Superconductivity and spin canting in spin-orbit-coupled trilayer graphene

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Materials

Background:

• Graphene and transition metal dichalcogenide flat-band systems exhibit complex phase diagrams with magnetic and superconducting properties.
• The interplay between magnetic ordering and superconductivity in these systems, particularly the mechanism of enhancement, remains an open question.
• Previous studies on Bernal bilayer graphene suggested spin-orbit coupling enhances superconductivity, but the underlying mechanism was unclear.

Purpose of the Study:

• To investigate the effect of spin-orbit coupling on superconductivity in rhombohedral trilayer graphene (RTG).
• To elucidate the mechanism by which spin-orbit coupling influences superconducting properties and critical temperatures in RTG.
• To explore the relationship between magnetic ordering, spin-orbit coupling, and superconductivity in this material system.

Main Methods:

• Fabrication of rhombohedral trilayer graphene (RTG) utilizing substrate proximity effect to introduce spin-orbit coupling.
• Electrical transport measurements to identify and characterize superconducting pockets and critical temperatures (T_c).
• Local magnetometry to probe the magnetic state and its transition in relation to superconductivity.
• Hartree-Fock calculations to model the electronic interactions and phase transitions.

Main Results:

• Introduction of spin-orbit coupling in RTG via substrate proximity generated new superconducting pockets for both electron and hole doping.
• Maximal critical temperature (T_c) reached approximately 300 mK, a threefold increase compared to RTG encapsulated by hexagonal boron nitride.
• Superconductivity was observed to straddle a transition between a spin-canted magnetic state and a spin-valley locked state.
• Hartree-Fock calculations reproduced this transition, attributing it to the competition between spin-orbit coupling and Hund's interaction.

Conclusions:

• Spin-orbit coupling significantly enhances superconductivity in rhombohedral trilayer graphene, leading to higher critical temperatures.
• The enhancement appears driven by a quantitative change in the spin-canted order parameter, rather than a change in the overall ground state symmetry.
• These findings support a mechanism where fluctuations in spin-canted order contribute to the superconducting pairing interaction.