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(2021) Nancy Sandler: Engineering Deformed Graphene Membranes: From confined charges to Valley Filters and Moire Structures

posted 4 Mar 2021, 03:46 by Peter Boggild   [ updated 9 Mar 2021, 06:45 ]
Department of Physics and Astronomy, and Nanoscale and Quantum Phenomena Institute, Ohio University, Athens, Ohio, USA.
*e-mail: sandler@ohio.edu

As an atomically thin membrane, graphene is a highly flexible material, a property that provides the opportunity to use strain engineering to control its electronic properties. Wrinkled or rippled graphene, either suspended or on a substrate, reveals inhomogeneous charge distributions originated by underlying strain fields that affect electron dynamics. Scanning tunneling microscopy (STM) measurements on deformed samples demonstrated electron confinement with peculiar charge distributions that break sublattice symmetry. The phenomena that differentiate carbon atoms in each unit cell results in local valley currents with essential applications in the field of valleytronics, i.e., the manipulation of the valley degree of freedom for electronic purposes. Because valley filtering properties in these structures are highly dependent on the type of deformation considered, it is crucial to identify the relevant factors determining the optimal operation and detection of valley currents. While local and extended deformations confine charges, the fold geometry serves as an electronic wave-guide, as revealed in recent transport measurements.

Furthermore, we showed that fold structures provide filtering in broader energy ranges and exhibit robust features against geometrical parameter variations and incident current directions. However, designing proper geometries is not enough to isolate valley states, fully embedded in the continuum that makes graphene a semimetal. Taking the strained membrane into the Quantum Hall regimes allows the separation of these states from the continuum and provides the flexibility of positioning them at different locations in the sample. More exciting is the possibility of developing band structure engineering protocols by designing substrates able to induce specific periodic strain patterns. Our recent studies reveal that Moire structures' characteristic features appear in images of electronic charge distributions of graphene samples deposited on regular arrays of deformations. These systems exhibit narrow bands at low energies reminiscent of those observed in twisted bilayer structures, suggesting the possibility for the emergence of novel correlated physics in single graphene membranes. 

Professor Nancy Sandler is a faculty in the Department of Physics and Astronomy at Ohio University (OU), in Athens, Ohio, USA. She is a member of the Nanoscale and Quantum Phenomena Institute (NQPI) and science editor of its newsletter and member of the Editorial College at SciPost. Prof. Sandler's research focuses on novel low-dimensional materials' electronic properties and the effects of strong correlations. She is active in outreach and education activities serving as a board member in the Margaret Boyd Scholar Program, a leadership program for women undergraduates at OU, and as the physics science director in the NSF-funded NOYCE teacher fellowship program. A native of Argentina, she obtained her Lic. en Ciencias Fisicas degree from the Universidad Nacional de Buenos Aires. She continued her studies at the University of Urbana-Champaign in Illinois, USA, where she received her Ph.D. in theoretical physics (1998). After holding postdoctoral positions at ENS and Orsay (France) and Brandeis and BU (USA), she joined the Ohio University faculty in 2005. She was a visiting professor at the Dahlem Center for Complex Quantum Systems at Freie Universitat, Berlin, Germany (2012-13), and is currently a visiting faculty in the Department of Physics at the Technical University of Denmark (DTU) and the Niels Bohr Institute at the University of Copenhagen, Denmark.