Abstract
We show that Friedel charge oscillation near an interface opens a gap at the Fermi energy for electrons with wave vectors perpendicular to the interface. If the Friedel gaps on two sides of the interface are different, a nonequilibrium effect—shifting of these gaps under bias—leads to asymmetric transport upon reversing the bias polarity. The predicted transport asymmetry is revealed by scanning tunneling potentiometry at monolayer-bilayer interfaces in epitaxial graphene on SiC(0001). This intriguing interfacial transport behavior opens a new avenue toward novel quantum functions such as quantum switching.
- Received 23 May 2013
DOI:https://doi.org/10.1103/PhysRevX.4.011021
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Published by the American Physical Society
Popular Summary
Electrons, by their quantum nature, are also waves. When scattered by static defects in solids such as metals and semiconductors, they form standing waves that can be seen on the solid surfaces. Such an interference pattern, often called Friedel oscillation, is not expected to impact how electrons conduct in the material because the large electron density can easily dwarf the Friedel oscillation. The situation can be different in materials such as recently discovered graphene and topological insulators, where the electron density is often low and the electronic interaction can become important. In this paper, we demonstrate that the Friedel oscillation can indeed open an energy gap for electron transport in graphene, which in turn can lead to asymmetric transport behavior across the interface in a composite monolayer-bilayer graphene system.
Our new finding is that the Friedel gap is opened because the charge oscillation, occurring at the interface, couples the right- and left-going electron waves near the Fermi energy. This gap opens both in the monolayer and in the bilayer of the composite graphene system formed epitaxially on SiC (0001), and it represents an extra energy cost for electron transmission across the monolayer-bilayer interface. The different strengths of the Coulomb interaction in the monolayer and bilayer make their gap sizes different, and that difference is accentuated by the bias voltage and manifested as asymmetric electrical transport across the interface. With our multiprobe scanning-tunneling-potentiometry measurements, we have demonstrated experimentally such a transport asymmetry.
In addition to the fundamental interest of our demonstration of the Friedel-gap-opening phenomenon, the sensitivity of the transport asymmetry to the scattering boundary conditions that we have shown makes scanning-tunneling potentiometry a suitable tool to probe the electron wave functions with respect to their chirality, Berry’s phase, and pseudospin polarization.