Transport evidence of surface states in magnetic topological insulator MnBi2Te4

TL;DR

Magnetic topological insulators face bulk-dominated transport that can obscure surface-state signatures, especially in thicker MnBi2Te4 nanostructures where disorder complicates spectroscopic identification. The authors perform magnetotransport measurements on exfoliated MnBi2Te4 Hall bars in fields up to $55~\mathrm{T}$ and analyze Shubnikov-de-Haas oscillations to isolate a 2D surface-state contribution. They observe SdHO above $40~\mathrm{T}$ with a frequency $f_B = 167~\mathrm{T}$, yielding a 2D carrier density $n_{2D}^{\mathrm{SdHO}} = \frac{e}{h} f_B = 4.1 \times 10^{12}~\mathrm{cm}^{-2}$ and an effective mass $m^* = 0.16~m_e$ from Lifshitz-Kosevich fits, with the angular dependence confirming a 2D origin. A back-gate experiment indicates the surface state responsible is on the top surface, and a band-bending model tied to the high bulk carrier density explains the large offset between surface and bulk bands, including a bulk chemical potential pinned near the bottom of CB3; together these results provide the first transport evidence of surface states in MnBi2Te4 and establish Landau-level spectroscopy as a practical alternative to photoemission for characterizing topological surface states.

Abstract

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.

Figures (4)

• Figure 1: Device characterization. a: Atomic force microscopy picture of an etched MnBi2Te4 nanostructure, with thickness 91 nm and roughness of about 1 nm. The original shape of the MnBi2Te4 exfoliated flake can still be faintly seen below the contacts. b: temperature dependence of the longitudinal resistance in zero magnetic field. The antiferromagnetic transition induces a resistance peak at 25.5 K.
• Figure 2: High-field magnetotransport in a MnBi2Te4 nanostructure at 4.2 K. a: Magnetoresistance and b: Hall effect up to 55 T. The magnetoresistance and Hall effect have been respectively symmetrized and antisymmetrized with magnetic field. Shubnikov-de-Haas oscillations are visible above about 40 T. Purple dashed lines represent respectively a cubic fit of the magnetoresistance, used afterward to extract Shubnikov-de-Haas oscillations, and a linear fit of the Hall effect asymptote at high field. Arrows indicate the spin-flop- and saturation fields.
• Figure 3: Temperature dependence of Shubnikov-de-Haas oscillations. a: Residual Shubnikov-de-Haas oscillations after removal of a cubic background, versus inverse magnetic field. The dashed lines show the position of the SdHO extrema, separated by 0.003 T$^{-1}$. b: Residual Shubnikov-de-Haas oscillations after removal of a cubic background, at different temperatures. The position of the minimum and maximum of the oscillation are indicated by dashed lines. c: Temperature dependence of the Shubnikov-de-Haas oscillation amplitude for the peaks at 43 and 49T. The error bars correspond to rapid noise in the raw data. Lifshitz-Kosevich fits lead effective masses $m^*$ of about $0.15 - 0.17 m_\mathrm{e}$.
• Figure 4: Angular dependence of Shubnikov-de-Haas oscillations and band bending. a: Angular dependence of the Shubnikov-de-Haas oscillations after removal of a cubic background, between 0° (transverse field configuration) to 30° tilt toward in-plane configuration, shown against the transverse magnetic field component $B_\perp$. Curves shifted for clarity. The schematic shows the tilting angle $\theta$ of the magnetic field. b: Schematic of the energy levels and band bending in the MnBi2Te4 nanostructure, as extracted from Shubnikov-de-Haas and Hall data (see main text), in a model with three conduction bands (CB1, CB2, CB3, see main text) and one topological surface state (TSS).