4 December 2015

MagLab users observe new type of fractional quantum Hall effect

New kind of quantum Hall state observed in graphene superlattices.

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TALLAHASSEE, Fla. — Building on the 2013 landmark discovery of the Hofstadter butterfly at the National High Magnetic Field Laboratory (National MagLab), researchers from Columbia University have discovered a new quantum state of matter. Their results are published this week in the journal Science.

Studying BN-supported graphene with a large wavelength moiré superlattice, the research team exposed the sample to high magnetic fields available at the National MagLab and revealed a quantum phase within the Hofstadter butterfly energy pattern in which electrons exhibit new and unusual quantum properties.

"This new state of matter, which we call a fractional Bloch band quantum Hall effect (FBQHE), was completely unexpected," says Lei Wang, a postdoctoral fellow in Mechanical Engineering at Columbia University and the lead author of the discovery.

The fractional quantum Hall effect is at the heart of one of the outstanding problems in modern condensed matter physics — understanding large collections of interacting quantum particles behaving in concert that give rise to new characteristics that are not a feature of the individual parts. This work identifies a new kind of quantum Hall state that does not fit within either of the two existing classes of quantum Hall states, but rather represents a new kind of phase, one that emerges under the specific condition of strongly interacting electrons moving in a spatially periodic structure under the influence of a magnetic field.

"We've seen how magnetic fields can fractionally quantize the Landau level index, which was a Nobel-Prize-winning discovery," said National MagLab Director Greg Boebinger. "Now, with this latest exciting result from graphene, we learn that magnetic fields can also impose fractional quantization on the Bloch index. Magnetic fields and two-dimensional electrons pull off another big surprise!"

The high-field measurements were performed at the DC Field Facility in Tallahassee, Florida. The unique capability of the hybrid magnet at this facility, able to produce fields up to 45 teslas, while cooling the sample to miliKelvin temperatures, was essential to this research.

"The combination of large fields, where electron interactions become strong, and low temperature, where small energy gaps can be observed, was crucial," explains Columbia professor Cory Dean, a collaborator on this research. "Not to mention the truly remarkable technical support at the MagLab, which makes this kind of extreme measurement possible."

Also crucial to this experiment was the ability to fabricate ultra clean layered crystal structures with nano-scale precision control. Collaborators from Columbia University’s Department of Mechanical Engineering created devices that eliminate disorder so sensitive quantum effects could be revealed.

The research team will now turn its attention to developing new measurement techniques to continue studying this unique collective behavior and new techniques to pattern even larger lattice structures. They hope that this system may one day provide access to entirely new kinds of quantum particles.

"Right now, we are very much at the discovery phase," said Dean. "The next challenge is to understand the precise properties associated with this phase. The opportunity for these kinds of unexpected discoveries is what makes experimental physics so tremendously exciting."

This work was supported by the Department of Defense through the National Defense Science and Engineering Graduate Fellowship Program, the National Science Foundation (DMR-1157490), the Air Force Office of Scientific Research (FA9550-09-1-0705), the office of Naval Research (N000141310662), the ONR Grant N000141110633 and the Defense Advanced Research Projects Agency (under ONR Grant N000141210814) and the Nano Material Technology Development Program through the National Research Foundation of Korea (2012M3A7B4049966).

The National High Magnetic Field Laboratory is the world’s largest and highest-powered magnet facility. Located at Florida State University, the University of Florida and Los Alamos National Laboratory, the interdisciplinary National MagLab hosts scientists from around the world to perform basic research in high magnetic fields, advancing our understanding of materials, energy and life. The lab is funded by the National Science Foundation (DMR-1157490) and the state of Florida. For more information, visit us online at nationalmaglab.org or follow us on Facebook, Twitter, Instagram and Pinterest at NationalMagLab.