Evolution of Mn1−xGexBi2Te4 Electronic Structure under Variation of Ge Content

Abstract

One of the approaches to manipulate MnBi2Te4 properties is the magnetic dilution, which inevitably affects the interplay of magnetism and band topology in the system. In this work, we carried out angle-resolved photoemission spectroscopy (ARPES) measurements and density functional theory (DFT) calculations for analysing changes in the electronic structure of Mn1−xGexBi2Te4 that occur under parameter x variation. We consider two ways of Mn/Ge substitution: (i) bulk doping of the whole system; (ii) surface doping of the first septuple layer. For the case (i), the experimental results reveal a decrease in the value of the bulk band gap, which should be reversed by an increase when the Ge concentration reaches a certain value. Ab-initio calculations show that at Ge concentrations above 50%, there is an absence of the bulk band inversion of the Te pz and Bi pz contributions at the Γ-point with significant spatial redistribution of the states at the band gap edges into the bulk, suggesting topological phase transition in the system. For case (ii) of the vertical heterostructure Mn1−xGexBi2Te4/MnBi2Te4, it was shown that an increase of Ge concentration in the first septuple layer leads to effective modulation of the Dirac gap in the absence of significant topological surface states of spatial redistribution. The results obtained indicate that surface doping compares favorably compared to bulk doping as a method for the Dirac gap value modulation.

Phase transitions, Dirac and WSM states in Mn1−xGexBi2Te4

“…While the formation of a plateau with a minimum energy gap in the extended concentration range of 30-50% is evident, the underlying physical processes responsible for these observed changes in the electronic structure remain to be investigated. One potential explanation, as suggested in previous studies 36,37,39 , is a possible transition to a WSM state within the region of the minimum energy gap. In this study, we aim to conduct a detailed analysis of the potential transition to such a state in Mn 1−x Ge x Bi 2 Te 4 and to identify the factors influencing the occurrence of various TPTs within this system, including the transition from the antiferromagnetic TI state to the WSM state.…”

“…As the Ge concentration approaches 90%, the states of the upper cone shift back towards lower binding energies, again forming a pronounced band gap. The ARPES data then resemble those more characteristic of Mn-doped GeBi 2 Te 4 , featuring the bulk band gap and TSS branches in the region of their intersection with the edge of the VB (for comparison, see 37,38 for GeBi 2 Te 4 ).…”

“…The observed decrease in the band gap, culminating in an apparent zero gap with increasing Ge concentration, and the formation of a plateau with a minimum band gap in the concentration range of 45-55%, followed by a transition to the electronic structure of Mn-doped GeBi 2 Te 4 (at 90%), is consistent with previously reported data in the literature. This pattern has been observed both in systems where Mn atoms are replaced by Ge atoms 37,38 , as well as for substitutions with other Group IV elements such as Sn 36 and Pb 39 . However, the underlying reason for the formation of the minimum band gap plateau remains unclear.…”

“…However, there is another possibility of modulating the energy gap at the DP and reaching the TPT point through the partial replacement of Mn atoms with IV-group elements (Ge, Sn, Pb) [36][37][38][39][40][41][42] . In Refs [36][37][38] , it was shown that when Mn is replaced by Ge, Sn, or Pb atoms in the Mn 1−x A x Bi 2 Te 4 (A = Ge, Sn, Pb) compounds, the bulk band gap decreases with an increase in the concentration of the substituting atoms. The gap decreases to practically zero at substituting element concentrations of 40-50%, with a possible transition from the topological state to the Weyl semimetal (WSM) state.…”

Comparative Study of Magnetic Properties of (Mn1−xAxIV)Bi2Te4 AIV = Ge, Pb, Sn

“…There are ternary TI compounds, such as A IV Bi 2 Te 4 , with the same R3m symmetry group as MnBi 2 Te 4 [13,27], allowing for arbitrary ratios of Mn substitution in solid solutions (Mn 1−x A IV x )Bi 2 Te 4 . Several studies [28][29][30][31][32] have demonstrated the synthesis of these crystals and confirmed the absence of additional substitution defects, as in the case of substitution of Sb for Bi atoms. Moreover, this substitution appears to be isovalent and does not introduce any additional charge according to ARPES studies [32].…”