping range of Ba1-xKxFe2As2 (BaK) and precisely determined the doping evolution of the superconducting (SC) gaps in this compound. The gap size of the outer hole Fermi surface (FS) sheet around the Brillioun zone (BZ) center shows an abrupt drop with overdoping (for x > 0.6) while the inner and middle FS gaps roughly scale with Tc. This is accompanied by the simultaneous disappearance of the electron FS sheet with similar orbital character at the BZ corner. These results browse the different contributions of X2-Y2 and XZ/YZ orbitals to superconductivity in BaK and can be hardly completely reproduced by the available theories on iron-based superconductors.

Bulk- and surface-sensitive high-resolution photoemission study of two mott-hubbard systems: SrVO3 and CaVO3.

We study the electronic structure of Mott-Hubbard systems SrVO3 and CaVO3 with bulk and surfacesensitive high-resolution photoemission spectroscopy, using a vacuum ultraviolet laser, synchrotron radiation, and a discharge lamp (h� 7–21 eV). A systematic suppression of the density of states (DOS) within 0: 2e Vof the Fermi level (EF) is found on decreasing photon energy, i.e., on increasing bulk sensitivity. The coherent band in SrVO3 and CaVO3 is shown to consist of surface and bulk-derived features, separated in energy. The stronger distortion on surface of CaVO3 compared to SrVO3 leads to a higher surface metallicity in the coherent DOS at EF, consistent with recent theory.

Laser-excited ultrahigh-resolution photoemission spectroscopy of Na x CoO 2 · y H 2 O : Evidence for pseudogap formation

We have studied the temperature

Evidence for a cos(4φ) modulation of the superconducting energy gap of optimally doped FeTe(0.6)Se(0.4) single crystals using laser angle-resolved photoemission spectroscopy.

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Energy band gap engineering in borate ultraviolet nonlinear optical crystals: ab?initio studies

-dependent electronic structure near the Fermi level of the layered cobaltate superconductor,

The sensitive magnetoimpedance effect in Fe-based soft ferromagnetic ribbons

{mathrm{Na}}_{0.35}{mathrm{CoO}}_{2}ifmmodecdotelsetextperiodcenteredfi{}1.3{mathrm{H}}_{2}mathrm{O}

Superconducting electronic state in optimally doped YBa 2 Cu 3 O 7 − δ observed with laser-excited angle-resolved photoemission spectroscopy

, and related materials, using laser-excited ultrahigh-resolution photoemission spectroscopy. We observe the formation of a pseudogap (PG) with an energy scale of

Quantitative determination of Eliashberg function and evidence of strong electron coupling with multiple phonon modes in heavily overdoped (Bi,Pb)(2)Sr2CuO6+delta

ensuremath{sim}20phantom{rule{0.3em}{0ex}}mathrm{meV}