||鹿野田 一司 教授
||NMR study of doped spin liquid material κ-(ET)4Hg2.89Br8
||κ-(ET)4Hg2.89Br8 is a layered material, in which conducting ET layers and insulating Hg2.89Br8 layers are alternately stacked. Due to incommensurability between the ET and Hg sublattices, the ET layer with a half-filled band is 11% hole-doped and is metallic. In the conduction layer, the ET molecules form dimers, which constitute a triangular lattice. Since the spin susceptibility of κ-(ET)4Hg2.89Br8 is similar to that of the spin-liquid Mott insulator, κ-(ET)2Cu2(CN)3, with a triangular lattice, κ-(ET)4Hg2.89Br8 is considered to be a "doped spin liquid" in which the spin degree of freedom is in a spin-liquid state even though it is a metal. In the present study, I have performed 13C NMR measurements to investigate the magnetism of this material in detail and obtained the following results.
First, I confirmed that the NMR spectral shift (Knight shift), which measures the magnitude of the local magnetic field created by the electron spins, is scaled to the spin susceptibility and found that the spectrum is widened with decreasing temperature, suggesting an increasing inhomogeneity of the local spin susceptibility upon cooling.
Second, the nuclear spin-lattice relaxation rate 1/T1, which measures the intensity of electron spin fluctuations, showed a temperature dependence similar to in the spin-liquid compound κ-(ET)2Cu2(CN)3. A previous study found both materials to behave similar in spin susceptibility; thus, κ-(ET)4Hg2.89Br8 is regarded as a doped spin liquid hosting both mobile carriers and a quantum spin liquid.
Furthermore, a decreases in the Knight shift and a cubic temperature dependence of 1/T1 below 10 K suggest that superconductivity emerging in this material has spin-singlet d-wave symmetry with highly enhanced fluctuations above Tc.