Andreev bound state fusion
C. Jünger1, S. Lehmann2, K.A. Dick2,3, C. Thelander2, C. Schönenberger1,4 and A. Baumgartner1,4
1 Department of Physics, University of Basel, Klingelbergstrasse 82, CH-4056 Basel, Switzerland
2 Division of Solid State Physics and NanoLund, Lund University, S-221 00 Lund, Sweden
3 Center for Analysis and Synthesis, Lund University, S-221 00 Lund, Sweden
4 Swiss Nanoscience Institute, University of Basel, Klingelbergstrasse 82, CH-4056, Basel, Switzerland
Semiconductor-superconductor hybrid structures form the basis for a large variety of novel quantum electronic devices, for example as a source of entangled electron pairs with anti-correlated spins [1]. However, the electrical control is often hampered by inhomogeneous or badly reproducible tunnel barriers. Recently, in-situ grown, atomically sharp and gate tunable barriers in InAs nanowires (NWs) were shown to form very stable and reproducible quantum dots (QDs) [2], used, for example, for tunnel spectroscopy of normal state lead states [3], but also to observe how the superconducting proximity effect is established in NW segments [4], or to investigate magnetic field independent subgap states [5].
Here, we use such built-in barriers to divide a NW into three segments, with superconducting contacts on the outer ones and a QD in between. In a sequence of tunneling spectroscopy measurements we show intermediate states in the process of merging individual Andreev bound states (ABSs) while the barrier height is lowered. For large barriers, the QD couples weakly to the two NW segments carrying individual ABSs, or “Andreev atoms”. For lower barrier strengths, these ABSs sequentially hybridize with the central QD, forming different types of “Andreev molecules”, with zero-bias conductance peaks as precursors of the Josephson effect. For very low barriers, we finally find gate tunable Josephson currents, suggesting ABSs spanning the complete NW, states one might call "fused" ABS or “Andreev helium”. Our results illustrate the merging of subgap states and may serve as a guide in future fusion experiments of topologically non-trivial bound states.
This work was supported by the Swiss National Science Foundation, the QuantEra project SuperTop, the Horizon 2020 projects AndQC and QUSTEC, the Swiss Nanoscience Institute, the Knut and Alice Wallenberg Foundation and the Swedish Research Council.
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[5] C. Jünger, R. Delagrange, D. Chevallier, S. Lehmann, K.A. Dick, C. Thelander, J. Klinovaja, D. Loss, A. Baumgartner, and C. Schönenberger, Phys. Rev. Lett. 125, 017701 (2020)
[6] C. Jünger, S. Lehmann, K.A. Dick, C. Thelander, C. Schönenberger, and A. Baumgartner, arXiv:2111.00651 (2021)