Probing Interlayer States in 2D Materials Randall M. Feenstra, Carnegie-Mellon University, DMR Two-dimensional (2D) materials possess a special type of electronic state that is localized between the atomic planes (not on the planes). The energies of such interlayer states are sensitive to the interplanar separation, as well as to the presence of e.g. intercalants between planes. We have measured the energies of interlayers states for graphene on Cu(001) and Cu(111) using a low- energy electron microscope. In spectra of electron reflectivity vs. energy, minima occur at the energies of interlayer states. By comparing the measured spectra with theoretical predictions, the separation between the graphene and the substrate is determined. Comparison of experiment and theory for low- energy electron reflectivity of graphene on copper. Srivastava, Gao, Widom, Feenstra, Nie, McCarty, and Vlassiouk, Phys. Rev. B 87, (2013).

WaveTrans – for obtaining wavefunctions from VASP Randall M. Feenstra, Carnegie-Mellon University, DMR VASP, the Vienna Ab-Initio Simulation Package, is used by hundreds of researchers worldwide to perform density-functional computations for solids. Remarkably, a computer program that enabled the extraction of complete, real-space wavefunctions from VASP was lacking. Rather, the decomposition of wavefunctions in terms of atomic basis states on specified atoms was provided, although that is insufficient for many applications. For example, the PI has studied interlayer states formed between graphene and metal surfaces such as copper. These wavefunctions, examples of which are shown in the figure, have peaks (marked by arrows) located between the atomic planes. To obtain the complete wavefunctions, the PI and his collaborator Michael Widom of CMU, developed a program that extracted the Fourier coefficients of the wavefunction from a binary file output by VASP. The routine is posted at and a link to that site has been placed on the VASP user bulletin board. Wavefunction for graphene on copper for different spacings and energies.

Undergrad Project Development of Vacuum System Randall M. Feenstra, Carnegie-Mellon University, DMR Photo shows Maddie McKay, an undergrad Physics major at CMU, who has worked in Prof. Feenstra’s laboratory during her junior and senior years developing a small vacuum system. This system will be employed in a Physics Laboratory Class to enable deposition of thin films that will be subsequently oxidized to form tunnel barriers. Maddie’s work included design of all components, procurement of parts (through purchase orders, or with mechanical drawings of pieces that were then fabricated), assembly, and testing. Use of the system to form tunnel junction is presently in progress. The photo was taken during the department’s annual poster session, held at the end of each Spring semester.