Quantum-limit Hall effect with large carrier density in topological semimetals

The quantum-limit Hall effect at ν = nh/eB ∼ O(1) that hosts a variety of exotic quantum phenomena requires demanding strong magnetic field B and low carrier density n. We propose to realize the quantum-limit Hall effect even in the presence of large carrier density residues n_e and n_h in a relatively weak and variable magnetic field B in topological semimetals, where a single Fermi-surface contour allows both electron-type and hole-type carriers and approaches charge neutrality when n_e ≈ n_h. The underlying filling factor ν = |n_e − n_h|h/eB also offers an example that violates Onsager's relation for quantum oscillations.