Inelastic neutron scattering has been used to study magnetic excitations in several single crystals of YBa2Cu3O6+x. In particular, an extensive study of spin waves in a large crystal with x=0.3 has been performed. As in La2CuO4, a very large superexchange is found within the CuO2 layers, with J?=80-30+60 meV. The optical-mode spin-wave energies are higher than 30 meV, indicating that JS 1, the coupling between nearest-neighbor CuO2 planes, is 2 meV. From the dispersion and splitting of the acoustic modes, we obtain values of 0.020±0.005 meV for the effective exchange between next-nearest-neighbor layers, JS2, and 0.035±0.005 meV for the anisotropy parameter D. The weak anisotropy gives the planes an XY-like symmetry; no evidence has been found for any in-plane anisotropy. The CuO2 bilayers remain strongly correlated well above TN, and dispersion of the acoustic modes appears 30 K above TN. The decrease of the magnetic order parameter below 30 K in the x=0.3 crystal is interpreted as the onset of an incoherent ordering of Cu moments in the oxygen-deficient CuOx layers. Low-energy inelastic scattering associated with the transition is also observed. Searches for low-energy magnetic fluctuations in several superconducting single crystals have so far been unsuccessful.
All Science Journal Classification (ASJC) codes
- Condensed Matter Physics