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.Comment: 4 pages, 5 figure

The nature of the nitrite disorder in Na3ONO2

The nature of the disorder present in Na3ONO2 has been studied using variable temperature solid state nuclear magnetic resonance techniques. Using N-15 MAS and static NMR line shape analysis, the disorder could be unambiguously ascribed to a reorientational motion of the nitrite anion. A detailed analysis of the line shapes revealed that at room temperature the dynamic is characterized by a rapid pseudo-isotropic reorientation of the NO2- group, in which the nitrogen atom occupies the six possible orientations with equal probability. The exchange process is not characterized by a continuous decrease in the exchange frequency with decreasing temperature, but has rather to be described by a dynamic order process. Upon decreasing the temperature, one of the six possible split positions for the nitrogen atom becomes increasingly favorable, leading to a preferential occupation of the two nitrite orientations related to this split position. Thus, the transition from H-Na3ONO2 (isotropically reorienting nitrite ions) to the M-phase is continuous and characterized by the continuous order process of the nitrite ion