Transport evidence of surface states in magnetic topological insulator MnBi2Te4

Magnetic topological insulators can host chiral 1D edge channels at zero magnetic field, when a magnetic gap opens at the Dirac point in the band structure of 2D topological surface states, leading to the quantum anomalous Hall effect in ultra-thin nanostructures. For thicker nanostructures, quantization is severely reduced by the co-existence of edge states with other quasi-particles, usually considered as bulk states. Yet, surface states also exist above the magnetic gap, but it remains difficult to identify electronic subbands by electrical measurements due to strong disorder. Here we unveil surface states in MnBi2Te4 nanostructures, using magneto-transport in very-high magnetic fields up to 55 T, giving evidence of Shubnikov-de-Haas oscillations above 40 T. A detailed analysis confirms the 2D nature of these quantum oscillations, thus establishing an alternative method to photoemission spectroscopy for the study of topological surface states in magnetic topological insulators, using Landau level spectroscopy.

💡 Research Summary

In this work the authors investigate the magnetotransport properties of exfoliated MnBi₂Te₄ Hall‑bar nanostructures up to an unprecedented magnetic field of 55 T. MnBi₂Te₄ is a magnetic topological insulator (MTI) in which a small exchange‑induced gap opens at the Dirac point of the topological surface states (TSS), giving rise to chiral edge channels and the quantum anomalous Hall effect (QAHE) in ultrathin films. In thicker flakes, however, bulk carriers usually dominate transport and mask the contribution of surface states, especially because strong disorder (Mn/Bi antisite defects) reduces carrier mobility. Previous transport studies up to 15 T never observed Shubnikov‑de‑Haas oscillations (SdHO) in pristine MnBi₂Te₄, and only weak resistance wiggles unrelated to surface states were reported.

The authors prepared high‑quality MnBi₂Te₄ single crystals, exfoliated them, and fabricated two Hall‑bar devices (≈91 nm thick, surface roughness ~1 nm) with Ti/Au ohmic contacts and a TiO₂ hard mask. Temperature‑dependent resistance shows a clear antiferromagnetic Néel transition at 25.5 K. Low‑field magnetotransport displays the expected spin‑flop transition at ~3.1 T and a sharp anomalous Hall jump, confirming the magnetic ordering.

At fields above 10 T the Hall resistance becomes linear, allowing extraction of a bulk carrier density nₕₐₗₗ≈6.7 × 10¹⁹ cm⁻³. Strikingly, above ~40 T clear SdHO appear in the longitudinal resistance. Only about one and a half periods are visible, but they are periodic in 1/B with a frequency f_B=167 T (Δ(1/B)=0.003 T⁻¹). The oscillation amplitude decays with temperature and disappears near 110 K. Lifshitz‑Kosevich analysis yields an effective mass m*≈0.16 mₑ, compatible with either the second bulk conduction band (CB2) or a Dirac‑type surface state.

To discriminate between 2D and 3D origins, the magnetic field was tilted. The SdHO positions depend only on the perpendicular component B⊥=B cosθ, a hallmark of a two‑dimensional Fermi surface. Interpreting the frequency as arising from a 3D parabolic band would give a bulk carrier density two orders of magnitude smaller than the Hall value, which is inconsistent. Assuming a Dirac surface state with a Fermi velocity v_F≈5.5 × 10⁵ m s⁻¹ (from ARPES) gives a 2D carrier density n₂D=e/h·f_B≈4.1 × 10¹² cm⁻² and a Fermi energy E_F≈255 meV above the Dirac point, in excellent agreement with previous ARPES measurements.

A back‑gate experiment (V_g=110 V) produced no shift of the SdHO, indicating that the observed surface state resides on the top surface rather than the bottom SiO₂‑faced surface, where charge traps likely suppress mobility. Using density‑functional‑theory (DFT) calculations, the authors model three bulk conduction bands (CB1, CB2, CB3) with effective masses 0.09 mₑ, 0.15 mₑ and 3 mₑ, respectively. The large bulk carrier density pins the chemical potential near the bottom of the heavy CB3 band. Combining Hall data (bulk density) with SdHO data (surface density) yields a band‑bending of ~150 meV between bulk and surface, a situation common in heavily doped 3D TIs where surface states are filled by charge transfer from the bulk.

From the extracted mobility µ≈250 cm² V⁻¹ s⁻¹ (quantum mobility from the SdHO onset) the mean free path is estimated as ℓ≈11 nm, shorter than in non‑magnetic Bi₂Se₃, reflecting stronger disorder and higher doping in MnBi₂Te₄. The effective mass calculated from the surface Fermi energy (m*=E_F/v_F²≈0.148 mₑ) matches the Lifshitz‑Kosevich result, further confirming the Dirac‑type 2D nature of the oscillations.

In summary, the authors provide the first transport‑based evidence of topological surface states in MnBi₂Te₄. By exploiting ultra‑high magnetic fields they resolve SdHO, demonstrate their 2D character through angular dependence, and quantitatively extract surface carrier density, effective mass, and band‑bending. This work establishes Landau‑level spectroscopy as a powerful complement to ARPES for probing surface states in magnetic topological insulators and highlights the crucial role of surface carriers in determining the transport properties of MnBi₂Te₄, especially for optimizing the QAHE in thin‑film devices.