Observation of Antiferromagnetic Resonance in an Organic Superconductor

Anomalous microwave absorption has been observed in the organic superconductor ${\mathrm{TMTSF}}_2$${\mathrm{AsF}}_6$ (TMTSF: tetramethyltetraselenafulvalene) below its metal-nonmetal transition near 12 K. This absorption is unambiguously identified as antiferromagnetic resonance by the excellent agreement between a spin-wave calculation and the pronounced dependence on temperature and magnetic field orientation. This result dramatically confirms earlier indications of magnetic ordering or spin-density waves and, furthermore, indicates that the spin-density-wave amplitude is considerably larger than previous estimates of < 1%.