Suyong Jung1, 2, Nikolai N. Klimov1-3, David N. Newell3, Nikolai B. Zhitenev1, Joseph A. Stroscio1


1Center for Nanoscale Science and Technology, NIST

2Maryland NanoCenter, University of Maryland

3Physical Measurement Laboratory, NIST



Bilayer graphene, the only known material to have an electrostatically tunable energy band gap, has drawn significant interests from both science and engineering communities due to the opening of the gap in its low energy electronic spectrum, which offers a route to future electronic applications.  Using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) measurements, we have found the microscopic nature of the bilayer band gap is very different from the characteristics observed in previous macroscopic measurements or expected from current theoretical models.  The bilayer graphene device was fabricated through mechanical exfoliation of natural graphite on SiO2/Si substrate and SiN stencil mask evaporation of metal contacts.   STM/STS studies were performed in ultra-high vacuum, liquid He temperature (4.3 K) and high magnetic fields (8 T).  The band gap, which is proportional to charge imbalance between the layers, shows strong spatial dependence on the disorder potential varying in both magnitude and sign on a microscopic level.  We have investigated these nanoscale variations of the energy gaps while controlling both the total charge density with an external gate electric field, and magnetic quantization strength with varying magnetic field.  Our measurements suggest that such gap behavior can be caused by spontaneous symmetry breaking with dipoles nucleated with opposite signs at minima and maxima of the disorder potential.