Coulomb pseudogap caused by partial localization of a three-dimensional electron system in the extreme quantum limit

Scanning tunneling spectroscopy is applied to a three-dimensional electron system in the extreme quantum limit (EQL). A parabolic pseudogap of a width of 2. 5meV is found at E₅ and interpreted as the Coulomb gap predicted by Efros and co-workers J. Phys. C 8, L49 (1975) ; Sov. Phys. Semicond. 14, 487 (1980) for localized particles. The fact that the depth of the gap is markedly above zero and fluctuates in space is interpreted as evidence for partial localization supporting our previous assumption that a distinct electron phase exists in the magnetic-field range between the transition to the EQL and the magnetic freeze-out.