Date: Wednesday, Nov. 5, 2025

Time: 11:40 am

Place: Aula 1.A1 (Faculty of Science & Technology, Leioa, UPV/EHU)

Title: Vortices in superconducting magic angle graphene

Speaker: Prof. Vadim Geshkenbein (ETH Zurich)

Abstract:

Superconductors only a few atoms thick offer an exciting platform for studying quantum phenomena in two dimensions. A major challenge is observing vortices—tiny whirlpools of supercurrent—which are central to understanding how these materials work. In bulk superconductors, vortices create measurable magnetic fields and can be imaged with various techniques. However, in atomically thin films, vortices produce almost no magnetic field, making them nearly impossible to detect directly.

We turn this weakness into a strength: precisely because the magnetic screening is so weak, we can use Josephson junctions (thin insulating barriers between superconductors) as ultrasensitive vortex detectors. Vortices in the superconducting leads change how current flows through the junction, creating a characteristic pattern as we vary the magnetic field.

At higher temperatures, thermal energy causes vortices to randomly jump in and out of the leads, scrambling this pattern. Our model explains these fluctuations quantitatively. By studying how fast vortices jump, we can measure fundamental properties like the superfluid stiffness and Berezinskii-Kosterlitz-Thouless transition temperature of magic-angle twisted four-layer graphene.