High-field electron spin resonance spectroscopy study of GdO$_{1-x}$F$_{x}$FeAs superconductors

摘要

We report a detailed investigation of GdO$_{1-x}$F$_{x}$FeAs (x = 0, 0.07 and0.14) samples by means of high-field/high-frequency electron spin resonance(HF-ESR) together with measurements of thermodynamic and transport properties.The parent GdOFeAs compound exhibits Fe long-range magnetic order below 128 K,whereas both doped samples do not show such order and are superconducting withT$_c$ = 20 K (x = 0.07) and T$_c$ = 45 K (x = 0.14). The Gd$^{3+}$ HF-ESRreveals an appreciable exchange coupling between Gd and Fe moments, throughwhich the static magnetic order is clearly seen in the parent compound. Owingto this coupling, HF-ESR can probe sensitively the evolution of the magnetismin the FeAs planes upon F doping. It is found that in both superconductingsamples, where the Fe long-range order is absent, there are short-range, staticon the ESR time scale magnetic correlations between Fe spins. Their occurrenceon a large doping scale may be indicative of the ground states' coexistence.