Abstract
We study the influence of negative spin-orbit coupling on the topological phase transition and properties of the topological insulator state in InGaN-based quantum wells grown along c axis of the wurtzite lattice. The realistic eight-band k·p method with relativistic and nonrelativistic linear-k terms is employed. Our calculations show that the negative spin-orbit coupling in InN is not an obstacle to obtain the topological insulator phase in InN/InGaN and InGaN/GaN quantum wells. The bulk energy gap in the topological insulator state can reach 2 meV, which allows experimental verification of the edge state transport in these materials. The topological phase transition occurs due to the band inversion between the highest light hole subband and the lowest conduction subband, and almost always is mediated by the two-dimensional Weyl semimetal, arising from an anticrossing of these subbands at zero in-plane wave vector. However, for certain InGaN/GaN quantum wells, we find that the magnitude of this anticrossing vanishes, leading to the appearance of the Dirac semimetal. The novel transition between the Weyl and Dirac semimetals originates from vanishing of the average in-plane spin-orbit interaction parameter, which decouples the conduction subband from the light hole subband at zero in-plane wave vector.
Highlights
The discovery of the time–reversal topological insulators (TIs) in two and three dimensions has greatly inspired the study of topological properties of the electronic band structure of crystalline materials[1]
In two-dimensional (2D) nanostructures, the topological phase transition (TPT) is always accompanied by the closing of the bulk band gap, and the character of the intermediate gapless states depends on the full crystal symmetry of the multilayer structures[6,7,8,9,10,11]
In conventional 2D topological materials, such as zinc-blende HgTe/CdTe and InAs/GaSb/AlSb quantum wells (QWs), the band inversion occurs in the center of the Brillouin zone (BZ), and the TPT is mediated either by the Dirac semimetal (DSM) or by the Weyl semimetal (WSM), depending whether the conduction band (CB) and the valence band (VB) states cross or anticross at zero in-plane wave vector (k⊥ = 0), respectively[8,9,10]
Summary
We consider first InN/GaN multi-QWs with the QW width, Lqw = 1.25 nm, corresponding to 4 monolayers of InN, for which the band structure can be inverted by the built-in electric field[13]. A disadvantage of these nanostructures is significant internal strain, which arises from large lattice misfit between GaN and InN, and causes difficulties in pseudomorphic growth of sufficiently thick wells[30].
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