Nanoscale Electronics and Mechanics in Low-Dimensional Material Systems

School of Physics Hard Condensed Matter & AMO Seminar: Prof. Marc Bockrath, University of California Riverside

We will discuss a number of our ongoing research projects aimed at understanding the properties of low-dimensional systems such as graphene and two-dimensional material heterostructures. We first 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, we find that Q is mainly determined by stochastic frequency broadening rather than frictional damping, in analogy to nuclear magnetic resonance. In addition, recently several research groups have demonstrated placing graphene on hexagonal BN (hBN) with crystallographic alignment. This not only creates a protected environment yielding high-mobility devices, but also due to the resulting superlattice formed in these heterostructures, an energy gap, secondary Dirac Points, and Hofstadter quantization in a magnetic field have been observed.

In these systems, we observe a p Berry’s phase shift in the magneto-oscillations when tuning the Fermi level past the secondary Dirac points, originating from a change in topological pseudospin winding number from odd to even when the Fermi-surface electron orbit begins to enclose the secondary Dirac points. We also observe a distinct hexagonal pattern in the longitudinal resistivity versus magnetic field and charge density, resulting 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 electrostatically gated structures such as twisted trilayers that are comprised of AB-stacked bilayer graphene contacting a graphene monolayer through a twist angle, and hBN-encapsulated graphene bilayers with large applied perpendicular electric field. In the twisted trilayers, which couple the massive bilayer spectrum to that of the massless monolayer spectrum, the interlayer interactions and screening produce a nonlinear monolayer graphene gate capacitance and renormalize the bilayer band structure. In the encapsulated bilayers, we perform Landau level spectroscopy, measure the layer polarizability of the electrons, and observe easy-axis quantum Hall ferromagnetism. Our latest results will be discussed.