  Graphene quantum dots (QDs) are intensively studied as platforms for the next
generation of quantum electronic devices. Fine tuning of the transport
properties in monolayer graphene QDs, in particular with respect to the
independent modulation of the tunnel barrier transparencies, remains
challenging and is typically addressed using electrostatic gating. We
investigate charge transport in back-gated graphene mechanical break junctions
and reveal Coulomb blockade physics characteristic of a single, high-quality QD
when a nanogap is opened in a graphene constriction. By mechanically
controlling the distance across the newly-formed graphene nanogap, we achieve
reversible tunability of the tunnel coupling to the drain electrode by five
orders of magnitude, while keeping the source-QD tunnel coupling constant.
These findings indicate that the tunnel coupling asymmetry can be significantly
modulated with a mechanical tuning knob and has important implications for the
development of future graphene-based devices, including energy converters and
quantum calorimeters.
