April 2022

Abstracts of the QSIT/Quantum Center, ETH Zurich Lunch Seminar, Thursday, April 7, 2022

Heteroepitaxial growth of semiconductor/superconductor hybrid structures

Erik Cheah - Advanced Semiconductor Quantum Materials Group (Wegscheider group), ETH Zurich

Over the last years, semiconductors with strong spin-orbit interaction coupled to a superconductor have received attention as a solid-state platform exhibiting topological superconductivity [1].

I will report on our findings in the molecular beam epitaxial (MBE) growth of shallow InAs and InSb two-dimensional semiconductor heterostructures and the deposition of aluminum as the superconductor. In the first part we will show our results of the growth of undoped InSb quantum wells on GaAs substrates. Despite the omission of doping, we show the quantum well displays high electron mobility which gives access to quantum Hall physics [2]. In the second part, we focus on planar InAs structures with in-situ, i.e., without breaking the vacuum, deposited aluminum thin-films [3, 4]. This approach has been shown to effectively increase the coupling between the semiconductor to the superconductor [5]. In addition to the separate semiconductor characterization of the shallow InAs quantum well, the interface and crystallinity of the combined semiconductor/aluminum structure is analyzed by scanning transmission electron microscopy. We illustrate on the performance of the hybrid system by electrical transport measurements.

[1] Frolov, S.M., Manfra, M.J. & Sau, J.D, Nat. Phys. 16, 718–724 (2020)
[2] Zijin Lei et al., arXiv:2110.11002 (2021)
[3] J. Shabani et al., Phys. Rev. B 93, 155402 (2016)
[4] Fornieri, A et al., Nature 569, 89–92 (2019)
[5] P. Krogstrup et al, Nature Materials volume 14, 400–406 (2015)

Metamaterials breaking time-reversal symmetry

Johan Andberger - Quantum Optoelectronics Group (Faist group), ETH Zurich

By coupling the linearly polarized modes of two orthogonal nano-antenna arrays to the cyclotron resonance of a two-dimensional electron gas embedded in a AlGaAs/GaAs heterostructure, a metamaterial cavity with non-degenerate chiral electromagnetic modes is achieved. The highly subwavelength cavity mode volume of the nano-antennas combined with the large dipole moment of the cyclotron resonance induced by a perpendicular magnetic field puts the system in the ultra-strong coupling regime. This gives rise to polaritonic states with opposite circular polarization, which is demonstrated using terahertz time-domain spectroscopy measurements.