We present a comprehensive infrared spectroscopic study of lattice dynamics in the pnictide parent compound BaFe$_2$As$_2$. In the tetragonal structural phase, we observe the two degenerate symmetry-allowed in-plane infrared active phonon modes. Foll
owing the structural transition from the tetragonal to orthorhombic phase, we observe splitting into four non-degenerate phonon modes and a significant phonon strength enhancement. These detailed data allow us to provide a physical explanation for the anomalous phonon strength enhancement as the result of anisotropic conductivity due to Hunds coupling.
We report an infrared optical study of the pnictide high-temperature superconductor BaFe$_{1.84}$Co$_{0.16}$As$_{2}$ and its parent compound BaFe$_{2}$As$_{2}$. We demonstrate that electronic correlations are moderately strong and do not change acros
s the spin-density wave transition or with doping. By examining the energy scale and direction of spectral weight transfer, we argue that Hunds coupling emph{J} is the primary mechanism that gives rise to correlations.
We present emph{c} axis infrared optical data on a number of Ba, Sr and Nd-doped cuprates of the La$_{2}$CuO$_{4}$ (La214) series in which we observe significant deviations from the universal Josephson relation linking the normal state transport (DC
conductivity $sigma_{DC}$ measured at $T_{c}$) with the superfluid density ($rho_{s}$): $rho_{s}proptosigma_{DC}(T_{c})$. We find the violation of Josephson scaling is associated with striking enhancement of the anisotropy in the superfluid density. The data allows us to link the breakdown of Josephson interlayer physics with the development of magnetic order in the CuO$_2$ planes.
We report a novel aspect of the competition and coexistence between magnetism and superconductivity in the high-$T_{c}$ cuprate La$_{2-x}$Sr$_{x}$CuO$_{4}$ (La214). With a modest magnetic field applied $H parallel c$-axis, we monitored the infrared s
ignature of pair tunneling between the CuO$_2$ planes and discovered the complete suppression of interlayer coupling in a series of underdoped La214 single crystals. We find that the in-plane superconducting properties remain intact, in spite of enhanced magnetism in the planes.
