Microwave spectroscopy of novel superconductors Original Research Article

We discuss the crossover of the form of the Cu Nuclear magnetic resonance (NMR) spin echo decay at the onset of Cu wipeout in lanthanum cuprates. Experimentally, the echo decay undergoes a crossover from Gaussian to exponential form below the temperature where the Cu NMR intensity drops. The wipeout and the change in behavior both arise because the nuclei experience spatially inhomogeneous spin fluctuations at low temperatures. We argue that regions where the spin fluctuations remain fast are localized on length scales of order 1–2 lattice spacings. The inhomogeneity is characterized by the local activation energy Ea(r); we estimate the functional form of Ea(r) for points where Ea>(r)∼0.