[Paper Review] Dominant non-local superconducting proximity effect due to electron-electron interaction in a ballistic double nanowire

Kento Ueda, Sadashige Matsuo+9 more|arXiv (Cornell University)|Oct 11, 2018

Physics of Superconductivity and Magnetism4 citations

TL;DR

This study demonstrates a dominant non-local superconducting proximity effect in a ballistic InAs double nanowire Josephson junction, where electron-electron interactions in one-dimensional electron gases suppress local pair tunneling and enhance Cooper pair splitting (CPS). The observed switching current into both wires exceeds the sum of individual currents, indicating inter-wire superconductivity dominates over intra-wire pairing, with a larger superconducting gap for inter-wire coupling—key for realizing time-reversal invariant topological superconductivity and Kramers pairs of Majorana fermions without external magnetic fields.

ABSTRACT

Cooper pair splitting (CPS) can induce non-local correlation between two normal conductors coupling to a superconductor. CPS into a double one-dimensional electron gas is an appropriate platform for extracting large amount of entangled electron pairs and one of the key ingredients for engineering Majorana Fermions with no magnetic field. Here we study CPS using a Josephson junction of a gate-tunable ballistic InAs double nanowire. The measured switching current into the two nanowires significantly larger than sum of that into the respective nanowires, indicating the inter-wire superconductivity dominant compared to the intra-wire superconductivity. From dependence on the number of propagating channels in the nanowires, the observed CPS is assigned to one-dimensional electron-electron interaction. Our results will pave the way for utilizing one-dimensional electron-electron interaction to reveal physics of high-efficient CPS and engineer Majorana Fermions in double nanowire systems via CPS.

Motivation & Objective

• To demonstrate high-efficiency Cooper pair splitting (CPS) in a ballistic double nanowire system with suppressed local pair tunneling (LPT).
• To investigate whether one-dimensional electron-electron interactions can dominate and enhance non-local superconducting correlations over local ones.
• To verify that inter-wire superconductivity exceeds intra-wire superconductivity in strength, a necessary condition for time-reversal invariant topological superconductivity.
• To provide a platform for engineering Majorana fermions without requiring strong external magnetic fields.

Proposed method

• Fabrication of a gate-tunable ballistic InAs double nanowire (DNW) Josephson junction with 20 nm separation between two Al electrodes.
• Measurement of switching current in both nanowires as a function of gate voltage and magnetic field to probe superconducting proximity effects.
• Use of Josephson junction characteristics to extract superconducting gap energies for intra-wire and inter-wire proximity effects via IcRn product analysis.
• Analysis of switching current dependence on the number of propagating channels to identify the role of one-dimensional electron-electron interactions.
• Comparison of measured CPS contribution to LPT using theoretical models of Tomonaga-Luttinger liquids coupled to a superconductor.
• Use of gate-tunable transport measurements to isolate non-local correlations from local ones.

Experimental results

Research questions

• RQ1 Can one-dimensional electron-electron interactions in a double nanowire suppress local pair tunneling and enhance non-local Cooper pair splitting?
• RQ2 Is the inter-wire superconducting proximity effect stronger than the intra-wire effect in a ballistic double nanowire Josephson junction?
• RQ3 Does the observed non-local superconducting correlation exhibit a larger superconducting gap than the local one, indicating a favorable condition for topological superconductivity?
• RQ4 Can electron-electron interactions in 1D systems serve as a dominant mechanism for high-efficiency CPS without relying on tunnel coupling or quantum dot electrostatics?

Key findings

• The measured switching current into both nanowires exceeds the sum of the individual switching currents, indicating a dominant non-local superconducting proximity effect.
• The inter-wire superconducting gap, derived from IcRn, is larger than the intra-wire gap, satisfying a key condition for time-reversal invariant topological superconductivity.
• The observed CPS is attributed to one-dimensional electron-electron interactions, as confirmed by the dependence on the number of propagating channels.
• Local pair tunneling is suppressed due to strong electron-electron repulsion, which screens the Coulomb interaction and enhances non-local entanglement.
• The system exhibits a high-efficiency CPS-to-LPT ratio without requiring strong tunnel barriers or large quantum dot charging energies.
• This work establishes a new platform for engineering Majorana fermions in double nanowires without the need for external magnetic fields.