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 YBa2Cu3Oy, y = 6.77 and 6.83. The dependence of the dynamic muon spin relaxation rate on applied magnetic field yields the rms magnitude B-loc(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*, as would be expected for a conventional magnetic transition; both B-loc(rms) and tau(c) are roughly constant between T* 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 YBa2Cu3Oy.