Competition between superconductivity and spin-density waves in an organic conductor, bis-tetramethyltetraselenafulvalenium hexafluorophosphate

The pressure-temperature phase diagram of the organic superconductor bis-tetramethyltetraselenafulvalenium hexafluorophosphate [${\mathrm{}\left(\mathrm{TMTSF}\right)\mathrm{}}_2$ ${\mathrm{PF}}_6$] has been carefully measured by electron-spin resonance is a single crystal at low magnetic fields. We present the first microscopic evidence for the transition from the spin-density-wave state to superconductivity as the temperature is lowered at constant pressure. Comparison is made with recent theoretical calculations on the competition between spin-densitywave and superconducting ground states.