Competition between superconductivity and spin-density

In-plane and c-axis optical spectroscopy study on
Nan Lin Wang
Institute of Physics, Chinese Academy of Sciences, Beijing, China
I present our optical spectroscopic measurement on both parent and hole or electron
doped BaFe$_2$As$_2$. We find that the optical conductivity spectra contain not
only the free carrier response at low frequency (which experiences dramatic change
upon entering the spin-density-wave (SDW) state for the parent compound), but also a
gap-like suppression at rather high energy scale near 0.6 eV in all measurement
temperatures. We elaborate that the feature is caused by the strong intra-atomic
Hund's rule coupling effect due to the presence of multiple orbitals. Our experiment
unambiguously demonstrates the coexistence of itinerant and localized electrons in
iron-pnictides. I shall also present our recent optical data on the newly discovered
superconducting system K0.8Fe2-xSe2. We identify that the parent compound is
neither a SDW metal nor a Mott insulator, but a semiconductor with a small energy
gap. The superconductivity is extremely close to the semiconductor phase. We
elaborate that the Fe vacancies and their orderings should play crucial roles in
understanding their physical properties. The measurement results should lead to a
more comprehensive understanding about the nature of magnetism in the materials.
Work done with Z. G. Chen, W. Z. Hu, G. Li, B. Cheng, J. Dong, T. Dong, R. H. Yuan,
P. Zheng, G. F. Chen, J. L. Luo, Z. Fang, X. Dai, C. L. Zhang and P. Dai
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