DC and microwave transport properties of TI nanoribbon-superconductor hybrid junctions

Tilo Bauch – Chalmers University of Technology (Göteborg)

The study of hybrid material systems with a conventional superconductor in proximity to a strong spin-orbit semiconductor or a Topological Insulator (TI) has lately received a dramatic boost. The potential of such hybrid systems to host exotic phenomena such as Majorana bound states makes them interesting for topological quantum computation architectures [1-2]. In a multimode hybrid TI Josephson junction with two terminal geometry, Majorana physics manifests as peculiar properties of a part of the Andreev bound states carrying the Josephson current. They give rise to an unconventional 4π periodic current phase relation (CPR) coexisting with a 2π periodic CPR resulting from the conventional Andreev bound states. Here we make use of Al-Bi2Se3-Al junctions fabricated using TI nano-ribbons grown by physical vapor deposition. [3-5] In a previous study we extracted the current phase relation (CPR) of our TI-junction by implementing an asymmetric dc-Superconducting Quantum Interference Device (SQUID) measurement technique [6]. We observed clear deviations from a standard sinusoidal CPR in all our devices pointing towards the presence of highly transparent modes in our TI junctions. To obtain more information about the bound state spectrum we have furthermore implemented a microwave rf-SQUID probing scheme for our Al-Bi2Se3-Al hybrid junctions. Here, we embedded a TI Josephson junction-based rf-SQUID in a superconducting coplanar waveguide resonator. The microwave response (frequency shift and losses) of the coupled resonator/junction system to an externally applied magnetic field (phase bias) at various temperatures is used to deduce information about the phase dependence of the bound state spectrum of the junction. We find that the bound state spectrum in our TI Josephson junctions is dominated by highly transparent modes with rather short lifetimes. 

References: 

[1] L. Fu et al., Phys. Rev. Lett. 100, 096407 (2008) 

[2] C. Nayak et al., Rev. Mod. Phys. 80, 1083–1159 (2008) 

[3] G. Kunakova et al., Nanoscale 10, 19595–19602 (2018) 

[4] G. Kunakova et al., Appl. Phys. Lett. 115, 172601 (2019) 

[5] G. Kunakova et al., J. Appl. Phys. 128, 194304 (2020)

[6] A. P. Surendran et al., Supercond. Sci. Technol. 36, 064003 (2023)