Magnetic excitation spectra of superconducting Ca-Fe-Pt-As system

By using the spectrometer 4SEASONS installed at the pulse neutron facility of J-PARC, magnetic excitation spectra have been measured for a large single crystal of the Ca-Fe-Pt-As system, which belongs to one of the series of superconducting systems with FeAs planes. The superconducting transition temperature T_c was found to be ∼30 K. The intensity map of the magnetic inelastic scattering exhibits two characteristics commonly observed for many Fe pnictide systems: One is that a peak of the scattering intensity exists at the reciprocal point [-(0.5;0)] in the (Q_x-Q_y) plane of the FeAs square lattice. The other is that the dispersion curves of this excitation are so steep that the two curves with the wave vectors Q = (0.5 ± δ 0) are not separately observed up to at least 20 meV. To identify the symmetry of the superconducting order parameter Δ, we have plotted the raw data in various forms and obtained the following. Although the spectral weight x"(Q; ΔE) of the magnetic excitation at Q = (0.5;0) is enhanced with decreasing temperature T through T_c in the broad transfer-energy (AE) region at around ∼12.5meV, it seems not to be very significant as compared with the sharp and strong enhancement in both the Q and ω (= ΔE) spaces expected for the so-called S_± symmetry of Δ. This result does not change after considering the resolutions of Q and ΔE. The observed shape of the peak structure is intrinsic, and seems to be favorable for the S₊₊ symmetry.