Coexistence of antiferromagnetic and paramagnetic electronic phases in quasi-one-dimensional (TMTSF)$_2$PF$_6$

Phase transitions occuring in a quasi-one-dimensional organic compound (TMTSF)$_2$PF$_6$ near the boundaries between the paramagnetic metallic (PM), antiferromagnetic insulator (AFI), and superconducting (SC) states were studied experimentally. A controlled transition through the phase boundary was achieved by maintaining the sample at fixed temperature $T$ and pressure $P$, while the critical pressure was tuned by varying a magnetic field $B$. When the PM/AFI phase boundary was crossed due to the variation of a magnetic field, history effects were observed: the resistance was found to depend on the trajectory described by the system before arriving at a given point ($P-B-T$) of the phase space. The results of the experiment give evidence for the formation of a macroscopically inhomogeneous state characterized by the inclusions of a minor phase that is spatially separated from the major phase. Away from the phase boundary, the homogeneous state is restored. After this, upon approaching the phase boundary in the back direction, the system exhibits no features of the minor phase up to the very boundary.