Magnetic-field-induced transition in quasi-two-dimensional systems

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 Tc=T0exp(-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)2X].