Kevin Myhro

In Situ Observation of Electrostatic and Thermal Manipulation of Suspended Graphene Membranes

Graphene is natures thinnest elastic membrane, and its morphology has important impacts on its electrical, mechanical, and electromechanical properties. Here we report manipulation of the morphology of suspended graphene via electrostatic and thermal control. By measuring the out-of-plane deflection as a function of applied gate voltage and number of layers, we show that graphene adopts a parabolic profile at large gate voltages with inhomogeneous distribution of charge density and strain. Unclamped graphene sheets slide into the trench under tension; for doubly clamped devices, the results are well-accounted for by membrane deflection with effective Youngs modulus E = 1.1 TPa. Upon cooling to 100 K, we observe buckling-induced ripples in the central portion and large upward buckling of the free edges, which arises from graphenes large negative thermal expansion coefficient.

Broken Symmetry Quantum Hall States in Dual-Gated ABA Trilayer Graphene

ABA-stacked trilayer graphene is a unique 2D electron system with mirror reflection symmetry and unconventional quantum Hall effect. We present low-temperature transport measurements on dual-gated suspended trilayer graphene in the quantum Hall (QH) regime. We observe QH plateaus at filling factors ν = -8, -2, 2, 6, and 10, which is in agreement with the full-parameter tight binding calculations. In high magnetic fields, odd-integer plateaus are also resolved, indicating almost complete lifting of the 12-fold degeneracy of the lowest Landau level (LL). Under an out-of-plane electric field E(perpendicular), we observe degeneracy breaking and transitions between QH plateaus. Interestingly, depending on its direction, E(perpendicular) selectively breaks the LL degeneracies in the electron-doped or hole-doped regimes. Our results underscore the rich interaction-induced phenomena in trilayer graphene.

Band gap and correlated phenomena in bilayer and trilayer graphene

Graphene and its few layer cousins are unique two-dimensional (2D) systems with extraordinary electrical, thermal, mechanical and optical properties, and they have become both fantastic platforms for exploring fundamental processes and some of the most promising material for next generation electronics. Here we present our transport studies of dual gated suspended bilayer and trilayer graphene devices. At the charge neutrality point, application of an electric field induces a gap in bilayer graphene’s band structure. For high mobility bilayer devices, we observe an intrinsic insulating state with a gap of 2-3 meV and a transition temperature ~5K, which arises from electronic interactions. In ABC-stacked trilayer devices, an insulating state with gap ~25 meV is observed. Our results underscore the rich interaction-induced collective states in few layer graphene and suggest a promising direction for THz technology and high speed low dissipation electronic devices.