Condensed Matter Physics Seminar

Electronic compressibility measurements provide a powerful tool to investigate correlated phenomena in two-dimensional electron systems.  Graphene serves as an especially intriguing system of study because its many underlying symmetries enable the formation of a rich variety of many-body states.  In this talk, I will describe local electronic compressibility measurements of exceptionally clean monolayer and bilayer graphene devices in the quantum Hall regime.  In both materials, we observe an unusual sequence of fractional quantum Hall states.  In monolayer graphene, even-numerator states are absent between filling factors |ν| = 1 and 2, and we observe a series of fractional quantum Hall phase transitions as a function of magnetic field.  Meanwhile, bilayer graphene exhibits electron-hole asymmetric integer and fractional quantum Hall effects.  I will describe how these surprising behaviors relate to the symmetries and symmetry-breaking terms in each material.