Observation of Local Field Distribution in the Mixed State of Cuprate Superconductor through ESR Experiments

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Temperature dependence of Cu^<2+> ESR in Tl_2Ba_2Ca_2Cu_3O_<10> superconductor was meqsured, and various anomalies of the spectroscopic g factor, linewidth and intensity were observed. The g factor began to decrease below the transition temperature T_c, the phenomena being explained by the superconducting shielding effect against the external field. On the other hand, the linewidth was observed to broaden below the flux melting temperature T_m rather than T_c, and interpreted with the frozen local field inhomogeneity in the mixed state. The motional narrowing was suggested between T_m and T_c. The ESR intensity also indicated drastic decrease below T_c with more enhanced reduction at lower modulation frequencies, and attributed to energy dissipation due to viscous flux motion.Next, sintered, single crystalline and oriented YBa_2Cu_3O_<7-delta> samples were prepared, and ESR of the DPPH free radical applied on each sample surface was observed (spin probe method) in order to investigate … More various factors contributing to the local field inhomogeneity and distribution. Asymmetrically broadened lineshape just below T_c for the sintered pellet evolved into clear double peak structures at lower temperatueres. The ESR parameters for the individual peaks were then evaluated with computer simulations and lineshape fittings. Combined examinations of temperature dependent ESR parameters with scanning micro-probe analyzes revealed large contribution of sample inhomogeneity (steady and weak superconductivity) to the line broadening as dominant one rather than the ideal field distribution in the mixed state. More enhanced ESR broadening was observed corresponding to the fine structures for the oriented sample, while broadening for the single crystal was fairly mild, indicating applicability and effectiveness of the present spin-probe method.Related measurements of magnetization and resistivity as well as ESR on Mn-doped Y-based superconductor were performed along with comprehensive examinations on the peculiar flux behaviors and characteristic local field variations in Suprate superconductors. Less