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

The quantum-limit Hall effect at v = nh/eB similar to 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) approximate to n(h). The underlying filling factor v = vertical bar n(e) - n(h)vertical bar h/eB also offers an example that violates Onsager's relation for quantum oscillations.