The Study Of 122 Iron-Based Superconductors By Optical Spectroscopy

The discovery of the disappearance of resistivity at about 4.2K in mercury by Kamerlingh Onnes, opened a new field of superconducting physics. Superconductivity has been one of the most fascinating subject in physics for more than a century. It has emerged a large number of experimental and theoretical physicists who have made a lot of achievements in this field. The most famous achievements should be Ginzburg-Landau theory and Bardeen-Cooper-Schrieff(BCS) theory. However, the discovery of the high temperature superconductivity in Perovskite structure LaBaCuO compound by Bednorz and Muller in 1986 broke the superconducting transition temperature limit predicted by BCS theory. Then, a series of copper oxide superconductors came out to increase the superconducting transition temperature continuously, which could not be explained by mechanism of the interaction between electron and phonon in BCS theo-ry. In 2006, by the discovery of the superconductivity in LaFePO-xFx compound by Japanese physicist Hosono's group, iron-based superconductors became a new member of the superconducting family. Iron-based superconductors have many novel properties and more complex band structure other than copper-based superconductors. To unveil the mechanism of these properties, we should understand its charge excitations and dynamics of relevant carriers.Using Raman spectroscopy, we report direct measurements of charge nematic fluc-tuations in the tetragonal phase of strain — free Ba(Fe1-x;Cox)2As2 single crystals. The strong enhancement of the Raman response at low temperatures unveils an un-derlying charge nematic state that extends to superconducting compositions and which has hitherto remained unnoticed. Comparison between the extracted charge nematic susceptibility and the elastic modulus allows us to disentangle the charge contribution to the nematic instability, and to show that charge nematic fluctuations are weakly coupled to the lattice. We also report Raman scattering measurements of charge ne-matic fluctuations in tetragonal phase of BaFe2As2 and Sr(Fe1-xCox)2As2(x=0.04) single crystals. In both systems, the observed nematic fluctuations are found to ex-hibit divergent Curie-Weiss like behavior with very similar characteristic temperature scales, indicating a universal tendency towards charge nematic order in 122 iron-based superconductors.We also report Raman scattering measurements of Sr(Fe1-xCox)2As2 single crys-tals in their magnetic-Spin Density Wave (SDW) phase. The spectra display multiple, polarization-resolved SDW gaps as expected in a band-folding itinerant picture for a multiband system. The temperature evolution of the SDW gaps reveals an unusual evolution of the reconstructed electronic structure with at least on gap being activated only well below the magnetic SDW transition TN. A comparison with temperature dependent Hall measurements allow us to assign this activated behavior to a change in the Fermi surface topology deep in the SDW phase, which we attribute to the disap-pearance of a hole-like Fermi pocket. Our results highlight the strong sensitivity of the low energy electronic structure to temperature in iron-arsenide superconductors.Finally, we measure the infrared reflectivity of Ba(Fe1-xNix)2As2(x= 0.048) single crystal from room temperature down to 20 K. Two Drude terms and a Lorentz term well describe the real part of the optical conductivity ?1(?). We fit the reciprocal of static optical conductivity 1/?1(O) by the power law ?(T)=?o+ATn with n=1.6. The "broad" Drude component exhibits an incoherent background with a T-independent scattering rate 1/?b, while the other "narrow" one reveals a T-quadratic scattering rate 1/?n, indicating a hidden Fermi-liquid behavior in Ba(Fe1-xNix)2As2 compound.