Magnetic-Field-Induced Metal-Insulator Transitions in Low-Carrier Density Materials

A metal-insulator transition (MIT) in a solid-state material reflects changes in the localization of charge carriers which normally coincides with the opening of an energy gap. In some systems, an external magnetic field can induce or enhance a MIT. When magnetic field is applied to a metal, electrons in the Fermi sea condense onto cyclotron orbits and form highly degenerate Landau levels separated by energy gaps. The gaps have magnitude (h/2π)(eH/m), where e, m, and H are the electron charge, effective mass, and magnetic field, respectively. This study applies a phenomenological parallel-resistor model incorporating the Landau level energy gap to metallic systems with low charge-carrier densities, including bismuth, graphite, and Li_0.9Mo₆O₁₇. Each system has a charge-carrier density in the range n ~ 10¹⁷–10¹⁸ cm^-3. Electrical resistivity of each material is measured and fit to the phenomenological model in the regime where magnetic field induces a MIT.