Enhanced conductance in the normal state of Fe pnictides {\&} chalcogenides measured by quasiparticle scattering spectroscopy (QPS): evidence of orbital fluctuations

QPS reveals a conductance enhancement at a temperature, T$_{onset}$,~for RFe$_{2}$As$_{2}$ (R=Ca, Sr, Ba) and Fe$_{1.13}$Te. For Ba/Sr Fe$_{2}$As$_{2}$ and Fe$_{1.13}$Te the enhancement survives well above the magnetic and structural transition temperatures (Ba: T$_{N}\sim $132 K, T$_{onset}\sim $175 K; Sr: T$_{N}\sim $192 K, T$_{onset}\sim $240 K; Fe$_{1.13}$Te: T$_{N}\sim $60 K, T$_{onset}\sim $75 K) while for most CaFe$_{2}$As$_{2 }$junctions it disappears below T$_{N}$ and T$_{S }$(11 junctions tested, only 2 showed weak enhancement above T$_{N})$. For Co underdoped BaFe$_{2}$As$_{2}$ the enhancement coexists with the superconducting Andreev peaks while it is not observed for Co overdoped Ba122. We construct a modified phase diagram for Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ to reflect the presence of this feature for the underdoped regime.\footnote{Arham et. al, arXiv:1108.2749.} We discuss this conductance enhancement in the context of non-Fermi liquid behavior of these compounds due to orbital fluctuations.\footnote{Lee et. al, arXiv:1110.5917.} This work is supported by the Center for Emergent Superconductivity, an Energy Frontier Research Center funded by the US DOE, Office of Science, Award No. DE-AC0298CH1088. Work at Cambridge supported by EPSRC. Work at BNL under DOE Award No.DE-AC0298CH10886. Ames Lab. operated by ISU for the U.S. DOE under Award No. DE-AC02-07CH11358.