Research Nuggets

This page will be filled regularly with our latest results:

Effects of doping in the s± gap

(Jan. 14, 2014)

Resistivity and scattering rate in BKFA
One liner: The s± nature of the superconducting gap in BaFe2As2 is such that in-plane doping by cobalt atoms depletes superconductivity, whereas out-of-plane potassium doping preserves the superconducting stiffness.

More details: Opposite to conventional materials, in an s± superconductor non magnetic impurity scattering mixes electron and hole states with opposing phases and, hence, is pair-breaking. The superconducting state optical conductivity (see figure) of Co doped BaFe2As2 shows a very large residual conductivity below the gap energy (hatched area). This is the signature of unpaired quasiparticles. Conversely, in the K doped material, a full gap (vanishingly small optical conductivity) is present. In the former compound, Co atoms go into the FeAs planes and, hence, act as diffusion centers. In the latter, K atoms enter the structure outside the plane and do not mix the bands at the Fermi level. The strong residual conductivity in the Co material opposed to the zero valued sub-gap optical conductivity in the K doped compound strongly supports a s± symmetry superconductor where impurity is pair-breaking.

Publication: Optical conductivity of Ba0.6K0.4Fe2As2: The effect of in-plane and out-of-plane doping the superconducting gap., Y.M. Dai, B. Xu, B. Shen, H.H. Wen, X.G. Qiu, and R.P.S.M. Lobo, EPL 104, 47006 (2013) [EPL page]

Linear scattering rate in Ba0.6K0.4Fe2As2

(Sep. 30, 2013)

Resistivity and scattering rate in BKFA
One liner: The curvature observed in the temperature dependence of the resistivity of Ba0.6K0.4Fe2As2 comes from an incoherent band with a flat lifetime and a coherent band T-linear scattering rate, which we associate to quantum critical point fluctuations.

More details: The temperature dependence of the resistivity of Ba0.6K0.4Fe2As2 has a negative curvature (top panel). We show that this can be explained by a multiband contribution to transport where one band has an incoherent-like large, temperature independent, scattering rate and another band has a sharp Drude peak with a scattering rate evolving linearly with temperature (bottom panel). The low temperature resistivity is dominated by the narrow peak whereas a crossover to the flat incoherent band takes over above ~170 K. The linear, hidden, scattering rate seems to originate from spin fluctuations close to an antiferromagnetic quantum critical point.

Publication: Hidden T-linear scattering rate in Ba0.6K0.4Fe2As2 revealed by optical spectroscopy, Y.M. Dai, B. Xu, B. Shen, X. Xiao, H.H. Wen, X.G. Qiu, C.C. Homes, and R.P.S.M. Lobo, Phys. Rev. Letters 111, 117001 (2013) [PRL page]