GaAs nanowires with hexagonal wurtzite (WZ) structure grown by molecular beam epitaxy (MBE) have been used as templates for deposition of (Pb,Sn)Te topological crystalline insulator (TCI) shells. In spite of substantial differences between WZ GaAs and rock-salt (Pb,Sn)Te there is a good matching between both crystal lattices along the axial NW direction. This enables growth of (Pb,Sn)Te TCI in a tubular-like geometry [1] maximizing the effects associated with topologically protected surface states - an immanent feature of topological insulators.

[1] Sania Dad, Piotr Dziawa, Wiktoria Zajkowska-Pietrzak, Sławomir Kret, Mirosław Kozłowski, Maciej Wójcik, and Janusz Sadowski, Axially lattice-matched wurtzite/rock-salt GaAs/Pb1-xSnxTe

SEM image of wurtzite GaAs nanowires with SnTe shells, grown by molecular beam epitaxy on GaAs(111)B

TEM image of wurtzite GaAs(core) – SnTe(shell) interface; dislocations at the GaAs-SnTe interface are marked with —I

2. Ferromagnetic MnAs to altermagnetic MnTe transformation

MnAs and MnTe crystallize in similar hexagonal (NiAs-type) structure. Both materials exhibit interesting magnetic properties. MnAs is a ferromagnet with a combined structural and ferromagnetic to paramagnetic phase transition at Curie temperature of 313 K. Hexagonal MnTe has long been considered as an antiferromagnet with Néel temperature of 307 K, recently it has been identified as an altermagnetic material sharing some properties with ferromagnets. We have noticed than thin layers of MnAs grown by molecular beam epitaxy can be transformed to MnTe by moderate temperature annealing (400 – 500 °C) in Te flux, in the ultra-high-vacuum environment (e.g. of the MBE growth chamber equipped with a tellurium source). This opens possibility of creating lateral heterostructures comprising two materials with ferromagnetic and antiferromagnetic properties. Such planar heterostructures enable formation of ferromagnetic patterns with laterally alternating exchange bias resulting in ferromagnetic layers with laterally tunable magnetic hysteresis curves.

High resolution transmission electron microscopy image of GaAs(111)B – MnTe interface.

MnTe was obtained by annealing MnAs layer in Te flux