Novel normal-state low field microwave absorption in SmFeAsO1−xFx iron pnictide superconductors

Abstract

We report on the systematic normal and superconducting states microwave absorption studies in polycrystalline samples of SmFeAsO1−xFx using an electron spin resonance (ESR) spectrometer. In the normal state, we observed two absorption spectra; at high fields (~3500 G) and low fields (around 0 G). The high field spectrum is in principle a resonance spectrum corresponding to a saturated state. It was observed that this signal broadens and its peak to peak intensity decreases on cooling the sample from room temperature. Ultimately, the signal becomes ESR ‘silent’ in the superconducting state. On the other hand, the low field microwave absorption (LFMA), centered around zero field, was observed for the first time, in a superconducting material. Based on the line-width, hysteresis and peak to peak signal intensity, we have shown that this spectrum is distinctively different from the already known LFMA signal that occurs in the superconducting state. These results present a system that shows LFMA spectrum in both normal and superconducting state thus providing an excellent platform to analyse the elusive LFMA phenomenon.