Cu NMR Study of Spin Dynamics in Low - dimensional Spin 1 / 2 Antiferromagnets

63 Cu and 170 nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) experiments are reported on copper-oxide compounds related to high temperature superconductors that are nearly ideal realizations of spin 1/2 Heisenberg antiferromagnets with different geometries of the magnetic interactions: 1 dimensional spin chains, 2 dimensional planes, two coupled chains (two-leg ladder), and three coupled chains (three-leg ladder). Comparison of the spin-lattice relaxation rate, 1/T 1 , for 6 3Cu and 170 reveals the wave-vector, q, dependence of low-energy magnetic fluctuations, and 1/T 2G, the Gaussian spin-spin relaxation rate provides information about the electron spin correlation length, . In the Id material, Sr 2 CuO 3, 171/T 1 (q = 0) oc aT + bT 2 over the whole temperature range 10 to 700 K. Frequency dependence measurements show that diffusive contributions dominate T1 (q r 0) for the double chain id material, SrCuO 2. For the undoped 2d copper oxide material, Sr 2 CuO 2 Cl2 , we demonstrate that 170 1/T 1 measures the spin wave damping in the undoped antiferromagnet for short wavelengths. We find that the spin wave damping is small, clarifying one of the unique properties of these 2d copper-oxide antiferromagnetic materials: there is a wide temperature range where short range spin excitations exist with long lifetimes, without long range 3-dimensional order. The two-leg ladder materials, SrCu 20 3 and A14 Cu 24 0 4 1 (A = La,Sr,Ca), have a large energy gap for spin excitations. There is a crossover in magnetic fluctuations from temperatures below the spin gap to above the spin gap. For the doped two-leg ladders, the effective doping of the ladders changes with temperature, and this temperature is correlated to the magnetic spin gap energy. The three-leg ladder material, Sr 2 Cu 3 0 5 , demonstrates a crossover in the temperature dependence of the spin correlation length, 6. At high temperatures, we find the 6~ 1/T behavior characteristic of a id structure (isolated three-leg ladders). At lower temperatures, the spin correlation length diverges exponentially, which suggests that weak coupling between ladders is creating an effective 2d system. Thesis Supervisor: Takashi Imai Title: Assistant Professor of Physics