Magnetic excitations in an array of $(\mathrm{VO}{)}_2{\mathrm{P}}_2{\mathrm{O}}_7$ single crystals have been measured using inelastic neutron scattering. Until now, $(\mathrm{VO}{)}_2{\mathrm{P}}_2{\mathrm{O}}_7$ has been thought of as a two-leg antiferromagnetic Heisenberg spin ladder with chains running in the $a$ direction. The present results show unequivocally that $(\mathrm{VO}{)}_2{\mathrm{P}}_2{\mathrm{O}}_7$ is best described as an alternating spin chain directed along the crystallographic $b$ direction. In addition to the expected magnon with magnetic zone-center energy gap $\Delta =3.1\mathrm{}\mathrm{meV}$, a second excitation is observed at an energy just below $2\Delta$. The higher mode may be a triplet two-magnon bound state.

• Received 9 April 1997

DOI:https://doi.org/10.1103/PhysRevLett.79.745