Mott Physics in Organic Conductors with Triangular Lattices

Electron correlation and spin frustration are among the central issues in condensed matter physics, and their interplay is expected to bring about exotic phases with both charge and spin fluctuations. Molecular materials are playgrounds suitable for this study. Fundamentals in physics of Mott transition and spin frustration on triangular lattices are seen in the organic materials ET and Pd(dmit)(2) compounds. We review the experimental studies on the criticality of Mott transition with a continuously controllable pressure technique and on the ground state of the quasi-triangular-lattice Mott insulator. Mott criticality is well characterized in both charge and spin channels with unconventional critical exponents of possibly quantum nature. The ground state of the triangular-lattice Mott insulator is changed from antiferromagnet to spin liquid as the triangular lattice becomes more isotropic. The various experiments probing the nature of spin liquid are described in the light of proposed mechanisms.