A magnetic origin of cuprate superconductivity? A MaxEnt-μSR view

The fundamental physics of cuprate superconductivity is still much deliberated after three decades of research. In contrast to phononic or polaronic roots, some major theories promote a magnetic origin. In this perspective, we review cuprate magnetism, as probed by muon-spin-rotation (μSR) in RBa2Cu3O7−δ (RBCO), Bi2Sr2CaCu2O8+x (Bi2212) and Tl2Ba2Ca2Cu3O10+x (Tl2223). Site-search RBCO studies show that muons localize and probe in locations away from the superconducting CuO2 planes. Maximum entropy (MaxEnt, ME) analysis of transverse field μSR data of GdBa2Cu3O7−δ (GdBCO) indicates that the muon probes in an undisturbed insulating environment, allowing μSR to detect (weak) magnetic features in these cuprates. Concerning Varma’s predicted loop currents, MaxEnt has shown weak μSR signals for GdBCO in zero field above and below the critical temperature, Tc; these are near the predicted ∼ 100 Oe. Concerning Zhang’s predicted antiferromagnetism (AF) connected to the vortex cores, we have observed Lorentzian relaxation of cuprate vortex signals below half Tc, consistent with AF-broadening effects. For both Bi2212 and Tl2223, Lorentzians describe the μSR vortex signals much better below 0.4Tc than Gaussians, indicating that extra AF fields occur near and in the vortex cores. In sum, both our MaxEnt-μSR (ME-μSR) studies point toward magnetic roots of cuprate superconductivity.

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