Optimizing Dirac fermions quasi-confinement by potential smoothness engineering

TL;DR

This paper demonstrates that the smoothness of potential barriers in graphene p-n junctions significantly influences Dirac fermions' quasi-confinement and interference, with smoother barriers enhancing interference contrast and confinement.

Contribution

It introduces a method to control p-n junction smoothness in graphene using a scanning gate microscope tip, revealing its impact on Dirac fermions' behavior through experiments and simulations.

Findings

Smoother potential barriers increase interference contrast.

Sharp barriers allow better transmission of Dirac fermions.

Smooth barriers induce quasi-confinement of Dirac fermions.

Abstract

With the advent of high mobility encapsulated graphene devices, new electronic components ruled by Dirac fermions optics have been envisioned and realized. The main building blocks of electron-optics devices are gate-defined p-n junctions, which guide, transmit and refract graphene charge carriers, just like prisms and lenses in optics. The reflection and transmission are governed by the p-n junction smoothness, a parameter difficult to tune in conventional devices. Here we create p-n junctions in graphene, using the polarized tip of a scanning gate microscope, yielding Fabry-P\'erot interference fringes in the device resistance. We control the p-n junctions smoothness using the tip-to-graphene distance, and show increased interference contrast using smoother potential barriers. Extensive tight-binding simulation reveal that smooth potential barriers induce a pronounced quasi-confinement of Dirac fermions below the tip, yielding enhanced interference contrast. On the opposite, sharp barriers are excellent Dirac fermions transmitters and lead to poorly contrasted interferences. Our work emphasizes the importance of junction smoothness for relativistic electron optics devices engineering.