[Paper Review] Magnetic Josephson Junctions and Superconducting Diodes in Magic Angle Twisted Bilayer Graphene

Jaime Díez-Mérida, A. Díez-Carlón+8 more|arXiv (Cornell University)|Oct 3, 2021

Physics of Superconductivity and Magnetism51 references28 citations

TL;DR

The paper demonstrates gate-defined magnetic Josephson junctions in MATBG near the correlated ν = -2 state, reveals unconventional, phase-shifted Fraunhofer patterns with magnetic hysteresis, and constructs a programmable zero-field superconducting diode.

ABSTRACT

The simultaneous co-existence and gate-tuneability of the superconducting (SC), magnetic and topological orders in magic angle twisted bilayer graphene (MATBG) open up entirely new possibilities for the creation of complex hybrid Josephson junctions (JJ). Here we report on the creation of gate-defined, magnetic Josephson junctions in MATBG, where the weak link is gate-tuned close to the correlated state at a moiré filling factor of ν=-2. A highly unconventional Fraunhofer pattern emerges, which is phase-shifted and asymmetric with respect to the current and magnetic field directions, and shows a pronounced magnetic hysteresis. Interestingly, our theoretical calculations of the JJ with a valley polarized ν=-2 with orbital magnetization as the weak link explain most of these unconventional features without fine tuning the parameters. While these unconventional Josephson effects persist up to the critical temperature Tc ~ 3.5K of the superconducting state, at temperatures below T < 800mK, we observed a pronounced magnetic hysteresis possibly due to further spin-polarization of the ν=-2 state. We demonstrate how the combination of magnetization and its current induced magnetization switching in the MATBG JJ allows us to realize a programmable zero field superconducting diode, which represents a major building block for a new generation of superconducting quantum electronics.

Motivation & Objective

• Motivate and explore coexisting superconducting, magnetic, and topological orders in MATBG for hybrid Josephson devices.
• Demonstrate gate-tuned weak links near the ν = -2 correlated state.
• Characterize unconventional Josephson effects including phase shifts and hysteresis.
• Show how magnetization and current-induced switching enable a zero-field superconducting diode.
• Discuss the potential for programmable superconducting quantum electronics using MATBG JJs.

Proposed method

• Gate-tune the weak link in MATBG to the correlated ν = -2 state.
• Fabricate and characterize magnetic Josephson junctions in MATBG.
• Measure Fraunhofer patterns and hysteresis as a function of current and magnetic field.
• Perform theoretical calculations for a valley-polarized ν = -2 with orbital magnetization as the weak link.
• Analyze temperature dependence up to Tc ~ 3.5 K and below 800 mK for spin-polarization effects.
• Demonstrate diode behavior via magnetization and current-induced switching in the MATBG JJ.

Experimental results

Research questions

• RQ1 Can MATBG host gate-defined magnetic Josephson junctions with a weak link tuned to the ν = -2 correlated state?
• RQ2 What unconventional features arise in Fraunhofer patterns when the weak link is valley-polarized with orbital magnetization?
• RQ3 Does the device exhibit magnetic hysteresis and how does it depend on temperature and spin polarization?
• RQ4 Can the combination of magnetization and current-induced switching realize a zero-field superconducting diode?
• RQ5 To what extent can the observed phenomena be explained by theory without fine-tuning parameters?

Key findings

• Unconventional Fraunhofer patterns emerge that are phase-shifted and asymmetric with respect to current and field directions.
• Pronounced magnetic hysteresis is observed in the Josephson response.
• Theoretical calculations with a valley-polarized ν = -2 and orbital magnetization explain many features without fine-tuning.
• Unconventional effects persist up to Tc ~ 3.5 K, the superconducting transition temperature.
• Below T < 800 mK, enhanced magnetic hysteresis suggests further spin-polarization of the ν = -2 state.
• Demonstration of a programmable zero-field superconducting diode using magnetization and current-induced switching in the MATBG JJ.