Band and momentum dependent electron dynamics in superconducting ${rm Ba(Fe_{1-x}Co_{x})_2As_2}$ as seen via electronic Raman scattering

We present details of carrier properties in high quality ${rm Ba(Fe_{1-x}Co_{x})_2As_2}$ single crystals obtained from electronic Raman scattering. The experiments indicate a strong band and momentum anisotropy of the electron dynamics above and below the superconducting transition highlighting the importance of complex band-dependent interactions. The presence of low energy spectral weight deep in the superconducting state suggests a gap with accidental nodes which may be lifted by doping and/or impurity scattering. When combined with other measurements, our observation of band and momentum dependent carrier dynamics indicate that the iron arsenides may have several competing superconducting ground states.

First-Order Type Effects in YBa$_2$Cu$_3$O$_{6+x}$ at the Onset of Superconductivity

We present results of Raman scattering experiments on tetragonal ${rm (Y_{1-y}Ca_{y})Ba_{2}Cu_{3}O_{6+x}}$ for doping levels $p(x,y)$ between 0 and 0.07 holes/CuO$_2$. Below the onset of superconductivity at $p_{rm sc1} approx 0.06$, we find evidence of a diagonal superstructure. At $p_{rm sc1}$, lattice and electron dynamics change discontinuously with the charge and spin properties being renormalized at all energy scales. The results indicate that charge ordering is intimately related to the transition at $p_{rm sc1}$ and that the maximal transition temperature to superconductivity at optimal doping $T_{c}^{rm max}$ depends on the type of ordering at $p>p_{rm sc1}$.