Electron Spin Resonance in the superconducting state of Ba_(0.6)K_(0.4)Fe_2As_2

摘要

We report the observation of electron spin resonance (ESR) signals in a single crystal of Ba_(0.6)K_(0.4)Fe2As_2 grown by self-flux method. We observed two narrow resonant absorption signals at g-values of 4.3 and 1.99. Significantly, these signals are stronger in intensity at 5 K. They become weaker as the temperature is increased and finally vanish at T_c. The resonance at g= 4.3 (signal I) shows different temperature dependence of intensity for parallel and perpendicular orientations of the magnetic field to the iron arsenide plane. However, the resonance at g= 1.99 (signal 2) does not show much difference in temperature dependence of intensity for the two orientations. Further, temperature dependence of the linewidth of the two signals are also different. We propose that these two signals have their origin in fluctuations in the spin system as magnetic fluctuations are believed to be the origin of superconductivity in iron pnictides. Temperature dependence of intensity of signal I is indicative of Fe cluster formation in the scenario of coexistence of spin density wave and superconducting phase for this composition of the crystal.