Charge Dynamics in 122 Iron-Based Superconductors by Aliaksei Charnukha

By Aliaksei Charnukha

This thesis combines hugely exact optical spectroscopy info at the lately came upon iron-based high-temperature superconductors with an incisive theoretical research. 3 impressive effects are pronounced: (1) The superconductivity-induced amendment of the far-infrared conductivity of an iron arsenide with minimum chemical sickness is quantitatively defined by way of a strong-coupling conception for spin fluctuation mediated Cooper pairing. The formalism built during this thesis additionally describes previous spectroscopic facts on extra disordered compounds. (2) a similar fabrics convey a pointy superconductivity-induced anomaly for photon energies round 2.5 eV, orders of value better than the superconducting power hole. the writer offers a qualitative interpretation of this unparalleled statement, that is in keeping with the multiband nature of the superconducting country. (3) The thesis additionally develops a finished description of a superconducting, but optically obvious iron chalcogenide compound. the writer indicates that this hugely strange habit should be defined because of the nanoscopic coexistence of insulating and superconducting stages, and he makes use of a mix of 2 complementary experimental tools - scanning near-field optical microscopy and low-energy muon spin rotation - to at once photograph the section coexistence and quantitatively be sure the section composition. those facts have very important implications for the translation of information from different experimental probes.

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Journal of the American Chemical Society, 128, 10012–10013. 2. , & Hosono, H. (2008). 12) with Tc = 26 K. Journal of the American Chemical Society, 130, 3296. 3. , & Akimitsu, J. (2001). Superconductivity at 39 K in magnesium diboride. Nature, 410, 63–64. 4. , et al. (2008). Superconductivity at 55 K in iron-based F-doped layered quaternary compound Sm[O1−x Fx ]FeAs. Chinese Physics Letters, 25, 2215. 5. , & Johrendt, D. (2008). Superconductivity at 38 K in the iron arsenide (Ba1−x Kx )Fe2 As2 .

3 Determination of the pairing strength is difficult and in most cases involves a significant amount of modeling, although in some cases some insight can be gained already from a simple comparing of the raw data. 32 Fe2 As2 single crystals to those of well-known superconductors, P. Popovich et al. came to the conclusion that this particular compound is a strongly-coupled superconductor [21]. Similar analysis has been applied to Ba(Fe1−x Cox )2 As2 [97] and other iron-based compounds. The strength of the superconducting pairing interaction can also be determined from the quasiparticle effective-mass renormalization, which can be inferred from QO measurements, as discussed earlier in this chapter.

Both ARPES and QO techniques allow for a comparison of the experimentally inferred band structure with the predictions of first-principles calculations. The information extracted from this comparison is twofold. Firstly, the overall adequacy of the ab initio description of a given class of materials can be assessed by comparing the gross band dispersions in the experimentally accessible energy range; the comparison of the widths of the bands tells one how strong high-energy correlations are in the system: the more compressed the experimental bands appear with respect to their calculated counterparts, the stronger the high-energy correlations are [51].

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