20 K superconductivity in heavily electron doped surface layer of FeSe bulk crystal

A superconducting transition temperature Tc as high as 100 K was recently discovered in 1 monolayer (1ML) FeSe grown on SrTiO3 (STO). The discovery immediately ignited efforts to identify the mechanism for the dramatically enhanced Tc from its bulk value of 7 K. Currently, there are two main views on the origin of the enhanced Tc; in the first view, the enhancement comes from an interfacial effect while in the other it is from excess electrons with strong correlation strength. The issue is controversial and there are evidences that support each view. Finding the origin of the Tc enhancement could be the key to achieving even higher Tc and to identifying the microscopic mechanism for the superconductivity in iron-based materials. Here, we report the observation of 20 K superconductivity in the electron doped surface layer of FeSe. The electronic state of the surface layer possesses all the key spectroscopic aspects of the 1ML FeSe on STO. Without any interface effect, the surface layer state is found to have a moderate Tc of 20 K with a smaller gap opening of 4 meV. Our results clearly show that excess electrons with strong correlation strength alone cannot induce the maximum Tc, which in turn strongly suggests need for an interfacial effect to reach the enhanced Tc found in 1ML FeSe/STO.Comment: 5 pages, 4 figure

Electronic structures of Zn1−x_{1-x}Cox_xO using photoemission and x-ray absorption spectroscopy

Electronic structures of Zn$_{1-x}$Co$_x$O have been investigated using photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS). The Co 3d states are found to lie near the top of the O $2p$ valence band, with a peak around $\sim 3$ eV binding energy. The Co $2p$ XAS spectrum provides evidence that the Co ions in Zn$_{1-x}$Co$_{x}$O are in the divalent Co$^{2+}$ ($d^7$) states under the tetrahedral symmetry. Our finding indicates that the properly substituted Co ions for Zn sites will not produce the diluted ferromagnetic semiconductor property.Comment: 3 pages, 2 figure

Extended Optical Model Analyses of Elastic Scattering and Fusion Cross Section Data for the 7Li+208Pb System at Near-Coulomb-Barrier Energies using the Folding Potential

Simultaneous $\chi^{2}$ analyses previously made for elastic scattering and fusion cross section data for the $^{6}$Li+$^{208}$Pb system is extended to the $^{7}$Li+$^{208}$Pb system at near-Coulomb-barrier energies based on the extended optical model approach, in which the polarization potential is decomposed into direct reaction (DR) and fusion parts. Use is made of the double folding potential as a bare potential. It is found that the experimental elastic scattering and fusion data are well reproduced without introducing any normalization factor for the double folding potential and that both the DR and fusion parts of the polarization potential determined from the $\chi^{2}$ analyses satisfy separately the dispersion relation. Further, we find that the real part of the fusion portion of the polarization potential is attractive while that of the DR part is repulsive except at energies far below the Coulomb barrier energy. A comparison is made of the present results with those obtained from the Continuum Discretized Coupled Channel (CDCC) calculations and a previous study based on the conventional optical model with a double folding potential. We also compare the present results for the $^7$Li+$^{208}$Pb system with the analysis previously made for the $^{6}$Li+$^{208}$Pb system.Comment: 7 figures, submitted to PR

Critical densities for the Skyrme type effective interactions

We use the stability conditions of the Landau parameters for the symmetric nuclear matter and pure neutron matter to calculate the critical densities for the Skyrme type effective nucleon-nucleon interactions. We find that the critical density can be maximized by adjusting appropriately the values of the enhancement factor $\kappa$ associated with isovector giant dipole resonance, the quantity $L$ which is directly related to the slope of the symmetry energy and the Landau parameter $G_0^\prime$. However, restricting $\kappa$, $L$ and $G_0^\prime$ to vary within acceptable limits reduces the maximum value for the critical density $\tilde\rho_{cr}$ by $\sim 25$. We also show that among the various quantities characterizing the symmetric nuclear matter, $\tilde\rho_{cr}$ depends strongly on the isoscalar effective mass $m^*/m$ and surface energy coefficient $E_s$. For realistic values of $m^*/m$ and $E_s$ we get $\tilde\rho_{cr} = 2\rho_0$ to $3\rho_0$ ($\rho_0 = 0.16$fm$^{-3}$).Comment: 10 pages, 3 figures. Physicsl Review C (in press