Density Wave Transitions in Na2Ti2As2 and Na2Ti2As2O Investigated by NMR Measurement G. Z. Fan，X. Zhang, Y. G. Shi, J. L. Luo Institute of Physics, Chinese Academy of Sciences Na2Ti2As2/Sb2O has attracted attention recently for it takes on different ordering states and has been proposed as possible candidates for new superconductors. Both compounds exhibit anomalies in resistivity, susceptibility and specific heat suggesting charge density wave (CDW) or spin density wave (SDW) phase transition. To investigate these anomalies, we performed 23Na NMR on Na2Ti2As2O and 75As NMR on Na2Ti2Sb2O. For single crystal of Na2Ti2Sb2O, 23Na NMR spectral lineshape are fairly narrow and symmetrical with no change across the transition temperature TA≈114 K. The measurement of spin-lattice relaxation time T1 suggests a gap opening behavior just below the transition temperature. 1/T1T obeys Fermi liquid behavior at low temperature with the loss of large part of electronic density of states. No spin fluctuations or magnetic order is found. The results suggest CDW phase transition occurring. 75As NMR for powder polycrystal of Na2Ti2As2O was performed from 10 K to 300 K. The NMR lineshape does not exhibit any significant difference across 40 K at which an anomaly appears in transport transition measurement. The temperature dependence of 1/T1T shows almost linear decrease from 300 K to 40 K indicating a gap opening behavior. Below 40 K it increases with the decrease of the temperature which may suggest the emergence of antiferromagnetic correlations.