Cuprate Pseudogap loop currents, observed by muon probing

Pertaining to Varma's loop currents (LpI) in the cuprate pseudogap phase, [1] we have observed [2] in zero field weak $\mu $SR LpI signals for two GdBCO samples above and below their T$_{\mathrm{c}}$'s of 81 K (underdoped) and 93 K (optimal doped). The measured fields are about the predicted 100 Oe. The question is, can the muon probe these fields. Shekhter \textit{et al} [3] found theoretically that positive muons destroy Varma's loop currents. In contrast, by analyzing transverse field $\mu $SR GdBCO data using MaxEnt, [4] we confirm the muon probes in \textit{insulating} regions near the BaO layers (Balmer sites) and CuO chain layers (Lin sites). [5] Thus, muons do \textit{not} destroy Varma's loop currents in the CuO$_{\mathrm{2}}$ planes, and can precisely probe their fields. In conclusion, well below RT we have detected LpI-$\mu $SR signals for GdBCO with a probability of occurrence of $\sim$ 65 {\%}. [2] Our ME$\mu $SR optimal doped GdBCO results indicate, the QCP in the cuprate phase diagram is located near the endpoint of the superconductivity dome. A magnetic origin for cuprate superconductivity is plausible. Research is supported by RSCA-SJSU and AFC San Jose. [1] CM Varma PRL \textbf{83} (1999) 3538; ME Simon, CM Varma PRL \textbf{89} (2002) 247003. [2] C Boekema \textit{et al,} 493 (2013) 136; T Songatikamas \textit{et al,} J Supercond {\&} Nov Magn \textbf{23} (2010) 793. [3] A Shekhter \textit{et al,} PRL \textbf{101} (2008) 227004. [4] C Boekema, MC Browne, AIP Conf Proc {\#}1073 (2008) 260; JC Lee \textit{et al}, J Appl Phys \textbf{95} (2004) 6906. [5] WK Dawson \textit{et al}, J Appl Phys \textbf{64} (1988) 5809; Hpf Interactions \textbf{63} (1990) 219.