$\mu $SR search for loop currents in GdBCO

In the cuprate pseudogap phase, Varma [1] predicts loop currents above T$_{c}$. We search for magnetic fields created by such currents in GdBa$_{2}$Cu$_{3}$O$_{(7-\delta )}$ (GdBCO). Using Maximum Entropy (ME) [2] we analyze zero-field (ZF) muon-spin-resonance ($\mu $SR) data [3] of GdBCO. ME-$\mu $SR applied to ZF-GdBCO data yields a frequency distribution with T-dependent signals at 0-MHz [3] and 0.3-MHz (f1) and hints of 1.4-MHz (f*) signals. To cancel systematic f1 effects, we analyze DS(t,T) $\equiv $ S(t,T$>$T$_{c})$ - S(t,T'$<<$T$_{c})$ by ME analysis. We find, f1 disappears and weak f* signals are seen above T$_{c}$. Their existence is similar to anomalous polar-Kerr behavior observed in YBCO. [4] We attribute the f* signals at $\sim $1.3 MHz (95 Oe) for underdoped GdBCO and $\sim $1.5 MHz (110 Oe) for optimal doped GdBCO to fields created by loop currents. Our TF $\mu $SR GdBCO evidence indicates, the muon probes at muon-O sites near the BaO and CuO-chain insulating layers without disturbing potential loop currents in the CuO$_{2}$ planes.~Validating predicted loop currents [1] is essential for a better understanding of the pseudogap phase. [1] CM Varma, PRL 83 (1999) 3538. [2] C Boekema and MC Browne, MaxEnt 2008, AIP Conf Proc {\#}1073 p260; JC Lee \textit{et al,} J Appl Phys 95 (2004) 6906. [3] DW Cooke \textit{et al}, PRB37 (1988) 9401. [4] J Xia \textit{et al}, PRL 100 (2008) 127002