Spin Dynamics of the 2D Spin $\frac{1}{2}$ Quantum Antiferromagnet Copper Deuteroformate Tetradeuterate (CFTD)

The magnetic excitation spectrum in the two-dimensional (2D) $S=1/2\mathrm{}$ Heisenberg antiferromagnet copper deuteroformate tetradeuterate has been measured for temperatures up to $T\sim J/2\mathrm{}$, where $J=6.31\mathrm{}\pm 0.02\mathrm{meV}$ is the 2D exchange coupling. For $T\ll J$, a dispersion of the zone boundary energy is observed, which is attributed to a wave vector dependent quantum renormalization. At higher temperatures, spin-wavelike excitations persist, but are found to broaden and soften. By combining our data with numerical calculations, and with existing theoretical work, a consistent description of the behavior of the model system is found over the whole temperature interval investigated.