A05

Technology and investigations of topological crystalline insulators (PbSn)(TeSe)

Summary

The project will continue the molecular beam epitaxy and study of topological crystalline insulator (TCI) (Pb,Sn)(Te,Se) layers and will focus on Se-alloyed quaternary compounds, thin multilayer struc- tures, as well as magnetic doping. Se alloying promises an inverted band structure with TCI properties at lower Sn content, a reduced density of intrinsic doping, and an improved control of biaxial strain in the layers. The four TCI surface states protected by crystal symmetry of the layer and possible time reversal symmetry are proposed to get partially gapped or shifted depending on surface orientation, layer thickness, lattice strain, and magnetic doping. Crystal symmetry, confinement, external fields and exchange coupling to magnetic dopants affect the TCI surface states in different ways and can result in Dirac-like TCI, topological insulator, normal insulator, Weyl semimetal or giant Rashba-split systems. In epitaxial layers, the crystal symmetry is first given by the orientation of the underlying substrate and can be locally reduced by surface steps, dislocations or hinges which can host topologically protected 1D states. The tuning of the topological surface states promises novel functionalities due to TCI phases with large variable Chern numbers, interfacial superconductivity or higher-order topological insulator properties with hinge states.

For reaching control of the different topological phases of the TCI layers, we deposite thin (Pb,Sn)(Te,Se) layers and multilayers of various composition on adequate substrates or buffer lay- ers (CdTe, BaF₂ , CdZnTe) and perform a detailed structural analysis of strain, surface properties, ferroelectric distortion and extended topological defects. Magnetic doping by d-transition metals such as Mn causes ferromagnetism at low temperatures and promises studies of massive Dirac-like states, the anomalous Hall effect and axion physics of a TCI with multiple surface states.

Sn-rich layers are intrinsically p-doped due to Sn vacancies. Moderately n-doped or fully compen- sated layers required for ARPES and transport studies of surface states are realized by optimizing the layer composition and MBE growth conditions, by intentional doping and by charge transfer in thin remote-doped heterostructures and multilayers, such as SnTe/Pb(Te,Se) and (Pb,Sn)Te/CdTe. Transport in TCI layers with superconducting contacts will also be studied in collaboration with the new Institute for Topological Insulators (ITI) Würzburg. Additionally, strained α-Sn layers will be epi- taxially deposited on adequate substrates such as CdTe and KBr and promise an elemental TI or Dirac semimetal system. The surface states of the (magnetic) 2D or 3D TCI and TI layer structures are studied in SFB 1170 cooperations by ARPES, STM, STS, magnetotransport at low temperature and according band structure calculations.