  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 nonequlibrium
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 towards novel quantum functions such as quantum switching.
