Observation of Van Hove singularities in twisted graphene layers

Electronic instabilities at the crossing of the Fermi energy with a Van Hove singularity in the density of states often lead to new phases of matter such as superconductivity, magnetism or density waves. However, in most materials this condition is difficult to control. In the case of single-layer graphene, the singularity is too far from the Fermi energy and hence difficult to reach with standard doping and gating techniques. Here we report the observation of low-energy Van Hove singularities in twisted graphene layers seen as two pronounced peaks in the density of states measured by scanning tunneling spectroscopy. We demonstrate that a rotation between stacked graphene layers can generate Van Hove singularities, which can be brought arbitrarily close to the Fermi energy by varying the angle of rotation. This opens intriguing prospects for Van Hove singularity engineering of electronic phases.Comment: 21 pages 5 figure

Microscopy capabilities of the Microscopy, Electrochemistry, and Conductivity Analyzer

The Phoenix microscopy station, designed for the study of Martian dust and soil, consists of a sample delivery system, an optical microscope, and an atomic force microscope. The combination of microscopies facilitates the study of features from the millimeter to nanometer scale. Light-emitting diode illumination allows for full color optical imaging of the samples as well as imaging of ultraviolet-induced visible fluorescence. The atomic force microscope uses an array of silicon tips and can operate in both static and dynamic mode. Copyright 2008 by the American Geophysical Union