Direct observation of the spin gap in NaV2O5 by submillimeter wave ESR

Transitions between the singlet ground state and the first excited triplet state in NaV2O5 have been observed by means of submillimeter wave ESR for t he first time. The distinct Zeeman splitting clearly indicates that the fir st excited state is a triplet state. The singlet-triplet energy gap at zero field has been determined as Delta = 8.13 meV at T = 4.2 K. A considerable anisotropy of the absorption intensity with respect to the magnetic field orientation has been observed. The intensity is strongest for B//a and weak est for B//c. We propose that this result can he explained by including in the Hamiltonian the Dzyaloshinsky-Moriya interaction which breaks the selec tion rule.