Andreev Reflection in the Quantum Hall Regime at an Al/InAs Junction on a Cleaved Edge

Takafumi Akiho, Hiroshi Irie, Yusuke Nakazawa, Satoshi Sasaki, Norio Kumada, Koji Muraki

Published 2024-07-31Version 1

We have fabricated a superconductor/semiconductor junction composed of Al and InAs using cleaved edge overgrowth (CEO). By exploiting the unique geometry with a thin Al/Pt/Al trilayer formed on the side surface of an in-situ cleaved heterostructure wafer containing an InAs quantum well (QW), we achieve a superconducting critical field of $\sim 5$~T, allowing superconductivity and quantum Hall (QH) effects to coexist down to filling factor $\nu = 3$. Andreev reflection at zero magnetic field shows a conductance enhancement that is limited solely by the Fermi velocity mismatch, demonstrating a virtually barrier-free, high-quality S/Sm junction. Bias spectroscopy in the QH regime reveals a clear bias dependence indicating the superconducting gap, with the reduced downstream resistance demonstrating that the electron-hole Andreev conversion probability consistently exceeds 50\%. Our results, obtained in a new experimental regime characterized by a clean edge-contacted junction, a much smaller superconducting gap, and a narrow superconducting electrode less than the coherence length, open a new avenue for both theoretical and experimental studies of the interplay between superconductivity and QH effects and the engineering of exotic quasiparticles.