Low-temperature NMR studies of Na2V3O7

We report low-temperature Na-23-NMR measurements on the quasi-one-dimensional spin 1/2 system Na2V3O7. The temperature dependence of the spin-lattice relaxation rate T-1(-1)(T), measured at a frequency of 76.5 MHz, exhibits a narrow peak near 2.5 K, indicating the onset of a cooperative phase transition to a state with a small gap in the spectrum of V spin excitations. Considering the results of the NMR spectra at low temperatures and of the magnetic susceptibility at higher temperatures, it seems unlikely that this transition reflects the onset of either ferromagnetic or antiferromagnetic order. The nature of the transition needs to be investigated in more detail. (C) 2002 Elsevier Science B.V. All rights reserved.