The Experimental Determination of Exchange Mass Terms in Surface States on Both Terminations of MnBi₄Te₇

The intrinsic antiferromagnetic topological insulators in the Mn-Bi-Te family, composed of superlattice-like MnBi₂Te₄/(Bi₂Te₃)_n (n = 0, 1, 2, 3, …) layered structure, present intriguing states of matter such as quantum anomalous Hall effect and the axion insulator. However, the surface state gap, which is the prerequisite for the observation of these states, remains elusive. Here by molecular beam epitaxy, we obtain two types of MnBi₄Te₇ films with the exclusive Bi₂Te₃ (BT) or MnBi₂Te₄ (MBT) terminations. By scanning tunneling spectroscopy, the mass terms in the surface states are identified on both surface terminations. Experimental results reveal the existence of a hybridization gap of approximately 23 meV in surface states on the BT termination. This gap comes from the hybridization between the surface states and the spin-split states in the adjacent MBT layer. On the MBT termination, an exchange mass term of about 28 ± 2 meV in surface states is identified by taking magnetic-field-dependent Landau level spectra as well as theoretical simulations. In addition, the mass term varies with the field in the film with a heavy Bi_Mn doping level in the Mn layers. These findings demonstrate the existence of mass terms in surface states on both types of terminations in our epitaxial MnBi₄Te₇ films investigated by local probes.