Shubnikov–de Haas and angular-dependent magnetoresistance oscillations have been measured to investigate the electronic state in the organic conductor $(\mathrm{BEDT}-\mathrm{TTF}{)}_2\mathrm{Br}(\mathrm{DIA}),$ where the supramolecular $\cdot \cdot \cdot \mathrm{Br}\cdot \cdot \cdot \mathrm{DIA}\cdot \cdot \cdot$ chains are formed $(\mathrm{DIA}=\mathrm{diiodoacetylene}).\mathrm{}$ Both results consistently show the presence of a large two-dimensional Fermi surface (FS), whose cross sectional area corresponds to about 50% of the first Brillouin zone. Very small FS’s are also found, which are not explained by the band calculation. When a large magnetic field of $\sim 14\mathrm{T}$ is applied along the longest (shortest) Fermi wave vector in the conduction plane, a dip (peak) in the interlayer resistance is observed. This behavior is understood in terms of coherent or incoherent transport of the electrons along the interlayer direction. As the field decreases, the dip changes to a peak, suggesting a crossover from the incoherent to coherent transport.

• Received 26 December 2002

DOI:https://doi.org/10.1103/PhysRevB.68.064420