CEM Special Seminar

Graphene Nanomechanics and Electronic Properties of Graphene/BN Heterostructures

Marc Bockrath
University of California, Riverside | Department of Physics and Astronomy

Marc Bockrath

We measure the quality factor Q of electrically-driven few-layer graphene drumhead resonators, providing an experimental demonstration that Q~1/T, where T is the temperature. Because the resonators are atomically thin, out-of-plane fluctuations are large. As a result Q is mainly determined by stochastic frequency broadening rather than frictional damping, in analogy to nuclear magnetic resonance. Additionally, at larger drives the resonance linewidth is enhanced by nonlinear damping, in qualitative agreement with recent theory of damping by radiation of in-plane phonons. Parametric amplification produced by periodic thermal expansion from the ac drive voltage yields an anomalously large linewidth at the largest drives. Our results contribute towards a general framework for understanding the mechanisms of dissipation and spectral line broadening in atomically thin membrane resonators.
Moreover, recently several research groups have demonstrated accurate placement of graphene on hexagonal BN (hBN) with crystallographic alignment. Due to the resulting superlattice formed in the graphene/hBN heterostructures, an energy gap, secondary Dirac Points, and Hofstadter quantization in a magnetic field have been observed. Using aligned layer transfer we are able to produce graphene/hBN heterostructures with ~1 degree alignment accuracy, and measure the transport properties of the resulting systems. We observe an additional Pi Berry’s phase shift in the magneto-oscillations when tuning the Fermi level past the secondary Dirac points, originating from a change in topological winding number from odd to even when the Fermi-surface electron orbit begins to enclose the secondary Dirac points. At large hole doping, inversion symmetry breaking generates a distinct hexagonal pattern in the longitudinal resistivity versus magnetic field and charge density. This results from a systematic pattern of replica Dirac points and gaps, reflecting the fractal spectrum of the Hofstadter butterfly. Finally, we study the properties of additional graphene/hBN layer structures such as twisted trilayers that are comprised of AB-stacked bilayer graphene contacting a graphene monolayer through a twist angle, coupling the massive bilayer spectrum to that of the massless monolayer spectrum. The interlayer interactions and screening produce a nonlinear monolayer graphene capacitance, and in a magnetic field enable Landau level spectroscopy to be performed. Our latest results will be discussed.