A model is presented which gives a unique magnetic-field-induced transition from a two-dimensional open-orbit metal to a semimetal. A magnetic field causes the open-orbit electron motion to become progressively more one dimensional, with an effective density of states proportional to H at low field. This leads to a Fermi-surface instability with a transition temperature which varies approximately as $T_c$=$T_0$exp(-A/H). The resulting gap, which is both temperature and magnetic field dependent, can explain some of the anomalous quantum oscillations and the Hall steps observed in the tetramethyltetraselenafulvalenium compounds [(TMTSF${)}_2$X].

• Received 12 June 1984

DOI:https://doi.org/10.1103/PhysRevB.31.4770