Muon-spin-rotation and relaxation studies in (TMTSF)2-X compounds

Muon-spin-rotation and relaxation (μSR) studies of the organic compounds (TMTSF)2-X (X=PF6, NO3, and ClO4) at ambient pressure are reported. We observe spin-density-wave (SDW) states in all three compounds under zero external magnetic field. The onset of the SDW is extremely sharp, which may indicate a first-order transition. The sublattice magnetization (or SDW amplitude) in the PF6 compound exhibits significant reduction with increasing temperature at low temperatures, which demonstrates the existence of collective low-energy excitations, in addition to the single-particle excitations across the SDW gap. The large spin-wave stiffness we observe in this system is incompatible with a Heisenberg model for a localized spin system; this demonstrates the importance of using an itinerant-electron picture to describe the magnetic behavior of this system. The broad distribution of local magnetic fields deduced from the μSR time spectra is consistent with that expected from an incommensurate SDW. The magnitude of the internal field at T→0 is approximately the same for the three systems, suggesting a common SDW amplitude in these systems. Transverse-field μSR measurements in the relaxed-state ClO4 system show no visible enhancement of the relaxation rate in the superconducting state down to 0.1 K, providing a lower limit for the superconducting penetration depth λab′⩾12 000 Å.