Surface states and Rashba-type spin polarization in antiferromagnetic ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$(0001)

The layered van der Waals antiferromagnet ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ has been predicted to combine the band ordering of archetypical topological insulators such as ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ with the magnetism of Mn, making this material a viable candidate for the realization of various magnetic topological states. We have systematically investigated the surface electronic structure of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$(0001) single crystals by use of spin- and angle-resolved photoelectron spectroscopy experiments. In line with theoretical predictions, the results reveal a surface state in the bulk band gap and they provide evidence for the influence of exchange interaction and spin-orbit coupling on the surface electronic structure.

Figure 1

(a) Crystal structure of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ along one unit cell (black lines). The arrow indicates a van der Waals gap between two septuple layers and thus a natural cleavage plane. (b)–(d), (f), (g) ARPES data along the $\overline{\mathrm{\Gamma }}\overline{M}$ direction at photon energies from 21 to $26\mathrm{eV}$. The excitation-energy-dependent evolution of bulk conduction- (BCB) and valence-band (BVB) states can be seen as well as the states S1 and S2 with two-dimensional character. All data sets were taken at $T=10\mathrm{K}$. (e) ARPES data set taken at $T=80\mathrm{K}$ and $h\nu =21\mathrm{eV}$. (h) Energy distribution curves (EDC) at the $\overline{\mathrm{\Gamma }}$ point and $h\nu =20–28\mathrm{eV}$. Markers trace the energy positions of the features BVB, S1 and S2. The dashed line indicates a weak shoulder at the low-binding-energy side of the BVB peak.

Figure 2

(a) Valence-band spectra for ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$(0001) obtained by resonant excitation at photon energies $h\nu$ near the Mn $2p\to 3d$ absorption edge. The inset shows the Mn $L_3$ absorption edge with colored arrows indicating the corresponding excitation energies. (b) Difference of the spectra measured under on-resonant (light blue) and off-resonant (black) conditions, showing the contribution of the Mn $3d$ states to the valence band. (c) Angle-resolved valence-band spectrum measured with $h\nu =79\mathrm{eV}$.

Figure 3

(a) Experimental geometry of the spin-resolved ARPES experiments for ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$(0001). Spin-resolved EDCs were measured at five different locations in $k_{\parallel }$ space, schematically indicated as (i)–(v) in (a). (b) Spin-resolved EDCs obtained at positive and negative wave vectors along $k_x$ and $k_y$. (c) Spin-resolved EDCs at $k_{\parallel }=0$, showing the absence of spin polarization at $\overline{\mathrm{\Gamma }}$. All data sets were acquired at $h\nu =20\mathrm{eV}$ and at $T=50$ K, i.e., in the paramagnetic regime.