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Unexpected Results in Graphene

MIT Postdoctoral Fellow Benjamin M. Hunt part of group to show new electronic behavior in layered graphene system.

Ashoori-Hunt-in-Lab 
Physics Professor Raymond C. Ashoori, left, and Postdoctoral Fellow Benjamin M. Hunt in an MIT lab where a dilution refrigerator is used to study electrical charges and conductivity of materials such as semiconductors at very low temperatures. Studies are conducted at 0.1 degrees (-273.05), just a fraction above absolute zero (-273.15 Celsius). Photo: Denis Paiste, Materials Processing Center

MIT Postdoctoral Fellow Benjamin M. Hunt points to an initial finding of unexpectedly high resistivity in graphene coupled to hexagonal boron nitride as the trigger that led to new understandings of electronic behavior in graphene.

Hunt was a key contributor and co-author of a recent paper in Science that demonstrated a unique bandgap in graphene coupled to hexagonal boron nitride that could be a precursor to developing the material for functional transistors. The researchers also showed in a paper in Nature that having a component of the applied magnetic field in the graphene plane forced electrons at the edge of graphene to move in opposite directions based on their spins. (See related story.)

“Andrea Young, Javier Sanchez-Yamagishi and I were essentially responsible for every aspect of those papers from sample fabrication to measurement to writing the paper,” Hunt says. Measurements were made both at MIT and at the National High Magnetic Field Lab in Tallahassee, Fla. Hunt notes that Ashoori and Mitsui Career Development Associate Professor of Physics Pablo Jarillo-Herrero were equal contributors to the Science and Nature papers, as were Hunt, Young and Sanchez-Yamagishi.

“The way that the study worked was we were attempting to get data for the second paper, for the Tunable Symmetry Breaking paper, and we cooled one of  our samples down and started measuring its resistance. What we found was that it became much more resistive than we ever expected. So that was the most surprising thing,” notes Hunt, 34.

“We had seen many samples of graphene in which the resistance was supposed to be a certain value and that certain value was related to Planck’s constant and the fundamental electron charge. What we found was the resistance was much, much greater than this. We really were not expecting to see this. So that was the discovery that led to the investigation of this moiré pattern that we saw in the first Science paper.

The research revealed the alignment of layered graphene and hexagonal boron nitride caused interactions between carbon atoms in one layer and boron atoms or nitrogen atoms in the other that changed the character of graphene, creating a gap that could alter its electronic behavior. “The gap changes the character of graphene from one that is always conducting to one that can be conducting or insulating,” Hunt explains.

“So it was the initial discovery that the system was much, much more resistive than we ever expected that led to all of the discoveries or all of the measurements, and the new physics that we saw for the Science paper,” Hunt says.

The Science paper also showed plots for the graphene-hexagonal boron nitride composite revealing the existence of a “Hofstadter Butterfly,” a graphical representation of electronic energy levels that resembles a butterfly. Hunt is interested in improving the time domain capacitance spectroscopy technique that Ashoori invented in order to study graphene and to study Hofstadter Butterfly in greater detail.

Gap-in-Graphene
By stacking graphene on a similarly patterned layer of boron nitride, MIT researchers found interactions between carbon and boron atoms on one sublattice and carbon and nitrogen atoms on the other resulted in adequate breaking of the sublattice symmetry to give the electrons an observable mass. In the above illustration, adjacent atoms are referred to as A sites and B sites. Illustration used with permission of Michael Fuhrer, Monash University.

Because of the very high magnetic fields and very low temperatures used in the recent studies, they don’t have direct applicability to room temperature semiconductor systems such as computers. “The exact system that we’re studying is probably not applicable, however, the physical principles that we are unveiling may be applicable to a system that you can study at much higher temperatures and much lower magnetic fields, something that might be attainable someday at room temperature. It’s really the physical principles that we’re interested in exposing,” Hunt says. The spin-oriented behavior of electrons at the edge of graphene may have potential application in spintronic devices.

Hunt, who received his doctorate at Cornell, is interviewing at universities for faculty positions. His MIT postdoctoral position is funded through a grant from the Gordon and Betty Moore Foundation. Hunt is working on several projects, among them capacitance studies in bi-layer graphene, time domain capacitance spectroscopy analysis of semiconductor structures, and topological insulators in conjunction with Jarillo-Herrero’s group.

A postdoctoral researcher in condensed matter physics has to be a bit of a jack-of-all-trades. The required skill set for  postdoctoral fellows, associates or graduate students includes sample design, measurement techniques, cryogenic and low temperature techniques and data analysis.  “It makes it enjoyable though. It’s nice to feel like you can do a lot of things,” Hunt says.

– Written by Denis Paiste, Materials Processing Center

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Materials Processing Center

Faculty Highlight: Raymond C. Ashoori

MIT researchers gearing up for Center for Integrated Quantum Materials

MIT News Office

Stacking 2-D materials produces surprising results  

Graphene can host exotic new quantum electronic states at its edges

Ashoori awarded $1.7 million grant by the Gordon and Betty Moore Foundation

Research Papers

Massive Dirac Fermions and Hofstadter Butterfly in a van der Waals Heterostructure

Tunable symmetry breaking and helical edge transport in a graphene quantum spin Hall state
Massachusetts Institute of Technology
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