Infrared and Raman study of the charge-ordered state in the vicinity of the superconducting state in the organic conductor β-(meso-DMBEDT-TTF)2PF6

We present spectroscopic evidence for charge ordering in beta-(meso-DMBEDT-TTF)(2)PF6 below similar to 70 K. Its infrared and Raman spectra change abruptly at similar to 70 K. The amplitude of the charge order was estimated to be 0.5 from the splitting of the infrared-active C=C stretching mode. The coexistence of both high-temperature and low-temperature signals was observed in a narrow temperature range (similar to 4 K) near the phase transition temperature. The pressure and temperature phase diagram was obtained in the vicinity of the superconducting phase. The checkerboard-type charge-order phase is not directly adjacent to the superconducting phase, but the superconducting phase is adjacent to the insulating phase where the short-range ordered domain of charge order is growing. The coexistent region expands significantly when hydrostatic pressure is applied. In the coexistent region, the crystal is inhomogeneous not just on a macroscopic scale, but also on a mesoscopic scale.