Neutron scattering study of magnetic excitations in YBa2Cu3O6+x.

Abstract

Inelastic neutron scattering has been used to study magnetic excitations in several single crystals of YBa{sub 2}Cu{sub 3}O{sub 6+{ital x}}. In particular, an extensive study of spin waves in a large crystal with {ital x}=0.3 has been performed. As in La{sub 2}CuO{sub 4}, a very large superexchange is found within the CuO{sub 2} layers, with {ital J}{sub {parallel}}=80{sub {minus}30}{sup +60} meV. The optical-mode spin-wave energies are higher than 30 meV, indicating that {ital J}{sub {perpendicular}1}, the coupling between nearest-neighbor CuO{sub 2} planes, is {approx gt}2 meV. From the dispersion and splitting of the acoustic modes, we obtain values of 0.020{plus minus}0.005 meV for the effective exchange between next-nearest-neighbor layers, {ital J}{sub {perpendicular}2}, and 0.035{plus minus}0.005 meV for the anisotropy parameter {ital D}. The weak anisotropy gives the planes an {ital XY}-like symmetry; no evidence has been found for any in-plane anisotropy. The CuO{sub 2} bilayers remain strongly correlated well above {ital T}{sub {ital N}}, and dispersion of the acoustic modes appears 30 K above {ital T}{sub {ital N}}. The decrease of the magnetic order parameter below 30 K in the {ital x}=0.3 crystal is interpreted as the onset of an incoherent ordering of Cu moments in the oxygen-deficient CuO{sub {italmore » x}} 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.« less