Macroscopically inhomogeneous state at the boundary between the superconducting, antiferromagnetic, and metallic phases in quasi-one-dimensional (TMTSF)₂PF₆

We report on experimental studies of the phase state and the character of phase transitions in the quasi-one-dimensional organic compound (TMTSF) ₂PF₆ in the close vicinity of the borders between the metal (M), antiferromagnetic (AF) insulator, and superconducting (SC) states. In order to drive the system precisely through the phase border P ₀(T₀), the sample was maintained at fixed temperature T and pressure P, whereas the critical pressure P₀ was tuned by applying the magnetic field B. In this approach, the magnetic field was used (i) for smooth and precise tuning δP = P - P₀ [thanks to a monotonic P₀(B) dependence] and (ii) for identifying the phase composition (due to qualitatively different magnetoresistance behavior in different phases). Experimentally, we measured magnetoresistance R(B) and its temperature dependence R(B,T) in the pressure range 0-1 GPa. Our studies focus on the features of the magnetoresistance at the phase transitions between the M and AF phases and in the close vicinity to the superconducting transition at T ≈ 1 K. We found pronounced history effects arising when the AF/M phase border is crossed by sweeping the magnetic field: the resistance depends on a trajectory which the system arrives at a given point of the P-B-T phase space. In the transition from the M to AF phase, driven by increasing magnetic field, the features of the M phase extend well into the AF phase. At the opposite transition from the AF to M phase, the features of the AF phase are observed in the M phase. These results evidence for a macroscopically inhomogeneous state, which contains macroscopic inclusions of the minority phase, spatially separated from the majority phase. When the system is driven away from the transition, the homogeneous state is restored; upon a return motion to the phase boundary, no signatures of the minority phase are observed up to the phase boundary.