Two-dimensional quantum transport: Tunnel vision

PatrickGallagher and colleagues demonstrate the rst successful manoeuvre to form a quantum mechanical tunnel under an ionic liquid that is extremely voluminous compared with the nanometre scale of the tunnel. The non-trivial shield is made of insulating Al2O3 and the tunnel is created by applying a voltage to theshield. The tunnel, more technically knownas a quantum point contact (QPC), is typically formed by pulling on a current-conducting wire until it snaps. Just beforeit breaks, the current is pinched o at the breakpoint. The metal tip of a scanning tunnelling microscope also forms a QPC against the surface of its contacting material. If the pinch-o width is comparable tothe wavelength of the electron (hole), the particle owing through such a narrowpath can no longer be considered classical. In particular, each mobile electron(hole) in the metal has a small electron wavelength given by =(2/ns)1/2, where ns is the sheet carrier densitythat is typically on the order of 110nm. The conductance is then quantized in multiples of the quantum conductance G0=2e2/h, where e is the elementary charge and h is Plancksconstant.