Observation of ultrafast photoinduced closing and recovery of the spin-density-wave gap in (TMTSF ) 2 PF6

Photoexcited dynamics of a spin-density-wave (SDW) state in an organic linear-chain compound bis-(tetramethyl-tetraselenafulvalene)hexafluorophosphate, (TMTSF) 2 PF6, is investigated by optical pump and terahertz probe experiments. After the ultrashort laser-pulse excitation, a sudden closing followed by a recovery of the SDW gap is clearly observed in the far-infrared spectrum. The recovery time is found to diverge toward the phase-transition temperature, TSDW, while the gap remains open at TSDW. The observed critical slowing down behavior is interpreted in terms of three-dimensional fluctuation which enlarges near the phase-transition temperature.