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 ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathrm{x}}$. In particular, an extensive study of spin waves in a large crystal with x=0.3 has been performed. As in ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$, a very large superexchange is found within the ${\mathrm{CuO}}_2$ layers, with $J_{?}$ ${=80\mathrm{}}_{\mathrm{-}30}^{+60\mathrm{}}$ meV. The optical-mode spin-wave energies are higher than 30 meV, indicating that $J_{\perp 1}$, the coupling between nearest-neighbor ${\mathrm{CuO}}_2$ 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, $J_{\perp 2}$, 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 ${\mathrm{CuO}}_2$ bilayers remain strongly correlated well above $T_N$, and dispersion of the acoustic modes appears 30 K above $T_N$. 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 ${\mathrm{CuO}}_{\mathrm{x}}$ 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.