Nonlinear microwave resonance behavior of itinerant holes in an organic conductor

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Abstract
The spin resonance of holes in the single-stack organic conductor (TMTSF)2PF6 is readily observed from room temperature down to ∼l2 K with monotonically decreasing linewidth. Below that temperature no signal is observed at low microwave power but if the sample is exposed to microwave electric fields parallel to the needle axis the resonance reappears suddenly at higher microwave power. The threshold power for spin ’’resurrection’’ rises on cooling below 12 K and, in samples of improved purity, a second threshold marked by a distinct change in line shape appears below ∼9 K. These nonlinear magnetic effects are also accompanied by sharp but weak changes in the sample’s loading of the microwave cavity and may be qualitatively correlated with a decrease in DC conductivity accompanied by nonlinear electric-field response, as well as anomalous variations of the magnetic susceptibility. These results suggest a spin-density-wave ground state which is very easily disturbed by experimental probes.