Contribution of small closed orbits to magnetoresistance in quasi-two-dimensional conductors

We find a striking peak structure in the magnetoresistance of the quasi-two-dimensional conductors $\beta -{\mathrm{}(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}-\mathrm{T}\mathrm{T}\mathrm{F})\mathrm{}}_2{\mathrm{I}}_3$ and $\alpha -{\mathrm{}(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}-\mathrm{T}\mathrm{T}\mathrm{F})\mathrm{}}_2{\mathrm{NH}}_4{\mathrm{Hg}\left(\mathrm{SCN}\right)\mathrm{}}_4$ for magnetic fields nearly parallel to the conducting plane. The peak structure can be ascribed to a Fermi-surface topological effect due to the small closed orbits on the side of the warped cylindrical Fermi surface. This effect provides a clue about how to evaluate the interlayer transfer integral.