Muon probing in optimal and under- doped GdBCO

By means of MaxEnt-Burg, transverse-field (TF) $\mu $SR data of underdoped ($\delta $1; T$_{c }$= 81 K) and optimal doped ($\delta $0; T$_{c }$= 93 K) GdBa$_{2}$Cu$_{3}$O$_{7-\delta }$ (GdBCO) are analyzed. Site search studies for RBCO generated plausible candidates for muon sites [1] called the Balmer and Lin sites. We wish to confirm the muon-probe sites in GdBCO.~One of the two Balmer sites and the Lin site are located near O vacancies. These two sites become unstable in underdoped GdBCO($\delta $1). Positive muons are repelled by the positive O vacancies. This repulsion effects would be more pronounced as temperature increases.~ At 120 K, three signals are present in GdBCO($\delta $1), while at roomtemperature (RT), only one dominant signal remains. In contrast, three signals occur at 120 K and RT for GdBCO($\delta $0), which has much less O vacancies.~ These results support earlier studies [1] of muon-O sites.~Thus, the muons probe away from the CuO$_{2}$ plane, allowing $\mu $SR to detect magnetic fields originating from potential loop currents in these planes. [2] By ME-$\mu $SR analysis, the Balmer {\&} Lin sites in GdBCO are confirmed. Research supported by NSF-REU. \\[0pt] [1] WK Dawson et al, J Appl Phys 64 (1988) 5809 {\&} Hpf Int 63 (1990) 219.\\[0pt] [2] CM Varma, PRL 83 (1999) 3538; T Songatikamas et al, J Superconductivity and Novel Magn 23 (2010) 793.