Muon spin relaxation and fluctuating magnetism in the pseudogap phase of YBa2Cu3Oy

We report the results of a muon spin relaxation study of slow magnetic fluctuations in the pseudogap phase of underdoped single-crystalline ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{O}_y,y=6.77$ and 6.83. The dependence of the dynamic muon spin relaxation rate on applied magnetic field yields the rms magnitude $B_{\mathrm{loc}}^{\mathrm{rms}}$ and correlation time ${\tau }_c$ of fluctuating local fields at muon sites. The observed relaxation rates do not decrease with decreasing temperature below the pseudogap onset at $T^{\ast }$, as would be expected for a conventional magnetic transition; both $B_{\mathrm{loc}}^{\mathrm{rms}}$ and ${\tau }_c$ are roughly constant between $T^{\ast }$ and the superconducting transition. NMR relaxation rates due to putative loop-current fluctuations are estimated and found to be too small to be observed. Our results put strong constraints on the theories of the anomalous pseudogap magnetism in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{O}_y$.

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