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UMD Discovery May Advance Magnetic 'Superpower' for Graphene

December 14, 2015

Jennifer Figgins Rooks 301-405-1458
Lee Tune 301-405-4679

COLLEGE PARK, Md. – Graphene, the superhero of materials, is stronger than steel, harder than diamond, flexible, essentially transparent and ready to transform technology. A University of Maryland-led team of researchers has made a new discovery advancing scientific efforts to add the power to generate strong magnetic fields to graphene’s list of super abilities. 

Teng LiAssociate Professor in the Department of Mechanical Engineeringand graduate student Shuze Zhu, along with National Institute of Standards and Technology (NIST) collaborator Joseph Stroscio, have developed a theoretical model that demonstrates how to shape and stretch graphene to create a powerful, adjustable and sustainable magnetic force.  Their finding could provide engineers and scientists a testbed for understanding how electrons move in extremely high magnetic fields.

The team’s model indicates that when stretched or strained, graphene's electrons behave as if they are in a strong magnetic field. This so-called pseudomagnetic effect could open up new possibilities in graphene electronics. 

 Illustration shows how applying a simple stretch to a specifically shaped sheet of graphene creates a stable and controllable pseudomagnetic field.So far, researchers have only been able to induce highly localized pseudomagnetic fields that need peculiar loading conditions that are prohibitively difficult to realize in practice. However, the new discovery by UMD researchers may show scientists how to shape a graphene ribbon so that simply pulling its two ends produces a uniform pseudomagnetic field. With today’s nanofabrication technologies, the team is confident that they will soon be able to transition their theoretical model into a design reality.

“Our findings reveal a facile yet effective solution to achieve an extremely high pseudomagnetic field in a planar graphene by a simple stretch," said research leader Associate Professor Teng Li.

The new UMD finding builds on an accidental discovery in 2010, by researchers at the Lawrence Berkeley National Laboratory, who – when handling a two-dimensional lattice of graphene,  created a tiny triangular, bubble shape in the material that caused a pseudomagnetic field in the bubble as high as 300 Tesla—well beyond what can be attained with stable laboratory magnets. The current record for a lab-produced magnetic field is the creation of an 85 tesla field for less than a small fraction of a second.

While it seems simple enough to stretch a material in two directions—like tugging on the ends of a rubber band—the UMD team discovered that the graphene sheet needed to not only be stretched, but that the sheet must also be shaped in a specific way. A simple rectangle or square of graphene, when stretched, would not create a pseudomagnetic field.

But, when the graphene was formed into a tapered shape like a trapezoid or pennant, pulling on the ends produces a strain that steadily increases along the length of the ribbon, and this constant strain gradient gives a uniform, and controllable, pseudomagnetic field. And the more strain applied to the material, the greater the magnetic force. The team’s model, which was verified across three computational models, predicts a tunable field magnitude from zero to 200 Tesla.

This type of controlled pseudomagnetic field creates the potential for new ways to study the motion of electrons in a controllable high magnetic field. Currently, there is no sustainable method for generating magnetic fields of this magnitude. The induced fields – if made more spatially uniform – could potentially enable new concepts of electronics, such as “valleytronics,” in which electrons separate between different valleys in the graphene band structure. 

Physical Review Letters published the team's research, "Programmable Extreme Pseudomagnetic Fields in Graphene by a Uniaxial Stretch," on December 8, 2015, and highlighted the work as an “Editors’ Suggestion.”  The findings are also featured on the Applied Physics Society's Physics site.