Ciao!
I am Filippo Gaggioli,
postdoc in the Condensed Matter Theory group
@ MIT.

The discovery of superconducting states in multilayer rhombohedral graphene with spin andvalley polarization [1] has raised an interesting question: how does superconductivity cope withtime-reversal symmetry breaking? In this work, using Ginzburg-Landau theory and microscopiccalculation, we predict the existence of a new superconducting state at low electron density, whichexhibits a spontaneously formed lattice of vortices and antivortices hosting Majorana zero-modes intheir cores. We further identify this vortex-antivortex lattice (VAL) state in the experimental phasediagram and describe its experimental manifestations.

The tunability of superconducting magic-angle twisted-layer graphene films elevates this material system to a promising candidate for superconducting electronics. We implement a gate-tuned Josephson junction in a magic-angle twisted four-layer graphene film. Field-dependent measurements of the critical current show a Fraunhofer-like pattern that differs from the standard pattern with characteristics typical for a weak transverse screener. We observe sudden shifts associated with vortices jumping into and out of the leads. By tuning the leads to the edge of the superconducting dome, we observe fast switching between superconducting and normal states, an effect associated with vortex dynamics. Time-dependent measurements provide us with the vortex energy scale and an estimate for the London penetration depth, in agreement with recent kinetic inductance measurements on twisted graphene films. Our results prove the utility of our junction as a sensor for vortex detection, allowing us to extract fundamental properties of the 2D superconductor.

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Nonreciprocal currents arise in a broad range of systems, from magnons and phonons to supercurrents, due to an interplay between spatial and temporal symmetry breakings. These find applications in devices, such as circulators and rectifiers, as well as in probing the interactions and states that underlie the nonreciprocity. An established symmetry argument anticipates the emergence of nonreciprocal currents along a direction perpendicular to the applied magnetic field that breaks the time-reversal symmetry. Here, motivated by recent experiments, we examine the emergence of nonreciprocity in vortex-limited superconducting critical currents along an applied magnetic field. Employing London’s equations for describing the Meissner response of a superconducting film, we find that an additional symmetry breaking due to a preferred vortex axis enables nonreciprocal critical currents along the applied magnetic field, consistent with the so far unexplained experimental observation. Building on our concrete theoretical model for supercurrents, we discuss a possible generalization of the prevailing symmetry consideration to encompass nonreciprocal currents along the time-reversal symmetry-breaking direction.

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