Magnetic-filed and angular dependence of magnetism in the triangular Mott insulator k-(BEDT-TTF)2Cu2(CN)3 investigated by 13C NMR

The organic conductors, $\kappa$-(BEDT-TTF)$_{2}$X, are prototype for investigating Mott physics and spin frustrations. The X=Cu[N(CN)$_{2}$]Cl is a Mott insulator which undergoes an antiferromagnetic phase transition. On the other hand, title compound which has a triangular lattice does not show a long range magnetic ordering. This suppression is believed to deeply relate to strong spin frustrations. While X=Cu$_{2}$(CN)$_{3}$ does not show magnetic ordering, we previously reported anomalous behaviors in $^{13}$C NMR around 6 K, where heat capacity, thermal conductivity and lattice constant show anomalies as well. So, the 6 K anomaly is a key phenomenon for understanding the origin of absence of magnetic order. We have measured external-filed angular dependence of $^{13}$C NMR under magnetic fields up to 15 T for clarifying the origins of the line broadening and the $1/T_{1}$ anomaly around 6K. At room temperature, angular dependence of spectra is well explained by a crystal structure. We will show the detailed experimental results and discuss the low temperature states.