Magnetic field-dependent interplay between incoherent and Fermi liquid transport mechanisms in low-dimensional τ phase organic conductors

We present an electrical transport study of the 2-dimensional (2D) organic conductor τ -(P-(S,S)-DMEDT-TTF)2(AuBr)2 (AuBr2)y (where y ∼ 0.75) at low temperatures and high magnetic fields. The inter-plane resistivity ρzz increases with decreasing temperature, with the exception of a slight anomaly at 12 K. Under a magnetic field B, both ρzz and the in-plane resistivity plane ρxx show a pronounced negative and hysteretic magnetoresistance. In spite of a negative residual resistivity ratio in zero field, Shubnikov de Haas (SdH) oscillations are observed in some (high quality) samples above 15 T. Furthermore, contrary to the single closed orbit Fermi surface predicted from band structure calculations (where a single star-shaped FS sheet with an area of ∼ 12.5% of AFBZ is expected), two fundamental frequencies Fl and Fh are detected in the SdH signal. These orbits correspond to 2.4% and 6.8% of the area of the first Brillouin zone (ABZ), with effective masses μl = 4.0 ± 0.5 and μh = 7.3 ± 0.1 respectively. The angular dependence, in tilted magnetic fields, of Fl and Fh, reveals a 2D character of the FS, but no evidence for warping along the kz direction ( e.g., the absence of a beating effect in the SdH signal) is observed. Angular dependent magnetoresistance (AMRO) fur-