Inelastic Neutron Scattering from the Spin Ladder Compound ${\mathrm{}\left(\mathrm{VO}\right)\mathrm{}}_2$${\mathrm{P}}_2$${\mathrm{O}}_7$

We present results from an inelastic neutron scattering experiment on the candidate Heisenberg spin ladder vanadyl pyrophosphate, ${\mathrm{}\left(\mathrm{VO}\right)\mathrm{}}_2$ ${\mathrm{P}}_2$ ${\mathrm{O}}_7$. We find evidence for a spin-wave excitation gap of $E_{\mathrm{gap}}=3.7\mathrm{}\pm 0.2$ meV at a band minimum near $Q=0.8\mathrm{}$ ${\mathrm{\AA }}^{-1\mathrm{}}$. This is consistent with expectations for triplet spin waves in ${\mathrm{}\left(\mathrm{VO}\right)\mathrm{}}_2$ ${\mathrm{P}}_2$ ${\mathrm{O}}_7$ in the spin-ladder model and represents the first confirmation in nature of a Heisenberg antiferromagnetic spin ladder.