Direct Observation of the Spin Gap in NaV_{<b> 2</b>}O_{<b> 5</b>} by Submillimeter Wave ESR

Abstract

Transitions between the singlet ground state and the first excited triplet state in NaV 2 O 5 have been observed by means of submillimeter wave ESR for the first time. The distinct Zeeman splitting clearly indicates that the first excited state is a triplet state. The singlet-triplet energy gap at zero field has been determined as Δ= 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 weakest for B // c . We propose that this result can be explained by including in the Hamiltonian the Dzyaloshinsky-Moriya interaction which breaks the selection rule.