Two-dimensional zone-center spin-wave excitations in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">La</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>Cu<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">O</mml:mi></mml:mrow><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

Abstract

We have studied the spin-wave excitations near the two-dimensional zone center at 80 K in a single crystal of ${\mathrm{La}}_{2}$Cu${\mathrm{O}}_{4}$ which orders at ${T}_{N}=195$ K. In-plane and out-of-plane modes are observed at energies of 1.0\ifmmode\pm\else\textpm\fi{}0.25 meV and 2.5\ifmmode\pm\else\textpm\fi{}0.5 meV, respectively, The in-plane mode energy is determined primarily by the antisymmetric exchange and it measures this term directly. The out-of-plane mode energy corresponds to a planar anisotropy energy of \ensuremath{\sim}0.016 meV. We observe, in addition, the appearance of significant $E\ensuremath{\simeq}0$ scattering below 30 K.