Neutron Scattering Study of Magnetic Excitation Spectra of Ba(Fe0.9Co0.1)2As2

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We present magnetic excitation spectra obtained for aligned crystals of Ba(Fe0.9Co0.1)2As2 with the superconducting transition temperature T_{\text{c}} \sim 23 K. The simple comparison between these data and those of model calculations for the so-called S_{++} symmetry of the superconducting order parameter indicates that the peak width of the spectra observed at the M point can be explained by the symmetry, although the material parameters used in the calculation may have certain ambiguity. Arguing the results in more detail, we point out that the possibility of the S_{++} symmetry cannot be excluded as the one realized in this Fe pnictide superconductor. It is consistent with our previous finding that the rate of T_{\text{c}}-suppression by nonmagnetic impurities is much smaller than the value expected for the S_{\pm} symmetry. It suggests the existence of a new pairing mechanism in Fe pnictide superconductors.
2011-07-15