Transport properties of superconducting hybrid structures based on quantum wells with normal and inverted band ordering
Final Report Abstract
In this project, we have investigated transport properties of NS (and NN’S) hybrid structures using a two-dimensional extended four-band model for HgTe quantum wells including Rashba spin-orbit interaction and bulk inversion asymmetry, induced s-wave superconductivity and a Zeeman field. We have shown how a decisive signature of a chiral topological superconductor (TSC) can be observed in an NS junction based on a QSH insulator and a TSC made of the same material. As a function of the Fermi energy the conductance resonances go through a sequence of 2 e^2 /h (nontrivial superconducting phase) and 4 e^2 /h (trivial superconducting phase) plateaux. These features occur only inside the 2D topological nontrivial phase with nonzero Chern number when the sample width largely exceeds the extent of the chiral Majorana edge channels. These signatures are a manifestation of the topological nature of the QSH effect and the TSC. We extended this study to the analysis of the supercurrent in Josephson junctions made of TSC/QSH insulator/TSC and found the signatures of the presence of Majorana modes that arise in the Fraunhofer pattern oscillations. In this setup, the QSH edges probe the Majorana modes giving rise to a local and non-local Andreev scattering. This leads to a doubling of the period of the Fraunhofer pattern in the nontrivial superconducting phase compared to the trivial phase. Next, we have investigated hybrid structures based on a bilayer QSH insulator in proximity to an s-wave superconductor where the induced pairing can be of intraor interlayer type, and domain walls of different types of pairing potentials host Kramers partners of Majorana bound states (MBS). We found that Majorana bound states at such topological interfaces emerge also in an otherwise homogeneous system by a spatially dependent interlayer gate voltage which suppresses the interlayer pairing. We identified transport signatures (zero-bias anomalies) in a five-terminal setup that can be related to the presence/absence of Kramers partners of MBS at the topological interfaces. Moreover, we found that it is possible to obtain experimentally the band structure of the 3D topological insulator based on a HgTe thin film from the analysis of the cyclotron resonance. The experimental band structure agrees reasonably well with that from k · p calculations. Both theoretical and experimental data revealed nonlinear dispersion of the surface states that could be explained by their strong hybridization with the bulk states.
Publications
- Voltage-tunable Majorana bound states in time-reversal symmetric bilayer quantum spin Hall hybrid systems. Physical Review B 100, 165420 (2019)
F. Schulz, J. C. Budich, E. G. Novik, P. Recher, and B. Trauzettel
(See online at https://doi.org/10.1103/PhysRevB.100.16542) - Band structure of a HgTe-based three-dimensional topological insulator, Phys. Rev. B 102, 115113 (2020)
J. Gospodarič, V. Dziom, A. Shuvaev, A. A. Dobretsova, N. N. Mikhailov, Z. D. Kvon, E. G. Novik, and A. Pimenov
(See online at https://doi.org/10.1103/PhysRevB.102.115113) - Transport signatures of a junction between a quantum spin Hall system and a chiral topological superconductor, Phys. Rev. B 101, 235308 (2020)
E. G. Novik, B. Trauzettel, and P. Recher
(See online at https://doi.org/10.1103/PhysRevB.101.235308)
