A low-dimensional spin S = 1/2 system at the quantum critical limit:   Na2V2O7
  We report the results of measurements of the dc-susceptibility and the 23Na-NMR response of Na2V2O7, a recently synthesized, non metallic low dimensional spin system. Our results indicate that upon reducing the temperature to below 100 K, the V^{4+} moments are gradually quenched, leaving only one moment out of 9 active. The NMR data reveal a phase transition at very low temperatures. With decreasing applied field H, the critical temperature shifts towards T = 0 K, suggesting that Na2V2O7 may be regarded as an insulator reaching a quantum critical point at H = 0. 
J. L. Gavilano, D. Rau, S. Mushkolaj, H. R. Ott, P. Millet, and F.   Mila