We report the electronic structure of the iron-chalcogenide superconductor, Fe 1.04 ͑Te 0.66 Se 0.34 ͒, obtained with high-resolution angle-resolved photoemission spectroscopy and density-functional calculations. In photoemission measurements, various photon energies and polarizations are exploited to study the Fermi surface topology and symmetry properties of the bands. The measured band structure and their symmetry characters qualitatively agree with our density-functional theory calculations of Fe͑Te 0.66 Se 0.34 ͒, although the band structure is renormalized by about a factor of three. We find that the electronic structures of this iron chalcogenides and the iron pnictides have many aspects in common; however, significant differences exist near the ⌫ point. For Fe 1.04 Te 0.66 Se 0.34 , there are clearly separated three bands with distinct even or odd symmetry that cross the Fermi energy ͑E F ͒ near the zone center, which contribute to three holelike Fermi surfaces. Especially, both experiments and calculations show a holelike elliptical Fermi surface at the zone center. Moreover, no sign of spin density wave was observed in the electronic structure and susceptibility measurements of this compound.
We adopt a personal approach here reviewing several calculations over the years in which we have experienced confrontations between cluster models and the shell model. In previous cluster conferences we have noted that cluster models go hand in hand with Skyrme Hartee-Fock calculations in describing states which cannot easily, if at all, be handled by the shell model. These are the highly deformed (many particle-many hole) intruder states, linear chain states e.t.c. In the present work we will consider several topics; the quadrupole moment of 6 Li, the non-existence of low lying intruders in 8 Be, and then jumping to the f 7/2 shell, we discuss the two-faceted nature of the nuclei-sometimes displaying shell model properties, other times cluster properties. I. THE QUADRUPOLE MOMENT OF THE J = 1 + STATE IN 6 LI Whereas the quadrupole moment of the deuteron is positive(Q = +2.74mb), that of the J=1 + state of 6 Li is negative, Q=-0.818(17)mb. The magnetic moment of the deuteron is µ = 0.85741 nm while that of 6 Li is 0.822 nm. There appears to be a big discrepancy between cluster model calculations and the shell model calculations. In nearly all cluster model calculations Q comes out positive. However in many shell model calculations Q comes out negative, sometimes too negative. This is an important problem that deserves further attention. See for example arguments in the literature between the cluster group[1] and the shell model group[2]. See also the recent compendium of A=6 by D.R. Tilley et. al.[3].
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