Muon spin relaxation studies of incommensurate magnetism and superconductivity in stage-4 La2CuO4.11 and La1.88Sr0.12CuO4 -: art. no. 014524

We report muon spin relaxation (muSR) measurements using single crystals of oxygen-intercalated stage-4 La2CuO4.11 (LCO:4.11) and La1.88Sr0.12CuO4 (LSCO:0.12), in which neutron scattering studies have found incommensurate magnetic Bragg reflections. In both systems, zero-field muSR measurements show muon spin precession below the Neel temperature T-N with frequency 3.6 MHz at T-->0, having a Bessel function line shape, characteristic of spin-density-wave systems. The amplitude of the oscillating and relaxing signals of these systems is less than half the value expected for systems with static magnetic order in 100% of the volume. Our results are consistent with a simulation of local fields for a heuristic model with (a) incommensurate spin amplitude modulation with the maximum ordered Cu moment size of similar to0.36mu(B), (b) static Cu moments on the CuO2 planes forming islands having typical radius 15-30 Angstrom, comparable to the in-plane superconducting coherence length, and (c) the measured volume fraction of magnetic muon sites V-mu increasing progressively with decreasing temperature below T-N towards V(mu)similar to40% for LCO:4.11 and 18% for LSCO:0.12 at T-->0. These results may be compared with correlation lengths in excess of 600 Angstrom and a long range ordered moment of 0.15+/-0.05mu(B) measured with neutron scattering techniques. In this paper we discuss a model that reconciles these apparently contradictory results. In transverse magnetic field muSR measurements, sensitive to the in-plane magnetic field penetration depth lambda(ab), the results for LCO:4.11 and LSCO:0.12 follow correlations found for underdoped, overdoped and Zn-doped high-T-c cuprate systems in a plot of T-c versus the superconducting relaxation rate sigma(T-->0). This indicates that the volume-integrated value of n(s)/m(*) (superconducting carrier density/effective mass) is a determining factor for T-c, not only in high-T-c cuprate systems without static magnetism, but also in the present systems where superconductivity coexists with static spin-density-wave spin order.