Emergence of inhomogeneous moments from spin liquid in the triangular-lattice Mott insulatorκ−(ET)2Cu2(CN)3

The static and dynamic local spin susceptibility of the organic Mott insulator $\kappa \text{−}\left(\mathrm{ET}{)}_2{\mathrm{Cu}}_2\right(\mathrm{CN}{)}_3$, a model material of the spin-1/2 triangular lattice, is studied by ${}^{13}\mathrm{C}$ NMR spectroscopy from room temperature down to $20\mathrm{mK}$. We observe an anomalous field-dependent spectral broadening with the continuous and bipolar shift distribution, appearing without the critical spin fluctuations. It is attributable to spatially nonuniform magnetizations induced in the spin liquid under magnetic fields. The amplitude of the magnetization levels off below $1\mathrm{K}$, while the low-lying spin fluctuations survive toward the ground state, as indicated by the temperature profile of the relaxation rates.