Itinerant-Electron Antiferromagnetism Precursor to Superconductivity in an Organic Conductor

Below 5.5 K minimally strained crystals of ${\mathrm{}\left(\mathrm{TMTSF}\right)\mathrm{}}_2$ ${\mathrm{ClO}}_4$ (TMTSF: tetramethyltetraselenafulvalene) exhibit vanishing spin-resonance intensity and reduced conductivity at low microwave power. More intense microwave electric fields along the needle axis nonlinearly restore both the resonance signal and the conductivity, indicating the presence of charged spin-density waves. Very anisotropic antiferromagnetic resonances are observed at 1.6 K, confirming that an itinerant spin-density-wave state precedes the onset of superconductivity at 1.3 K.